Information

Descendants of dinosaurs


It is commonly accepted that birds are descendants of dinosaurs. It is nice that not all dinosaurs died out.

Are there other animals, descendants of dinosaurs, that survived ? (Maybe not till our time, but survived the time when most dinosaurs died).

By the way, what made flying dinosaurs survive and not die like other dinosaurs?


No. No other group found in the fossil record after the K-T boundary (the extinction "event") descends from dinosaurs.

It is likely that the extinction event was not itself instantaneous so if you wanted to be extremely picky you could argue that small numbers of individuals survived the K-T boundary but, apart from birds, none of these survivors went on to form long lasting lineages.

Non-avian dinosaurs that survived the extinction event are referred to as "palaeocene dinosaurs", there are some links on Wikipedia that might interest you.

You can see a diagram linking vertebrate groups here, similar diagrams can be found in most evolution texts.


Can Dinosaurs Be Resurrected from Extinction?

If you could visit a theme park that offered you a chance to view and even interact with real-life dinosaurs, would you go? I think I might. Who wants to swim with dolphins when you can hang out with dinosaurs? Maybe even ride one?

Well, if legendary paleontologist Jack Horner has his way, we just might get our wish—and, it could be much sooner than any of us realize. Horner is a champion of the scientific proposal to resurrect dinosaurs from extinction. And it looks like this idea might have a real chance at success.

Horner’s not taking the “Jurassic Park/World” approach of trying to clone dinosaurs from ancient DNA (which won’t work for myriad technical reasons). He wants to transform birds into dinosaur-like creatures by experimentally manipulating their developmental processes in a laboratory setting.

The Evolutionary Connection between Birds and Dinosaurs

The basis for Horner’s idea rises out of the evolutionary paradigm. Most paleontologists think that birds and dinosaurs share an evolutionary history. These scientists argue that shared anatomical features (a key phrase we’ll return to) between birds and certain dinosaur taxa demonstrate their evolutionary connection. Currently, paleontologists place dinosaurs into two major groups: avian and nonavian dinosaurs. Accordingly, paleontologists think that birds are the evolutionary descendants of dinosaurs.

So, if Horner and others are successful, what does this mean for creation? For evolution?

Reverse Evolution

In effect, Horner and other interested scientists seek to reverse what they view as the evolutionary process, converting birds into an evolutionarily ancestral state. Dubbed reverse evolution, this approach will likely become an important facet of paleontology in the future. Evolutionary biologists believe that they can gain understanding of how biological transformations took place during life’s history by experimentally reverting organisms to their ancestral state. Reverse evolution experiments fuse insights from paleontology with those from developmental biology, molecular biology, comparative embryology, and genomics. Many life scientists are excited, because, for the first time, researchers can address questions in evolutionary biology using an experimental strategy.

Proof-of-Principle Studies

The first bird that researchers hope to reverse-evolve into a dinosaur-like creature is the chicken (Gallus gallus). This makes sense. We know a whole lot about chicken biology, and life scientists can leverage this understanding to precisely manipulate the embryonic progression of chicks so that they develop into dinosaur-like creatures.

As I described previously (see Resources for Further Exploration), in 2015 researchers from Harvard and Yale Universities moved the scientific community one step closer to creating a “chickenosaurus” by manipulating chickens in ovo to develop snout-like structures, instead of beaks, just like dinosaurs. 1

Now, two additional proof-of-principle studies demonstrate the feasibility of creating a chickenosaurus. Both studies were carried out by a research team from the Universidad de Chile.

In one study, the research team coaxed chicken embryos to develop a dinosaur-like foot structure, instead of the foot structure characteristic of birds. 2 A bird’s foot has a perching digit that points in the backward direction, in opposition to the other toes. The perching digit allows birds to grasp. In contrast, the corresponding toe in dinosaurs is nonopposable, pointing forward.

Figure 1: Dinosaur Foot Structure. Image credit: Shutterstock

Figure 2: Bird Foot Structure. Image credit: Shutterstock

The researchers took advantage of the fact that vertebrate skeletons are plastic, meaning that their structure can be altered by muscle activity. These types of skeletal alterations most commonly occur during embryonic and juvenile stages of growth and development.

Investigators discovered that muscle activity causes the perching toe of birds to reorient during embryonic development from originally pointing forward to adopting an opposable orientation. Specifically, the activity of three muscles (flexor hallucis longus, flexor hallucis brevis, and musculus extensor hallucis longus) creates torsion that twists the first metatarsal, forcing the perching digit into the opposable position.

The team demonstrated that by injecting the compound decamethonium bromide into a small opening in the eggshell just before the torsional twisting of the first metatarsal takes place, they could prevent this foot bone from twisting. The compound causes muscle paralysis, which limits the activity of the muscles that cause the torsional stress on the first metatarsal. The net result: the chick developed a dinosaur-like foot structure.

In a second study, this same research team was able to manipulate embryonic development of chicken embryos to form a dinosaur-like leg structure. 3 The lower legs of vertebrates consist of two bones: the tibia and the fibula. In most vertebrates, the fibula is shaped like a tube, extending all the way to the ankle. In birds, the fibula is shorter than the tibia and has a spine-like morphology (think chicken drumsticks).

Figure 3: The Lower Leg of a Chicken. Image credit: Shutterstock

Universidad de Chile researchers discovered that the gene encoding the Indian Hedgehog protein becomes active at the distal end of the fibula during embryonic development of the lower leg in chicks, causing the growth of the fibula to cease. They also learned that the event triggering the increased activity of the Indian H edgehog gene likely relates to the depletion of the Parathyroid Hormone-Related Protein near the distal end of the fibula. This protein plays a role in stimulating bone growth.

The researchers leveraged this insight to experimentally create a chick with dinosaur-like lower legs. Specifically, they injected the amniotic region of the chicken embryo with cyclopamine. This compound inhibits the activity of Indian Hedgehog. They discovered that this injection altered fibula development so that it was the same length as the tibia, contacting the ankle, just like in dinosaurs.

These two recent experiments on foot structure along with the previous one on snout structure represent s cience at its best. While the experiments reside at the proof-of-principle stage, they still give scientists like Jack Horner reason to think that we just might be able to resurrect dinosaurs from extinction one day. These experiments also raise scientific and theological questions.

Do Studies in Reverse Evolution Support the Evolutionary Paradigm?

On the surface, these studies seemingly make an open-and-shut case for the evolutionary origin of birds. It is impressive that researchers can rewind the tape of life and convert chickens into dinosaur-like creatures.

But deeper reflection points in a different direction.

All three studies highlight the amount of knowledge and insight about the developmental process required to carry out the reverse evolution experiments. The ingenious strategy the researchers employed to alter the developmental trajectory is equally impressive. They had to precisely time the addition of chemical agents at the just-right levels in order to influence muscle activity in the embryo’s foot or gene activity in the chick’s developing lower legs. Recognizing the knowledge, ingenuity, and skill required to alter embryological development in a coherent way that results in a new type of creature forces the question: Is it really reasonable to think that unguided, historically contingent processes could carry out such transformations when small changes in development can have profound effects on an organism’s anatomy?

It seems that the best the evolutionary process could achieve would be the generation of “monsters” with little hope of survival. Why? Because evolutionary mechanisms can only change gene expression patterns in a random, haphazard manner. I would contend that the coherent, precisely coordinated genetic changes needed to generate one biological system from another signals a Creator’s handiwork, not undirected evolutionary mechanisms, as the explanation for life’s history.

Can a Creation Model Approach Explain the Embryological Similarities?

Though the work in reverse evolution seems to fit seamlessly within an evolutionary framework, observations from these studies can be explained from a creation model perspective.

Key to this explanation is the work of Sir Richard Owen, a preeminent biologist who preceded Charles Darwin. In contemporary biology, scientists view shared features possessed by related organisms as evidence of common ancestry. Birds and theropod dinosaurs would be a case in point. But for Owen, shared anatomical features reflected an archetypal design that originated in the Mind of the First Cause. Toward this end, the anatomical features shared by birds and theropods can be understood as reflecting common design, not common descent.

Though few biologists embrace Owen’s ideas today, it is important to note that his ideas were not tried and found wanting. They simply were abandoned in favor of Darwin’s theory, which many biologists preferred because it provided a mechanistic explanation for life’s history and the origin of biological systems. In fact, Darwin owes a debt of gratitude to Owen’s thinking. Darwin coopted the idea of the archetype, but then replaced the canonical blueprint that existed in the Creator’s Mind (per Owen) with a hypothetical common ancestor.

This archetypal approach to biology can account for the results of reverse-evolution studies. Accordingly, the researchers have discovered differences in the developmental program that affect variations in the archetype, yielding differences in modern birds and long-extinct dinosaurs.

The idea of the archetype can extend to embryonic growth and development. One could argue that the Creator appears to have developed a core (or archetypal) developmental algorithm that can be modified to yield disparate body plans. From a creation model standpoint, then, the researchers from Harvard and Yale Universities and the Universidad de Chile didn’t reverse the evolutionary process. They unwittingly reverse-engineered a dinosaur-like developmental algorithm from a bird-like developmental program.

Why Would God Create Using the Same Design Templates?

There may well be several reasons why a Creator would design living systems around a common set of templates. In my estimation, the most significant reason is discoverability.

Shared anatomical and physiological features, as well as shared features of embryological development make it possible to apply what we learn by studying one organism to others. This shared developmental program makes it possible to use our understanding of embryological growth and development to reengineer a bird into a dinosaur-like creature. Discoverability makes it easier to appreciate God’s glory and grandeur, as evinced in biochemical systems by their elegance, sophistication, and ingenuity.

Discoverability also reflects God’s providence and care for humanity. If not for the shared features, it would be nearly impossible for us to learn enough about the living realm for our benefit. Where would biomedical science be without the ability to learn fundamental aspects about our biology by studying model organisms such as chickens? And where would our efforts to re-create dinosaurs be if not for the biological designs they share with birds?


Meet Cassowary Brothers Irwin and Dundee

Standing eye-to-eye with a southern cassowary, it is easy to see how this giant bird is considered the dinosaurs’ closest living relative. With an imposing frame that is more than 5.5 feet tall, a helmet-like casque atop its head and a 5-inch-long razor-sharp claw on each foot, the cassowary is both impressive and intimidating.

In October 2020, we took two of these avian ambassadors under our wing: brothers Irwin and Dundee. Considering all of the new sights, sounds and construction happening at the Bird House and Great Flight Aviary, they settled into their new home remarkably well. In June, they will turn 2 years old—Irwin on June 11 and Dundee on June 14.

Southern cassowary brothers Irwin (left) and Dundee (right).

Both are similar in size, but Dundee’s skin is lighter in color than Irwin’s—though that may change as they mature. Cassowaries’ heads and necks are mostly feather-bare, but their skin is a vibrant palette of turquoise blue and indigo. Two bright red wattles dangle from the base of their neck, which is why this species is also known as the double-wattled cassowary.

One of the cassowary’s most unique and dinosaur-like features is the casque atop its head. It is a hollow, pointed structure made of keratin, the same substance as our hair and nails. The purpose of the casque eluded scientists for years, but recent studies suggest that it serves as a thermal window that lets heat escape from their bodies—similar to humans sweating or dogs panting.

The cassowary's helmet-like casque is made of keratin, the same material that makes up our hair and fingernails.

A common misconception is that cassowaries use their casques as a weapon. They do not, but they have some seriously amazing defense mechanisms in place. To start, they have extremely powerful legs. Cassowaries cannot fly, but they are incredible jumpers and can launch themselves up to 7 feet of the ground!

When cassowaries feel threatened, they jump . . . and kick! The inside claw on both feet is razor-sharp and can grow up to 5 inches in length. With one swipe, the cassowary can eviscerate its target. This is why we always interact with these birds through a safety barrier and never enter their space when they are present.

Cassowaries have a 5-inch long razer sharp claw on each foot.

Despite not being able to interact with the cassowary brothers directly, we have come to know their personalities over the last seven months. Irwin is a natural born leader. He is almost always the first to investigate new things. Although cassowaries are usually cautious, Irwin is more curious. He seems to study the way that things move and operate, like their doors and feeders, which run on pulley systems.

Where Irwin leads, Dundee follows. Our younger cassowary is much more reserved and prefers to stand back and watch Irwin do things before he tries them. He is cautious in all respects with one exception: food. While both are food-motivated, Dundee’s enthusiasm about food is even more apparent.

Cassowaries are frugivores—that is, fruit comprises the majority of their diet. They are a keystone species in the rainforests of Australia as they eat fruit and move through their territory, cassowaries deposit the seeds in their droppings. This enables new plants to grow, complete with natural cassowary-made fertilizer!

As part of their daily diet, our cassowaries receive an assortment of grapes, tomatoes, squash, papaya, bananas, apples, sweet potatoes and carrots. When Irwin and Dundee first arrived, we trained them to come into their indoor enclosure on cue by placing their food inside and ringing a bell to let them know it was mealtime.

They quickly associated the bell with food, and would even try to run through the door at the same time. Given their large size, though, they couldn’t enter side-by-side, so they would jockey to be the first one to the food!

Many of our birds are trained to voluntarily participate in their own healthcare in exchange for a favorite food reward. Once they mastered shifting indoors on cue, we trained them to stand on a scale. This helps us monitor their weights and ensure they are growing at a good pace.

We usually save the tomatoes, grapes, sweet potato and papaya for these training sessions, since those are Irwin and Dundee’s favorite foods. Recently, they both weighed-in at approximately 74 pounds (34 kilograms) each. As adults, male cassowaries typically weigh 121 pounds (55 kilograms), so they’ve still got plenty of growing to do!

In the wild, cassowaries are solitary birds. When they first arrived at the Zoo, Irwin and Dundee got along well and even played with each other. Now that they are approaching their second birthdays, the boys are becoming more independent.

In early April, Irwin moved into a new habitat of his own accord, while Dundee stayed behind. Initially, we left the door between the habitats open, but the brothers did not seem interested in reuniting. Part of the process of growing up is going their separate ways, so this was a normal and expected progression. Their habitats are adjacent, so they can still see and interact with one another as they choose.

Watching these magnificent birds maneuver through their environment, one cannot help but sense the link between them and their ancient ancestors. The way that they interact with and study their surroundings is endlessly fascinating. We are honored to have Irwin and Dundee as ambassadors for their species and cannot wait to introduce them to visitors when the Bird House reopens in spring 2022!

This story appears in the May 2021 issue of National Zoo News. The Smithsonian’s National Zoo is temporarily closed to help prevent the spread of COVID-19 and will reopen May 21, 2021. Our whole team works diligently to care for our animals and keep you connected to the Zoo. With your support, our conservation mission continues. If you can, please join us in this important work by making a donation today. On behalf of the animals we care for and work to protect: thank you.


Contents

Under phylogenetic nomenclature, dinosaurs are usually defined as the group consisting of the most recent common ancestor (MRCA) of Triceratops and modern birds (Neornithes), and all its descendants. [7] It has also been suggested that Dinosauria be defined with respect to the MRCA of Megalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria. [8] Both definitions result in the same set of animals being defined as dinosaurs: "Dinosauria = Ornithischia + Saurischia". This definition includes major groups such as ankylosaurians (armored herbivorous quadrupeds), stegosaurians (plated herbivorous quadrupeds), ceratopsians (bipedal or quadrupedal herbivores with neck frills), pachycephalosaurians (bipedal herbivores with thick skulls), ornithopods (bipedal or quadrupedal herbivores including "duck-bills"), theropods (mostly bipedal carnivores and birds), and sauropodomorphs (mostly large herbivorous quadrupeds with long necks and tails). [9]

Birds are now recognized as being the sole surviving lineage of theropod dinosaurs. In traditional taxonomy, birds were considered a separate class that had evolved from dinosaurs, a distinct superorder. However, a majority of contemporary paleontologists concerned with dinosaurs reject the traditional style of classification in favor of phylogenetic taxonomy this approach requires that, for a group to be natural, all descendants of members of the group must be included in the group as well. Birds are thus considered to be dinosaurs and dinosaurs are, therefore, not extinct. [10] Birds are classified as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs. [11]

Research by Matthew G. Baron, David B. Norman, and Paul M. Barrett in 2017 suggested a radical revision of dinosaurian systematics. Phylogenetic analysis by Baron et al. recovered the Ornithischia as being closer to the Theropoda than the Sauropodomorpha, as opposed to the traditional union of theropods with sauropodomorphs. They resurrected the clade Ornithoscelida to refer to the group containing Ornithischia and Theropoda. Dinosauria itself was re-defined as the last common ancestor of Triceratops horridus, Passer domesticus and Diplodocus carnegii, and all of its descendants, to ensure that sauropods and kin remain included as dinosaurs. [12] [13]

General description

Using one of the above definitions, dinosaurs can be generally described as archosaurs with hind limbs held erect beneath the body. [14] Other prehistoric animals, including pterosaurs, mosasaurs, ichthyosaurs, plesiosaurs, and Dimetrodon, while often popularly conceived of as dinosaurs, are not taxonomically classified as dinosaurs. [15] Pterosaurs are distantly related to dinosaurs, being members of the clade Ornithodira. The other groups mentioned are, like dinosaurs and pterosaurs, members of Sauropsida (the reptile and bird clade), except Dimetrodon (which is a synapsid). None of them had the erect hind limb posture characteristic of true dinosaurs. [16]

Dinosaurs were the dominant terrestrial vertebrates of the Mesozoic Era, especially the Jurassic and Cretaceous periods. Other groups of animals were restricted in size and niches mammals, for example, rarely exceeded the size of a domestic cat, and were generally rodent-sized carnivores of small prey. [17] They have always been recognized as an extremely varied group of animals over 900 non-avian dinosaur genera have been identified with certainty as of 2018, and the total number of genera preserved in the fossil record has been estimated at around 1850, nearly 75% of which remain to be discovered, and 1124 species by 2016. [18] [19] [20] A 1995 study predicted that about 3,400 dinosaur genera ever existed, including many that would not have been preserved in the fossil record. [21]

In 2016, the estimated number of dinosaur species that existed in the Mesozoic was 1,543–2,468. [22] [23] In 2021, the number of modern-day birds (avian dinosaurs) was estimated to be at 10,806 species. [24] Some are herbivorous, others carnivorous, including seed-eaters, fish-eaters, insectivores, and omnivores. While dinosaurs were ancestrally bipedal (as are all modern birds), some prehistoric species were quadrupeds, and others, such as Anchisaurus and Iguanodon, could walk just as easily on two or four legs. Cranial modifications like horns and crests are common dinosaurian traits, and some extinct species had bony armor. Although known for large size, many Mesozoic dinosaurs were human-sized or smaller, and modern birds are generally small in size. Dinosaurs today inhabit every continent, and fossils show that they had achieved global distribution by at least the Early Jurassic epoch. [25] Modern birds inhabit most available habitats, from terrestrial to marine, and there is evidence that some non-avian dinosaurs (such as Microraptor) could fly or at least glide, and others, such as spinosaurids, had semiaquatic habits. [26]

Distinguishing anatomical features

While recent discoveries have made it more difficult to present a universally agreed-upon list of their distinguishing features, nearly all dinosaurs discovered so far share certain modifications to the ancestral archosaurian skeleton, or are clearly descendants of older dinosaurs showing these modifications. Although some later groups of dinosaurs featured further modified versions of these traits, they are considered typical for Dinosauria the earliest dinosaurs had them and passed them on to their descendants. Such modifications, originating in the most recent common ancestor of a certain taxonomic group, are called the synapomorphies of such a group. [27]

A detailed assessment of archosaur interrelations by Sterling Nesbitt [28] confirmed or found the following twelve unambiguous synapomorphies, some previously known:

  • In the skull, a supratemporal fossa (excavation) is present in front of the supratemporal fenestra, the main opening in the rear skull roof , obliquely backward-pointing processes on the rear top corners of the anterior (front) neck vertebrae behind the atlas and axis, the first two neck vertebrae
  • Apex of a deltopectoral crest (a projection on which the deltopectoral muscles attach) located at or more than 30% down the length of the humerus (upper arm bone) , a lower arm bone, shorter than 80% of humerus length (projection where the caudofemoralis muscle attaches on the inner rear shaft) on the femur (thigh bone) is a sharp flange
  • Fourth trochanter asymmetrical, with distal, lower, margin forming a steeper angle to the shaft
  • On the astragalus and calcaneum, upper ankle bones, the proximal articular facet, the top connecting surface, for the fibula occupies less than 30% of the transverse width of the element
  • Exoccipitals (bones at the back of the skull) do not meet along the midline on the floor of the endocranial cavity, the inner space of the braincase
  • In the pelvis, the proximal articular surfaces of the ischium with the ilium and the pubis are separated by a large concave surface (on the upper side of the ischium a part of the open hip joint is located between the contacts with the pubic bone and the ilium) on the tibia (protruding part of the top surface of the shinbone) arcs anterolaterally (curves to the front and the outer side)
  • Distinct proximodistally oriented (vertical) ridge present on the posterior face of the distal end of the tibia (the rear surface of the lower end of the shinbone)
  • Concave articular surface for the fibula of the calcaneum (the top surface of the calcaneum, where it touches the fibula, has a hollow profile)

Nesbitt found a number of further potential synapomorphies and discounted a number of synapomorphies previously suggested. Some of these are also present in silesaurids, which Nesbitt recovered as a sister group to Dinosauria, including a large anterior trochanter, metatarsals II and IV of subequal length, reduced contact between ischium and pubis, the presence of a cnemial crest on the tibia and of an ascending process on the astragalus, and many others. [7]

A variety of other skeletal features are shared by dinosaurs. However, because they are either common to other groups of archosaurs or were not present in all early dinosaurs, these features are not considered to be synapomorphies. For example, as diapsids, dinosaurs ancestrally had two pairs of Infratemporal fenestrae (openings in the skull behind the eyes), and as members of the diapsid group Archosauria, had additional openings in the snout and lower jaw. [29] Additionally, several characteristics once thought to be synapomorphies are now known to have appeared before dinosaurs, or were absent in the earliest dinosaurs and independently evolved by different dinosaur groups. These include an elongated scapula, or shoulder blade a sacrum composed of three or more fused vertebrae (three are found in some other archosaurs, but only two are found in Herrerasaurus) [7] and a perforate acetabulum, or hip socket, with a hole at the center of its inside surface (closed in Saturnalia tupiniquim, for example). [30] [31] Another difficulty of determining distinctly dinosaurian features is that early dinosaurs and other archosaurs from the Late Triassic epoch are often poorly known and were similar in many ways these animals have sometimes been misidentified in the literature. [32]

Dinosaurs stand with their hind limbs erect in a manner similar to most modern mammals, but distinct from most other reptiles, whose limbs sprawl out to either side. [33] This posture is due to the development of a laterally facing recess in the pelvis (usually an open socket) and a corresponding inwardly facing distinct head on the femur. [34] Their erect posture enabled early dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that surpassed those of "sprawling" reptiles. [35] Erect limbs probably also helped support the evolution of large size by reducing bending stresses on limbs. [36] Some non-dinosaurian archosaurs, including rauisuchians, also had erect limbs but achieved this by a "pillar-erect" configuration of the hip joint, where instead of having a projection from the femur insert on a socket on the hip, the upper pelvic bone was rotated to form an overhanging shelf. [36]

Pre-scientific history

Dinosaur fossils have been known for millennia, although their true nature was not recognized. The Chinese considered them to be dragon bones and documented them as such. For example, Huayang Guo Zhi ( 華陽國志 ), a gazetteer compiled by Chang Qu ( 常璩 ) during the Western Jin Dynasty (265–316), reported the discovery of dragon bones at Wucheng in Sichuan Province. [37] Villagers in central China have long unearthed fossilized "dragon bones" for use in traditional medicines. [38] In Europe, dinosaur fossils were generally believed to be the remains of giants and other biblical creatures. [39]

Early dinosaur research

Scholarly descriptions of what would now be recognized as dinosaur bones first appeared in the late 17th century in England. Part of a bone, now known to have been the femur of a Megalosaurus, [40] was recovered from a limestone quarry at Cornwell near Chipping Norton, Oxfordshire, in 1676. The fragment was sent to Robert Plot, Professor of Chemistry at the University of Oxford and first curator of the Ashmolean Museum, who published a description in his The Natural History of Oxford-shire (1677). [41] He correctly identified the bone as the lower extremity of the femur of a large animal, and recognized that it was too large to belong to any known species. He, therefore, concluded it to be the femur of a huge human, perhaps a Titan or another type of giant featured in legends. [42] [43] Edward Lhuyd, a friend of Sir Isaac Newton, published Lithophylacii Britannici ichnographia (1699), the first scientific treatment of what would now be recognized as a dinosaur when he described and named a sauropod tooth, "Rutellum impicatum", [44] [45] that had been found in Caswell, near Witney, Oxfordshire. [46]

Between 1815 and 1824, the Rev William Buckland, the first Reader of Geology at the University of Oxford, collected more fossilized bones of Megalosaurus and became the first person to describe a non-avian dinosaur in a scientific journal. [40] [47] The second non-avian dinosaur genus to be identified, Iguanodon, was discovered in 1822 by Mary Ann Mantell – the wife of English geologist Gideon Mantell. Gideon Mantell recognized similarities between his fossils and the bones of modern iguanas. He published his findings in 1825. [48] [49]

The study of these "great fossil lizards" soon became of great interest to European and American scientists, and in 1841 the English paleontologist Sir Richard Owen coined the term "dinosaur", using it to refer to the "distinct tribe or sub-order of Saurian Reptiles" that were then being recognized in England and around the world. [50] [51] The term is derived from Ancient Greek δεινός (deinos) 'terrible, potent or fearfully great', and σαῦρος (sauros) 'lizard or reptile'. [50] [52] Though the taxonomic name has often been interpreted as a reference to dinosaurs' teeth, claws, and other fearsome characteristics, Owen intended it to also evoke their size and majesty. [53] Owen recognized that the remains that had been found so far, Iguanodon, Megalosaurus and Hylaeosaurus, shared a number of distinctive features, and so decided to present them as a distinct taxonomic group. With the backing of Prince Albert, the husband of Queen Victoria, Owen established the Natural History Museum, London, to display the national collection of dinosaur fossils and other biological and geological exhibits. [54]

Discoveries in North America

In 1858, William Parker Foulke discovered the first known American dinosaur, in marl pits in the small town of Haddonfield, New Jersey. (Although fossils had been found before, their nature had not been correctly discerned.) The creature was named Hadrosaurus foulkii. It was an extremely important find: Hadrosaurus was one of the first nearly complete dinosaur skeletons found (the first was in 1834, in Maidstone, England), and it was clearly a bipedal creature. This was a revolutionary discovery as, until that point, most scientists had believed dinosaurs walked on four feet, like other lizards. Foulke's discoveries sparked a wave of interests in dinosaurs in the United States, known as dinosaur mania. [55]

Dinosaur mania was exemplified by the fierce rivalry between Edward Drinker Cope and Othniel Charles Marsh, both of whom raced to be the first to find new dinosaurs in what came to be known as the Bone Wars. This fight between the two scientists lasted for over 30 years, ending in 1897 when Cope died after spending his entire fortune on the dinosaur hunt. Unfortunately, many valuable dinosaur specimens were damaged or destroyed due to the pair's rough methods: for example, their diggers often used dynamite to unearth bones. Modern paleontologists would find such methods crude and unacceptable, since blasting easily destroys fossil and stratigraphic evidence. Despite their unrefined methods, the contributions of Cope and Marsh to paleontology were vast: Marsh unearthed 86 new species of dinosaur and Cope discovered 56, a total of 142 new species. Cope's collection is now at the American Museum of Natural History, while Marsh's is at the Peabody Museum of Natural History at Yale University. [56]

"Dinosaur renaissance" and beyond

The field of dinosaur research has enjoyed a surge in activity that began in the 1970s and is ongoing. This was triggered, in part, by John Ostrom's discovery and 1969 description of Deinonychus, an active predator that may have been warm-blooded, in marked contrast to the then-prevailing image of dinosaurs as sluggish and cold-blooded. [57] [58] [59] [60] [61] [62] Vertebrate paleontology has become a global science. Major new dinosaur discoveries have been made by paleontologists working in previously unexploited regions, including India, South America, Madagascar, Antarctica, and most significantly China (the well-preserved feathered dinosaurs in China have further consolidated the link between dinosaurs and their living descendants, modern birds). The widespread application of cladistics, which rigorously analyzes the relationships between biological organisms, has also proved tremendously useful in classifying dinosaurs. Cladistic analysis, among other modern techniques, helps to compensate for an often incomplete and fragmentary fossil record. [63]

Soft tissue and DNA

One of the best examples of soft-tissue impressions in a fossil dinosaur was discovered in the Pietraroia Plattenkalk in southern Italy. The discovery was reported in 1998, and described the specimen of a small, juvenile coelurosaur, Scipionyx samniticus. The fossil includes portions of the intestines, colon, liver, muscles, and windpipe of this dinosaur. [64]

In the March 2005 issue of Science, the paleontologist Mary Higby Schweitzer and her team announced the discovery of flexible material resembling actual soft tissue inside a 68-million-year-old Tyrannosaurus rex leg bone from the Hell Creek Formation in Montana. After recovery, the tissue was rehydrated by the science team. [65] When the fossilized bone was treated over several weeks to remove mineral content from the fossilized bone-marrow cavity (a process called demineralization), Schweitzer found evidence of intact structures such as blood vessels, bone matrix, and connective tissue (bone fibers). Scrutiny under the microscope further revealed that the putative dinosaur soft tissue had retained fine structures (microstructures) even at the cellular level. The exact nature and composition of this material, and the implications of Schweitzer's discovery, are not yet clear. [65]

In 2009, a team including Schweitzer announced that, using even more careful methodology, they had duplicated their results by finding similar soft tissue in a duck-billed dinosaur, Brachylophosaurus canadensis, found in the Judith River Formation of Montana. This included even more detailed tissue, down to preserved bone cells that seem to have visible remnants of nuclei and what seem to be red blood cells. Among other materials found in the bone was collagen, as in the Tyrannosaurus bone. The type of collagen an animal has in its bones varies according to its DNA and, in both cases, this collagen was of the same type found in modern chickens and ostriches. [66]

The extraction of ancient DNA from dinosaur fossils has been reported on two separate occasions [67] upon further inspection and peer review, however, neither of these reports could be confirmed. [68] However, a functional peptide involved in the vision of a theoretical dinosaur has been inferred using analytical phylogenetic reconstruction methods on gene sequences of related modern species such as reptiles and birds. [69] In addition, several proteins, including hemoglobin, [70] have putatively been detected in dinosaur fossils. [71] [72]

In 2015, researchers reported finding structures similar to blood cells and collagen fibers, preserved in the bone fossils of six Cretaceous dinosaur specimens, which are approximately 75 million years old. [73] [74]

Origins and early evolution

Dinosaurs diverged from their archosaur ancestors during the Middle to Late Triassic epochs, roughly 20 million years after the devastating Permian–Triassic extinction event wiped out an estimated 96% of all marine species and 70% of terrestrial vertebrate species approximately 252 million years ago. [75] [76] Radiometric dating of the Ischigualasto Formation of Argentina where the early dinosaur genus Eoraptor was found date it as 231.4 million years old. [77] Eoraptor is thought to resemble the common ancestor of all dinosaurs if this is true, its traits suggest that the first dinosaurs were small, bipedal predators. [78] [79] [80] The discovery of primitive, dinosaur-like ornithodirans such as Lagosuchus and Lagerpeton in Argentina in the Carnian epoch of the Triassic, around 233 million years ago, [81] supports this view analysis of recovered fossils suggests that these animals were indeed small, bipedal predators. Dinosaurs may have appeared as early as the Anisian epoch of the Triassic, 245 million years ago, as evidenced by remains of the genus Nyasasaurus from that period. However, its known fossils are too fragmentary to tell if it was a dinosaur or only a close relative. [82] Paleontologist Max C. Langer et al. (2018) determined that Staurikosaurus from the Santa Maria Formation dates to 233.23 million years ago, making it older in geologic age than Eoraptor. [83]

When dinosaurs appeared, they were not the dominant terrestrial animals. The terrestrial habitats were occupied by various types of archosauromorphs and therapsids, like cynodonts and rhynchosaurs. Their main competitors were the pseudosuchians, such as aetosaurs, ornithosuchids and rauisuchians, which were more successful than the dinosaurs. [84] Most of these other animals became extinct in the Triassic, in one of two events. First, at about 215 million years ago, a variety of basal archosauromorphs, including the protorosaurs, became extinct. This was followed by the Triassic–Jurassic extinction event (about 201 million years ago), that saw the end of most of the other groups of early archosaurs, like aetosaurs, ornithosuchids, phytosaurs, and rauisuchians. Rhynchosaurs and dicynodonts survived (at least in some areas) at least as late as early –mid Norian and late Norian or earliest Rhaetian stages, respectively, [85] [86] and the exact date of their extinction is uncertain. These losses left behind a land fauna of crocodylomorphs, dinosaurs, mammals, pterosaurians, and turtles. [7] The first few lines of early dinosaurs diversified through the Carnian and Norian stages of the Triassic, possibly by occupying the niches of the groups that became extinct. [9] Also notably, there was a heightened rate of extinction during the Carnian Pluvial Event. [87]

Evolution and paleobiogeography

Dinosaur evolution after the Triassic followed changes in vegetation and the location of continents. In the Late Triassic and Early Jurassic, the continents were connected as the single landmass Pangaea, and there was a worldwide dinosaur fauna mostly composed of coelophysoid carnivores and early sauropodomorph herbivores. [88] Gymnosperm plants (particularly conifers), a potential food source, radiated in the Late Triassic. Early sauropodomorphs did not have sophisticated mechanisms for processing food in the mouth, and so must have employed other means of breaking down food farther along the digestive tract. [89] The general homogeneity of dinosaurian faunas continued into the Middle and Late Jurassic, where most localities had predators consisting of ceratosaurians, megalosauroids, and allosauroids, and herbivores consisting of stegosaurian ornithischians and large sauropods. Examples of this include the Morrison Formation of North America and Tendaguru Beds of Tanzania. Dinosaurs in China show some differences, with specialized metriacanthosaurid theropods and unusual, long-necked sauropods like Mamenchisaurus. [88] Ankylosaurians and ornithopods were also becoming more common, but primitive sauropodomorphs had become extinct. Conifers and pteridophytes were the most common plants. Sauropods, like earlier sauropodomorphs, were not oral processors, but ornithischians were evolving various means of dealing with food in the mouth, including potential cheek-like organs to keep food in the mouth, and jaw motions to grind food. [89] Another notable evolutionary event of the Jurassic was the appearance of true birds, descended from maniraptoran coelurosaurians. [11]

By the Early Cretaceous and the ongoing breakup of Pangaea, dinosaurs were becoming strongly differentiated by landmass. The earliest part of this time saw the spread of ankylosaurians, iguanodontians, and brachiosaurids through Europe, North America, and northern Africa. These were later supplemented or replaced in Africa by large spinosaurid and carcharodontosaurid theropods, and rebbachisaurid and titanosaurian sauropods, also found in South America. In Asia, maniraptoran coelurosaurians like dromaeosaurids, troodontids, and oviraptorosaurians became the common theropods, and ankylosaurids and early ceratopsians like Psittacosaurus became important herbivores. Meanwhile, Australia was home to a fauna of basal ankylosaurians, hypsilophodonts, and iguanodontians. [88] The stegosaurians appear to have gone extinct at some point in the late Early Cretaceous or early Late Cretaceous. A major change in the Early Cretaceous, which would be amplified in the Late Cretaceous, was the evolution of flowering plants. At the same time, several groups of dinosaurian herbivores evolved more sophisticated ways to orally process food. Ceratopsians developed a method of slicing with teeth stacked on each other in batteries, and iguanodontians refined a method of grinding with dental batteries, taken to its extreme in hadrosaurids. [89] Some sauropods also evolved tooth batteries, best exemplified by the rebbachisaurid Nigersaurus. [90]

There were three general dinosaur faunas in the Late Cretaceous. In the northern continents of North America and Asia, the major theropods were tyrannosaurids and various types of smaller maniraptoran theropods, with a predominantly ornithischian herbivore assemblage of hadrosaurids, ceratopsians, ankylosaurids, and pachycephalosaurians. In the southern continents that had made up the now-splitting supercontinent Gondwana, abelisaurids were the common theropods, and titanosaurian sauropods the common herbivores. Finally, in Europe, dromaeosaurids, rhabdodontid iguanodontians, nodosaurid ankylosaurians, and titanosaurian sauropods were prevalent. [88] Flowering plants were greatly radiating, [89] with the first grasses appearing by the end of the Cretaceous. [91] Grinding hadrosaurids and shearing ceratopsians became very diverse across North America and Asia. Theropods were also radiating as herbivores or omnivores, with therizinosaurians and ornithomimosaurians becoming common. [89]

The Cretaceous–Paleogene extinction event, which occurred approximately 66 million years ago at the end of the Cretaceous, caused the extinction of all dinosaur groups except for the neornithine birds. Some other diapsid groups, such as crocodilians, sebecosuchians, turtles, lizards, snakes, sphenodontians, and choristoderans, also survived the event. [92]

The surviving lineages of neornithine birds, including the ancestors of modern ratites, ducks and chickens, and a variety of waterbirds, diversified rapidly at the beginning of the Paleogene period, entering ecological niches left vacant by the extinction of Mesozoic dinosaur groups such as the arboreal enantiornithines, aquatic hesperornithines, and even the larger terrestrial theropods (in the form of Gastornis, eogruiids, bathornithids, ratites, geranoidids, mihirungs, and "terror birds"). It is often stated that mammals out-competed the neornithines for dominance of most terrestrial niches but many of these groups co-existed with rich mammalian faunas for most of the Cenozoic Era. [93] Terror birds and bathornithids occupied carnivorous guilds alongside predatory mammals, [94] [95] and ratites are still fairly successful as mid-sized herbivores eogruiids similarly lasted from the Eocene to Pliocene, only becoming extinct very recently after over 20 million years of co-existence with many mammal groups. [96]

Dinosaurs belong to a group known as archosaurs, which also includes modern crocodilians. Within the archosaur group, dinosaurs are differentiated most noticeably by their gait. Dinosaur legs extend directly beneath the body, whereas the legs of lizards and crocodilians sprawl out to either side. [27]

Collectively, dinosaurs as a clade are divided into two primary branches, Saurischia and Ornithischia. Saurischia includes those taxa sharing a more recent common ancestor with birds than with Ornithischia, while Ornithischia includes all taxa sharing a more recent common ancestor with Triceratops than with Saurischia. Anatomically, these two groups can be distinguished most noticeably by their pelvic structure. Early saurischians—"lizard-hipped", from the Greek sauros ( σαῦρος ) meaning "lizard" and ischion ( ἰσχίον ) meaning "hip joint"—retained the hip structure of their ancestors, with a pubis bone directed cranially, or forward. [34] This basic form was modified by rotating the pubis backward to varying degrees in several groups (Herrerasaurus, [97] therizinosauroids, [98] dromaeosaurids, [99] and birds [11] ). Saurischia includes the theropods (exclusively bipedal and with a wide variety of diets) and sauropodomorphs (long-necked herbivores which include advanced, quadrupedal groups). [26] [100]

By contrast, ornithischians—"bird-hipped", from the Greek ornitheios (ὀρνίθειος) meaning "of a bird" and ischion (ἰσχίον) meaning "hip joint"—had a pelvis that superficially resembled a bird's pelvis: the pubic bone was oriented caudally (rear-pointing). Unlike birds, the ornithischian pubis also usually had an additional forward-pointing process. Ornithischia includes a variety of species that were primarily herbivores.

Despite the terms "bird hip" (Ornithischia) and "lizard hip" (Saurischia), birds are not part of Ornithischia. Birds instead belong to Saurischia, the “lizard-hipped” dinosaurs—birds evolved from earlier dinosaurs with "lizard hips". [27]

Taxonomy

The following is a simplified classification of dinosaur groups based on their evolutionary relationships, and organized based on the list of Mesozoic dinosaur species provided by Holtz (2007). [101] A more detailed version can be found at Dinosaur classification. The dagger (†) is used to signify groups with no living members.

  • Dinosauria
    ("lizard-hipped" includes Theropoda and Sauropodomorpha)
  • †Herrerasauria (early bipedal carnivores) (all bipedal most were carnivorous)
  • †Coelophysoidea (small, early theropods includes Coelophysis and close relatives)
  • †Ceratosauria (generally elaborately horned, the dominant southern carnivores of the Cretaceous) ("stiff tails" includes most theropods)
  • †Megalosauroidea (early group of large carnivores including the semiaquatic spinosaurids)
  • †Carnosauria (Allosaurus and close relatives, like Carcharodontosaurus)
  • †Megaraptora (group of medium to large sized theropods, often with large hand claws) (feathered theropods, with a range of body sizes and niches) [63]
  • †Compsognathidae (early coelurosaurs with reduced forelimbs)
  • †Tyrannosauroidea (Tyrannosaurus and close relatives)
  • †Ornithomimosauria ("ostrich-mimics" mostly toothless carnivores to possible herbivores)
  • †Alvarezsauroidea (small insectivores with reduced forelimbs each bearing one enlarged claw) ("hand snatchers" had long, slender arms and fingers)
  • †Therizinosauria (bipedal herbivores with large hand claws and small heads)
  • †Oviraptorosauria (mostly toothless their diet and lifestyle are uncertain)
  • †Deinonychosauria (small- to medium-sized bird-like, with a distinctive toe claw) (modern birds and extinct relatives)
  • †Archaeopterygidae (small, winged theropods or primitive birds)
  • †Scansoriopterygidae (small primitive avialans with long third fingers)
  • †Omnivoropterygidae (large, early short-tailed avialans)
  • †Confuciusornithidae (small toothless avialans)
  • †Enantiornithes (primitive tree-dwelling, flying avialans) (advanced flying birds)
  • †Yanornithiformes (toothed Cretaceous Chinese birds)
  • †Hesperornithes (specialized aquatic diving birds) (modern, beaked birds and their extinct relatives)
  • †Sauropodomorpha (herbivores with small heads, long necks, long tails)
  • †Guaibasauridae (small, primitive, omnivorous sauropodomorphs)
  • †Plateosauridae (primitive, strictly bipedal "prosauropods")
  • †Riojasauridae (small, primitive sauropodomorphs)
  • †Massospondylidae (small, primitive sauropodomorphs)
  • †Sauropoda (very large and heavy, usually over 15 m (49 ft) long quadrupedal)
  • †Vulcanodontidae (primitive sauropods with pillar-like limbs)
  • †Eusauropoda ("true sauropods")
  • †Cetiosauridae ("whale reptiles")
  • †Turiasauria (group of Jurassic and Cretaceous sauropods)
  • †Neosauropoda ("new sauropods")
  • †Diplodocoidea (skulls and tails elongated teeth typically narrow and pencil-like)
  • †Macronaria (boxy skulls spoon- or pencil-shaped teeth)
  • †Brachiosauridae (long-necked, long-armed macronarians)
  • †Titanosauria (diverse stocky, with wide hips most common in the Late Cretaceous of southern continents)
  • †Ornithischia ("bird-hipped" diverse bipedal and quadrupedal herbivores)
  • †Heterodontosauridae (small basal ornithopod herbivores/omnivores with prominent canine-like teeth)
  • †Thyreophora (armored dinosaurs mostly quadrupeds)
  • †Ankylosauria (scutes as primary armor some had club-like tails)
  • †Stegosauria (spikes and plates as primary armor)
  • †Neornithischia ("new ornithischians")
  • †Ornithopoda (various sizes bipeds and quadrupeds evolved a method of chewing using skull flexibility and numerous teeth)
  • †Marginocephalia (characterized by a cranial growth)
  • †Pachycephalosauria (bipeds with domed or knobby growth on skulls)
  • †Ceratopsia (bipeds and quadrupeds with neck frills many also had horns)

Knowledge about dinosaurs is derived from a variety of fossil and non-fossil records, including fossilized bones, feces, trackways, gastroliths, feathers, impressions of skin, internal organs and other soft tissues. [64] [65] Many fields of study contribute to our understanding of dinosaurs, including physics (especially biomechanics), chemistry, biology, and the Earth sciences (of which paleontology is a sub-discipline). [102] [103] Two topics of particular interest and study have been dinosaur size and behavior. [104]

Current evidence suggests that dinosaur average size varied through the Triassic, Early Jurassic, Late Jurassic and Cretaceous. [79] Predatory theropod dinosaurs, which occupied most terrestrial carnivore niches during the Mesozoic, most often fall into the 100 to 1 000 kg (220 to 2 200 lb) category when sorted by estimated weight into categories based on order of magnitude, whereas recent predatory carnivoran mammals peak in the 10 to 100 kg (22 to 220 lb) category. [105] The mode of Mesozoic dinosaur body masses is between 1 to 10 metric tons (1.1 to 11.0 short tons). [106] This contrasts sharply with the average size of Cenozoic mammals, estimated by the National Museum of Natural History as about 2 to 5 kg (4.4 to 11.0 lb). [107]

The sauropods were the largest and heaviest dinosaurs. For much of the dinosaur era, the smallest sauropods were larger than anything else in their habitat, and the largest was an order of magnitude more massive than anything else that has since walked the Earth. Giant prehistoric mammals such as Paraceratherium (the largest land mammal ever) were dwarfed by the giant sauropods, and only modern whales approach or surpass them in size. [108] There are several proposed advantages for the large size of sauropods, including protection from predation, reduction of energy use, and longevity, but it may be that the most important advantage was dietary. Large animals are more efficient at digestion than small animals, because food spends more time in their digestive systems. This also permits them to subsist on food with lower nutritive value than smaller animals. Sauropod remains are mostly found in rock formations interpreted as dry or seasonally dry, and the ability to eat large quantities of low-nutrient browse would have been advantageous in such environments. [109]

In early 2021, paleontologists unearthed fossils in China that reveal a new species of prehistoric giant rhinoceros, now believed to be the largest known land mammal in the history of the Earth. [110]

Largest and smallest

Scientists will probably never be certain of the largest and smallest dinosaurs to have ever existed. This is because only a tiny percentage of animals were ever fossilized and most of these remain buried in the earth. Few of the specimens that are recovered are complete skeletons, and impressions of skin and other soft tissues are rare. Rebuilding a complete skeleton by comparing the size and morphology of bones to those of similar, better-known species is an inexact art, and reconstructing the muscles and other organs of the living animal is, at best, a process of educated guesswork. [111]

The tallest and heaviest dinosaur known from good skeletons is Giraffatitan brancai (previously classified as a species of Brachiosaurus). Its remains were discovered in Tanzania between 1907 and 1912. Bones from several similar-sized individuals were incorporated into the skeleton now mounted and on display at the Museum für Naturkunde in Berlin [112] this mount is 12 meters (39 ft) tall and 21.8 to 22.5 meters (72 to 74 ft) long, [113] [114] and would have belonged to an animal that weighed between 30 000 and 60 000 kilograms ( 70 000 and 130 000 lb). The longest complete dinosaur is the 27 meters (89 ft) long Diplodocus, which was discovered in Wyoming in the United States and displayed in Pittsburgh's Carnegie Museum of Natural History in 1907. [115] The longest dinosaur known from good fossil material is the Patagotitan: the skeleton mount in the American Museum of Natural History in New York is 37 meters (121 ft) long. The Museo Municipal Carmen Funes in Plaza Huincul, Argentina, has an Argentinosaurus reconstructed skeleton mount that is 39.7 meters (130 ft) long. [116]

There were larger dinosaurs, but knowledge of them is based entirely on a small number of fragmentary fossils. Most of the largest herbivorous specimens on record were discovered in the 1970s or later, and include the massive Argentinosaurus, which may have weighed 80 000 to 100 000 kilograms (90 to 110 short tons) and reached lengths of 30 to 40 meters (98 to 131 ft) some of the longest were the 33.5-meter (110 ft) long Diplodocus hallorum [109] (formerly Seismosaurus), the 33-to-34-meter (108 to 112 ft) long Supersaurus, [117] and 37-meter (121 ft) long Patagotitan and the tallest, the 18-meter (59 ft) tall Sauroposeidon, which could have reached a sixth-floor window. The heaviest and longest dinosaur may have been Maraapunisaurus, known only from a now lost partial vertebral neural arch described in 1878. Extrapolating from the illustration of this bone, the animal may have been 58 meters (190 ft) long and weighed 122 400 kg ( 270 000 lb). [109] However, as no further evidence of sauropods of this size has been found, and the discoverer, Cope, had made typographic errors before, it is likely to have been an extreme overestimation. [118]

The largest carnivorous dinosaur was Spinosaurus, reaching a length of 12.6 to 18 meters (41 to 59 ft), and weighing 7 to 20.9 metric tons (7.7 to 23.0 short tons). [119] [120] Other large carnivorous theropods included Giganotosaurus, Carcharodontosaurus and Tyrannosaurus. [120] Therizinosaurus and Deinocheirus were among the tallest of the theropods. The largest ornithischian dinosaur was probably the hadrosaurid Shantungosaurus giganteus which measured 16.6 meters (54 ft). [121] The largest individuals may have weighed as much as 16 metric tons (18 short tons). [122]

The smallest dinosaur known is the bee hummingbird, [123] with a length of only 5 centimeters (2.0 in) and mass of around 1.8 g (0.063 oz). [124] The smallest known non-avialan dinosaurs were about the size of pigeons and were those theropods most closely related to birds. [125] For example, Anchiornis huxleyi is currently the smallest non-avialan dinosaur described from an adult specimen, with an estimated weight of 110 g (3.9 oz) [126] and a total skeletal length of 34 centimeters (1.12 ft). [125] [126] The smallest herbivorous non-avialan dinosaurs included Microceratus and Wannanosaurus, at about 60 centimeters (2.0 ft) long each. [101] [127]

Behavior

Many modern birds are highly social, often found living in flocks. There is general agreement that some behaviors that are common in birds, as well as in crocodiles (closest living relatives of birds), were also common among extinct dinosaur groups. Interpretations of behavior in fossil species are generally based on the pose of skeletons and their habitat, computer simulations of their biomechanics, and comparisons with modern animals in similar ecological niches. [102]

The first potential evidence for herding or flocking as a widespread behavior common to many dinosaur groups in addition to birds was the 1878 discovery of 31 Iguanodon, ornithischians that were then thought to have perished together in Bernissart, Belgium, after they fell into a deep, flooded sinkhole and drowned. [128] Other mass-death sites have been discovered subsequently. Those, along with multiple trackways, suggest that gregarious behavior was common in many early dinosaur species. Trackways of hundreds or even thousands of herbivores indicate that duck-billed (hadrosaurids) may have moved in great herds, like the American bison or the African Springbok. Sauropod tracks document that these animals traveled in groups composed of several different species, at least in Oxfordshire, England, [129] although there is no evidence for specific herd structures. [130] Congregating into herds may have evolved for defense, for migratory purposes, or to provide protection for young. There is evidence that many types of slow-growing dinosaurs, including various theropods, sauropods, ankylosaurians, ornithopods, and ceratopsians, formed aggregations of immature individuals. One example is a site in Inner Mongolia that has yielded remains of over 20 Sinornithomimus, from one to seven years old. This assemblage is interpreted as a social group that was trapped in mud. [131] The interpretation of dinosaurs as gregarious has also extended to depicting carnivorous theropods as pack hunters working together to bring down large prey. [132] [133] However, this lifestyle is uncommon among modern birds, crocodiles, and other reptiles, and the taphonomic evidence suggesting mammal-like pack hunting in such theropods as Deinonychus and Allosaurus can also be interpreted as the results of fatal disputes between feeding animals, as is seen in many modern diapsid predators. [134]

The crests and frills of some dinosaurs, like the marginocephalians, theropods and lambeosaurines, may have been too fragile to be used for active defense, and so they were likely used for sexual or aggressive displays, though little is known about dinosaur mating and territorialism. Head wounds from bites suggest that theropods, at least, engaged in active aggressive confrontations. [135]

From a behavioral standpoint, one of the most valuable dinosaur fossils was discovered in the Gobi Desert in 1971. It included a Velociraptor attacking a Protoceratops, [136] providing evidence that dinosaurs did indeed attack each other. [137] Additional evidence for attacking live prey is the partially healed tail of an Edmontosaurus, a hadrosaurid dinosaur the tail is damaged in such a way that shows the animal was bitten by a tyrannosaur but survived. [137] Cannibalism amongst some species of dinosaurs was confirmed by tooth marks found in Madagascar in 2003, involving the theropod Majungasaurus. [138]

Comparisons between the scleral rings of dinosaurs and modern birds and reptiles have been used to infer daily activity patterns of dinosaurs. Although it has been suggested that most dinosaurs were active during the day, these comparisons have shown that small predatory dinosaurs such as dromaeosaurids, Juravenator, and Megapnosaurus were likely nocturnal. Large and medium-sized herbivorous and omnivorous dinosaurs such as ceratopsians, sauropodomorphs, hadrosaurids, ornithomimosaurs may have been cathemeral, active during short intervals throughout the day, although the small ornithischian Agilisaurus was inferred to be diurnal. [139]

Based on fossil evidence from dinosaurs such as Oryctodromeus, some ornithischian species seem to have led a partially fossorial (burrowing) lifestyle. [140] Many modern birds are arboreal (tree climbing), and this was also true of many Mesozoic birds, especially the enantiornithines. [141] While some early bird-like species may have already been arboreal as well (including dromaeosaurids) such as Microraptor [142] ) most non-avialan dinosaurs seem to have relied on land-based locomotion. A good understanding of how dinosaurs moved on the ground is key to models of dinosaur behavior the science of biomechanics, pioneered by Robert McNeill Alexander, has provided significant insight in this area. For example, studies of the forces exerted by muscles and gravity on dinosaurs' skeletal structure have investigated how fast dinosaurs could run, [102] whether diplodocids could create sonic booms via whip-like tail snapping, [143] and whether sauropods could float. [144]

Communication

Modern birds are known to communicate using visual and auditory signals, and the wide diversity of visual display structures among fossil dinosaur groups, such as horns, frills, crests, sails, and feathers, suggests that visual communication has always been important in dinosaur biology. [145] Reconstruction of the plumage color of Anchiornis, suggest the importance of color in visual communication in non-avian dinosaurs. [146] The evolution of dinosaur vocalization is less certain. Paleontologist Phil Senter has suggested that non-avian dinosaurs relied mostly on visual displays and possibly non-vocal acoustic sounds like hissing, jaw grinding or clapping, splashing and wing beating (possible in winged maniraptoran dinosaurs). He states they were unlikely to have been capable of vocalizing since their closest relatives, crocodilians and birds, use different means to vocalize, the former via the larynx and the latter through the unique syrinx, suggesting they evolved independently and their common ancestor was mute. [145]

The earliest remains of a syrinx, which has enough mineral content for fossilization, was found in a specimen of the duck-like Vegavis iaai dated 69 –66 million years ago, and this organ is unlikely to have existed in non-avian dinosaurs. However, in contrast to Senter, other researchers have suggested that dinosaurs could vocalize and that the syrinx-based vocal system of birds evolved from a larynx-based one, rather than the two systems evolving independently. [147] A 2016 study suggests that dinosaurs produced closed mouth vocalizations like cooing, which occur in both crocodilians and birds as well as other reptiles. Such vocalizations evolved independently in extant archosaurs numerous times, following increases in body size. [148] The crests of the Lambeosaurini and nasal chambers of ankylosaurids have been suggested to have functioned in vocal resonance, [149] [150] though Senter stated that the presence of resonance chambers in some dinosaurs is not necessarily evidence of vocalization as modern snakes have such chambers which intensify their hisses. [145]

Reproductive biology

All dinosaurs laid amniotic eggs with hard shells made mostly of calcium carbonate. [151] Dinosaur eggs were usually laid in a nest. Most species create somewhat elaborate nests which can be cups, domes, plates, beds scrapes, mounds, or burrows. [152] Some species of modern bird have no nests the cliff-nesting common guillemot lays its eggs on bare rock, and male emperor penguins keep eggs between their body and feet. Primitive birds and many non-avialan dinosaurs often lay eggs in communal nests, with males primarily incubating the eggs. While modern birds have only one functional oviduct and lay one egg at a time, more primitive birds and dinosaurs had two oviducts, like crocodiles. Some non-avialan dinosaurs, such as Troodon, exhibited iterative laying, where the adult might lay a pair of eggs every one or two days, and then ensured simultaneous hatching by delaying brooding until all eggs were laid. [153]

When laying eggs, females grow a special type of bone between the hard outer bone and the marrow of their limbs. This medullary bone, which is rich in calcium, is used to make eggshells. A discovery of features in a Tyrannosaurus skeleton provided evidence of medullary bone in extinct dinosaurs and, for the first time, allowed paleontologists to establish the sex of a fossil dinosaur specimen. Further research has found medullary bone in the carnosaur Allosaurus and the ornithopod Tenontosaurus. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that the production of medullary tissue is a general characteristic of all dinosaurs. [154]

Another widespread trait among modern birds (but see below in regards to fossil groups and extant megapodes) is parental care for young after hatching. Jack Horner's 1978 discovery of a Maiasaura ("good mother lizard") nesting ground in Montana demonstrated that parental care continued long after birth among ornithopods. [155] A specimen of the oviraptorid Citipati osmolskae was discovered in a chicken-like brooding position in 1993, [156] which may indicate that they had begun using an insulating layer of feathers to keep the eggs warm. [157] An embryo of the basal sauropodomorph Massospondylus was found without teeth, indicating that some parental care was required to feed the young dinosaurs. [158] Trackways have also confirmed parental behavior among ornithopods from the Isle of Skye in northwestern Scotland. [159]

However, there is ample evidence of precociality or superprecociality among many dinosaur species, particularly theropods. For instance, non-ornithuromorph birds have been abundantly demonstrated to have had slow growth rates, megapode-like egg burying behavior and the ability to fly soon after birth. [160] [161] [162] [163] Both Tyrannosaurus and Troodon had juveniles with clear superprecociality and likely occupying different ecological niches than the adults. [153] Superprecociality has been inferred for sauropods. [164]

Genital structures are unlikely to fossilize as they lack scales that may allow preservation via pigmentation or residual calcium phosphate salts. In 2021, the best preserved specimen of a dinosaur's cloacal vent exterior was described for Psittacosaurus, demonstrating lateral swellings similar to crocodylian musk glands used in social displays by both sexes and pigmented regions which could also reflect a signalling function. However, this specimen on its own does not offer enough information to determine whether this dinosaur had sexual signalling functions it only supports the possibility. Cloacal visual signalling can occur in either males or females in living birds, making it unlikely to be useful to determine sex for extinct dinosaurs. [165]

Physiology

Because both modern crocodilians and birds have four-chambered hearts (albeit modified in crocodilians), it is likely that this is a trait shared by all archosaurs, including all dinosaurs. [166] While all modern birds have high metabolisms and are endothermic ("warm-blooded"), a vigorous debate has been ongoing since the 1960s regarding how far back in the dinosaur lineage this trait extended. Various researchers have supported dinosaurs as being endothermic, ectothermic ("cold-blooded"), or somewhere in between. [167] An emerging consensus among researchers is that, while different lineages of dinosaurs would have had different metabolisms, most of them had higher metabolic rates than other reptiles but lower than living birds and mammals, [168] which is termed mesothermy by some. [169] Evidence from crocodiles and their extinct relatives suggests that such elevated metabolisms could have developed in the earliest archosaurs, which were the common ancestors of dinosaurs and crocodiles. [170] [171]

After non-avian dinosaurs were discovered, paleontologists first posited that they were ectothermic. This was used to imply that the ancient dinosaurs were relatively slow, sluggish organisms, even though many modern reptiles are fast and light-footed despite relying on external sources of heat to regulate their body temperature. The idea of dinosaurs as ectothermic remained a prevalent view until Robert T. Bakker, an early proponent of dinosaur endothermy, published an influential paper on the topic in 1968. Bakker specifically used anatomical and ecological evidence to argue that sauropods, which had hitherto been depicted as sprawling aquatic animals with their tails dragging on the ground, were endotherms that lived vigorous, terrestrial lives. In 1972, Bakker expanded on his arguments based on energy requirements and predator-prey ratios. This was one of the seminal results that led to the Dinosaur Renaissance (see § "Dinosaur renaissance"). [58] [59] [61] [172]

One of the greatest contributions to the modern understanding of dinosaur physiology has been paleohistology, the study of microscopic tissue structure in dinosaurs. [173] [174] From the 1960s forward, Armand de Ricqlès suggested that the presence of fibrolamellar bone—bony tissue with an irregular, fibrous texture and filled with blood vessels—was indicative of consistently fast growth and therefore endothermy. Fibrolamellar bone was common in both dinosaurs and pterosaurs, [175] [176] though not universally present. [177] [178] This has led to a significant body of work in reconstructing growth curves and modeling the evolution of growth rates across various dinosaur lineages, [179] which has suggested overall that dinosaurs grew faster than living reptiles. [174] Other lines of evidence suggesting endothermy include the presence of feathers and other types of body coverings in many lineages (see § Feathers) more consistent ratios of the isotope oxygen-18 in bony tissue compared to ectotherms, particularly as latitude and thus air temperature varied, which suggests stable internal temperatures [180] [181] (although these ratios can be altered during fossilization [182] ) and the discovery of polar dinosaurs, which lived in Australia, Antarctica, and Alaska when these places would have had cool, temperate climates. [183] [184] [185] [186]

In saurischian dinosaurs, higher metabolisms were supported by the evolution of the avian respiratory system, characterized by an extensive system of air sacs that extended the lungs and invaded many of the bones in the skeleton, making them hollow. [187] Such respiratory systems, which may have appeared in the earliest saurischians, [188] would have provided them with more oxygen compared to a mammal of similar size, while also having a larger resting tidal volume and requiring a lower breathing frequency, which would have allowed them to sustain higher activity levels. [108] The rapid airflow would also have been an effective cooling mechanism, which in conjunction with a lower metabolic rate [189] would have prevented large sauropods from overheating. These traits may have enabled sauropods to grow quickly to gigantic sizes. [190] [191] Sauropods may also have benefitted from their size—their small surface area to volume ratio meant that they would have been able to thermoregulate more easily, a phenomenon termed gigantothermy. [108] [192]

Like other reptiles, dinosaurs are primarily uricotelic, that is, their kidneys extract nitrogenous wastes from their bloodstream and excrete it as uric acid instead of urea or ammonia via the ureters into the intestine. This would have helped them to conserve water. [168] In most living species, uric acid is excreted along with feces as a semisolid waste. [193] [194] However, at least some modern birds (such as hummingbirds) can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia. [195] This material, as well as the output of the intestines, emerges from the cloaca. [196] [197] In addition, many species regurgitate pellets, [198] and fossil pellets are known as early as the Jurassic from Anchiornis. [199]

The size and shape of the brain can be partly reconstructed based on the surrounding bones. In 1896, Marsh calculated ratios between brain weight and body weight of seven species of dinosaurs, showing that the brain of dinosaurs was proportionally smaller than in today's crocodiles, and that the brain of Stegosaurus was smaller than in any living land vertebrate. This contributed to the widespread public notion of dinosaurs as being sluggish and extraordinarily stupid. Harry Jerison, in 1973, showed that proportionally smaller brains are expected at larger body sizes, and that brain size in dinosaurs was not smaller than expected when compared to living reptiles. [200] Later research showed that relative brain size progressively increased during the evolution of theropods, with the highest intelligence – comparable to that of modern birds – calculated for the troodontid Troodon. [201]

The possibility that dinosaurs were the ancestors of birds was first suggested in 1868 by Thomas Henry Huxley. [202] After the work of Gerhard Heilmann in the early 20th century, the theory of birds as dinosaur descendants was abandoned in favor of the idea of them being descendants of generalized thecodonts, with the key piece of evidence being the supposed lack of clavicles in dinosaurs. [203] However, as later discoveries showed, clavicles (or a single fused wishbone, which derived from separate clavicles) were not actually absent [11] they had been found as early as 1924 in Oviraptor, but misidentified as an interclavicle. [204] In the 1970s, Ostrom revived the dinosaur–bird theory, [205] which gained momentum in the coming decades with the advent of cladistic analysis, [206] and a great increase in the discovery of small theropods and early birds. [29] Of particular note have been the fossils of the Yixian Formation, where a variety of theropods and early birds have been found, often with feathers of some type. [63] [11] Birds share over a hundred distinct anatomical features with theropod dinosaurs, which are now generally accepted to have been their closest ancient relatives. [207] They are most closely allied with maniraptoran coelurosaurs. [11] A minority of scientists, most notably Alan Feduccia and Larry Martin, have proposed other evolutionary paths, including revised versions of Heilmann's basal archosaur proposal, [208] or that maniraptoran theropods are the ancestors of birds but themselves are not dinosaurs, only convergent with dinosaurs. [209]

Feathers

Feathers are one of the most recognizable characteristics of modern birds, and a trait that was also shared by several non-avian dinosaurs. Based on the current distribution of fossil evidence, it appears that feathers were an ancestral dinosaurian trait, though one that may have been selectively lost in some species. [210] Direct fossil evidence of feathers or feather-like structures has been discovered in a diverse array of species in many non-avian dinosaur groups, [63] both among saurischians and ornithischians. Simple, branched, feather-like structures are known from heterodontosaurids, primitive neornithischians, [211] and theropods, [212] and primitive ceratopsians. Evidence for true, vaned feathers similar to the flight feathers of modern birds has been found only in the theropod subgroup Maniraptora, which includes oviraptorosaurs, troodontids, dromaeosaurids, and birds. [11] [213] Feather-like structures known as pycnofibres have also been found in pterosaurs, [214] suggesting the possibility that feather-like filaments may have been common in the bird lineage and evolved before the appearance of dinosaurs themselves. [210] Research into the genetics of American alligators has also revealed that crocodylian scutes do possess feather-keratins during embryonic development, but these keratins are not expressed by the animals before hatching. [215]

Archaeopteryx was the first fossil found that revealed a potential connection between dinosaurs and birds. It is considered a transitional fossil, in that it displays features of both groups. Brought to light just two years after Charles Darwin's seminal On the Origin of Species (1859), its discovery spurred the nascent debate between proponents of evolutionary biology and creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, at least one specimen was mistaken for the small theropod Compsognathus. [216] Since the 1990s, a number of additional feathered dinosaurs have been found, providing even stronger evidence of the close relationship between dinosaurs and modern birds. Most of these specimens were unearthed in the lagerstätte of the Yixian Formation, Liaoning, northeastern China, which was part of an island continent during the Cretaceous. Though feathers have been found in only a few locations, it is possible that non-avian dinosaurs elsewhere in the world were also feathered. The lack of widespread fossil evidence for feathered non-avian dinosaurs may be because delicate features like skin and feathers are seldom preserved by fossilization and thus often absent from the fossil record. [217]

The description of feathered dinosaurs has not been without controversy perhaps the most vocal critics have been Alan Feduccia and Theagarten Lingham-Soliar, who have proposed that some purported feather-like fossils are the result of the decomposition of collagenous fiber that underlaid the dinosaurs' skin, [218] [219] [220] and that maniraptoran dinosaurs with vaned feathers were not actually dinosaurs, but convergent with dinosaurs. [209] [219] However, their views have for the most part not been accepted by other researchers, to the point that the scientific nature of Feduccia's proposals has been questioned. [221]

Skeleton

Because feathers are often associated with birds, feathered dinosaurs are often touted as the missing link between birds and dinosaurs. However, the multiple skeletal features also shared by the two groups represent another important line of evidence for paleontologists. Areas of the skeleton with important similarities include the neck, pubis, wrist (semi-lunate carpal), arm and pectoral girdle, furcula (wishbone), and breast bone. Comparison of bird and dinosaur skeletons through cladistic analysis strengthens the case for the link. [222]

Soft anatomy

Large meat-eating dinosaurs had a complex system of air sacs similar to those found in modern birds, according to a 2005 investigation led by Patrick M. O'Connor. The lungs of theropod dinosaurs (carnivores that walked on two legs and had bird-like feet) likely pumped air into hollow sacs in their skeletons, as is the case in birds. "What was once formally considered unique to birds was present in some form in the ancestors of birds", O'Connor said. [223] [224] In 2008, scientists described Aerosteon riocoloradensis, the skeleton of which supplies the strongest evidence to date of a dinosaur with a bird-like breathing system. CT scanning of Aerosteon's fossil bones revealed evidence for the existence of air sacs within the animal's body cavity. [187] [225]

Behavioral evidence

Fossils of the troodonts Mei and Sinornithoides demonstrate that some dinosaurs slept with their heads tucked under their arms. [226] This behavior, which may have helped to keep the head warm, is also characteristic of modern birds. Several deinonychosaur and oviraptorosaur specimens have also been found preserved on top of their nests, likely brooding in a bird-like manner. [227] The ratio between egg volume and body mass of adults among these dinosaurs suggest that the eggs were primarily brooded by the male, and that the young were highly precocial, similar to many modern ground-dwelling birds. [228]

Some dinosaurs are known to have used gizzard stones like modern birds. These stones are swallowed by animals to aid digestion and break down food and hard fibers once they enter the stomach. When found in association with fossils, gizzard stones are called gastroliths. [229]

All non-avian dinosaurs and most lineages of birds [230] became extinct in a mass extinction event, called the Cretaceous–Paleogene (K-Pg) extinction event, at the end of the Cretaceous period. Above the Cretaceous-Paleogene boundary, which has been dated to 66.038 ± 0.025 million years ago, [231] fossils of non-avian dinosaurs disappear abruptly the absence of dinosaur fossils was historically used to assign rocks to the ensuing Cenozoic. The nature of the event that caused this mass extinction has been extensively studied since the 1970s, leading to the development of two mechanisms that are thought to have played major roles: an extraterrestrial impact event in the Yucatán Peninsula, along with flood basalt volcanism in India. However, the specific mechanisms of the extinction event and the extent of its effects on dinosaurs are still areas of ongoing research. [232] Alongside dinosaurs, many other groups of animals became extinct: pterosaurs, marine reptiles such as mosasaurs and plesiosaurs, several groups of mammals, ammonites (nautilus-like mollusks), rudists (reef-building bivalves), and various groups of marine plankton. [233] [234] In all, approximately 47% of genera and 76% of species on Earth became extinct during the K-Pg extinction event. [235] The relatively large size of most dinosaurs and the low diversity of small-bodied dinosaur species at the end of the Cretaceous may have contributed to their extinction [236] the extinction of the bird lineages that did not survive may also have been caused by a dependence on forest habitats or a lack of adaptations to eating seeds for survival. [237] [238]

Pre-extinction diversity

Just before the K-Pg extinction event, the number of non-avian dinosaur species that existed globally has been estimated at between 628 and 1078. [239] It remains uncertain whether the diversity of dinosaurs was in gradual decline before the K-Pg extinction event, or whether dinosaurs were actually thriving prior to the extinction. Rock formations from the Maastrichtian epoch, which directly preceded the extinction, have been found to have lower diversity than the preceding Campanian epoch, which led to the prevailing view of a long-term decline in diversity. [233] [234] [240] However, these comparisons did not account either for varying preservation potential between rock units or for different extents of exploration and excavation. [232] In 1984, Dale Russell carried out an analysis to account for these biases, and found no evidence of a decline [241] another analysis by David Fastovsky and colleagues in 2004 even showed that dinosaur diversity continually increased until the extinction, [242] but this analysis has been rebutted. [243] Since then, different approaches based on statistics and mathematical models have variously supported either a sudden extinction [232] [239] [244] or a gradual decline. [245] [246] End-Cretaceous trends in diversity may have varied between dinosaur lineages: it has been suggested that sauropods were not in decline, while ornithischians and theropods were in decline. [247] [248]

Impact event

The bolide impact hypothesis, first brought to wide attention in 1980 by Walter Alvarez, Luis Alvarez, and colleagues, attributes the K-Pg extinction event to a bolide (extraterrestrial projectile) impact. [249] Alvarez and colleagues proposed that a sudden increase in iridium levels, recorded around the world in rock deposits at the Cretaceous-Paleogene boundary, was direct evidence of the impact. [250] Shocked quartz, indicative of a strong shockwave emanating from an impact, was also found worldwide. [251] The actual impact site remained elusive until a crater measuring 180 km (110 mi) wide was discovered in the Yucatán Peninsula of southeastern Mexico, and was publicized in a 1991 paper by Alan Hildebrand and colleagues. [252] Now, the bulk of the evidence suggests that a bolide 5 to 15 kilometers (3.1 to 9.3 miles) wide impacted the Yucatán Peninsula 66 million years ago, forming this crater [253] and creating a "kill mechanism" that triggered the extinction event. [254] [255] [256]

Within hours, the Chicxulub impact would have created immediate effects such as earthquakes, [257] tsunamis, [258] and a global firestorm that likely killed unsheltered animals and started wildfires. [259] [260] However, it would also have had longer-term consequences for the environment. Within days, sulphate aerosols released from rocks at the impact site would have contributed to acid rain and ocean acidification. [261] [262] Soot aerosols are thought to have spread around the world over the ensuing months and years they would have cooled the surface of the Earth by reflecting thermal radiation, and greatly slowed photosynthesis by blocking out sunlight, thus creating an impact winter. [232] [263] [264] (This role was ascribed to sulphate aerosols until experiments demonstrated otherwise. [262] ) The cessation of photosynthesis would have led to the collapse of food webs depending on leafy plants, which included all dinosaurs save for grain-eating birds. [238]

Deccan Traps

At the time of the K-Pg extinction, the Deccan Traps flood basalts of India were actively erupting. The eruptions can be separated into three phases around the K-Pg boundary, two prior to the boundary and one after. The second phase, which occurred very close to the boundary, would have extruded 70 to 80% of the volume of these eruptions in intermittent pulses that occurred around 100,000 years apart. [265] [266] Greenhouse gases such as carbon dioxide and sulphur dioxide would have been released by this volcanic activity, [267] [268] resulting in climate change through temperature perturbations of roughly 3 °C (5.4 °F) but possibly as high as 7 °C (13 °F). [269] Like the Chicxulub impact, the eruptions may also have released sulphate aerosols, which would have caused acid rain and global cooling. [270] However, due to large error margins in the dating of the eruptions, the role of the Deccan Traps in the K-Pg extinction remains unclear. [231] [232] [271]

Before 2000, arguments that the Deccan Traps eruptions—as opposed to the Chicxulub impact—caused the extinction were usually linked to the view that the extinction was gradual. Prior to the discovery of the Chicxulub crater, the Deccan Traps were used to explain the global iridium layer [267] [272] even after the crater's discovery, the impact was still thought to only have had a regional, not global, effect on the extinction event. [273] In response, Luis Alvarez rejected volcanic activity as an explanation for the iridium layer and the extinction as a whole. [274] Since then, however, most researchers have adopted a more moderate position, which identifies the Chicxulub impact as the primary progenitor of the extinction while also recognizing that the Deccan Traps may also have played a role. Walter Alvarez himself has acknowledged that the Deccan Traps and other ecological factors may have contributed to the extinctions in addition to the Chicxulub impact. [275] Some estimates have placed the start of the second phase in the Deccan Traps eruptions within 50,000 years after the Chicxulub impact. [276] Combined with mathematical modelling of the seismic waves that would have been generated by the impact, this has led to the suggestion that the Chicxulub impact may have triggered these eruptions by increasing the permeability of the mantle plume underlying the Deccan Traps. [277] [278]

Whether the Deccan Traps were a major cause of the extinction, on par with the Chicxulub impact, remains uncertain. Proponents consider the climatic impact of the sulphur dioxide released to have been on par with the Chicxulub impact, and also note the role of flood basalt volcanism in other mass extinctions like the Permian-Triassic extinction event. [279] [280] They consider the Chicxulub impact to have worsened the ongoing climate change caused by the eruptions. [281] Meanwhile, detractors point out the sudden nature of the extinction and that other pulses in Deccan Traps activity of comparable magnitude did not appear to have caused extinctions. They also contend that the causes of different mass extinctions should be assessed separately. [282] In 2020, Alfio Chiarenza and colleagues suggested that the Deccan Traps may even have had the opposite effect: they suggested that the long-term warming caused by its carbon dioxide emissions may have dampened the impact winter from the Chicxulub impact. [256]

Possible Paleocene survivors

Non-avian dinosaur remains have occasionally been found above the K-Pg boundary. In 2000, Spencer Lucas and colleagues reported the discovery of a single hadrosaur right femur in the San Juan Basin of New Mexico, and described it as evidence of Paleocene dinosaurs. The rock unit in which the bone was discovered has been dated to the early Paleocene epoch, approximately 64.8 million years ago. [283] If the bone was not re-deposited by weathering action, it would provide evidence that some dinosaur populations may have survived at least half a million years into the Cenozoic. [284] Other evidence includes the presence of dinosaur remains in the Hell Creek Formation up to 1.3 m (4.3 ft) above the Cretaceous–Paleogene boundary, representing 40,000 years of elapsed time. This has been used to support the view that the K-Pg extinction was gradual. [285] However, these supposed Paleocene dinosaurs are considered by many other researchers to be reworked, that is, washed out of their original locations and then re-buried in younger sediments. [286] [287] [288] The age estimates have also been considered unreliable. [289]

By human standards, dinosaurs were creatures of fantastic appearance and often enormous size. As such, they have captured the popular imagination and become an enduring part of human culture. The entry of the word "dinosaur" into the common vernacular reflects the animals' cultural importance: in English, "dinosaur" is commonly used to describe anything that is impractically large, obsolete, or bound for extinction. [290]

Public enthusiasm for dinosaurs first developed in Victorian England, where in 1854, three decades after the first scientific descriptions of dinosaur remains, a menagerie of lifelike dinosaur sculptures was unveiled in London's Crystal Palace Park. The Crystal Palace dinosaurs proved so popular that a strong market in smaller replicas soon developed. In subsequent decades, dinosaur exhibits opened at parks and museums around the world, ensuring that successive generations would be introduced to the animals in an immersive and exciting way. [291] The enduring popularity of dinosaurs, in its turn, has resulted in significant public funding for dinosaur science, and has frequently spurred new discoveries. In the United States, for example, the competition between museums for public attention led directly to the Bone Wars of the 1880s and 1890s, during which a pair of feuding paleontologists made enormous scientific contributions. [292]

The popular preoccupation with dinosaurs has ensured their appearance in literature, film, and other media. Beginning in 1852 with a passing mention in Charles Dickens ' Bleak House, [293] dinosaurs have been featured in large numbers of fictional works. Jules Verne's 1864 novel Journey to the Center of the Earth, Sir Arthur Conan Doyle's 1912 book The Lost World, the 1914 animated film Gertie the Dinosaur (featuring the first animated dinosaur), the iconic 1933 film King Kong, the 1954 Godzilla and its many sequels, the best-selling 1990 novel Jurassic Park by Michael Crichton and its 1993 film adaptation are just a few notable examples of dinosaur appearances in fiction. Authors of general-interest non-fiction works about dinosaurs, including some prominent paleontologists, who have often sought to use the animals as a way to educate readers about science in general. Dinosaurs are ubiquitous in advertising numerous companies have referenced dinosaurs in printed or televised advertisements, either in order to sell their own products or in order to characterize their rivals as slow-moving, dim-witted, or obsolete. [294] [295]

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Stop Saying That Dinosaurs Went Extinct. They Didn't and You Sound Ignorant.

Dinosaur phylogeny is fascinating, and not being grotesquely misinformed is cool too.

Everyone knows that about 65 million years ago, a massive asteroid hit the Earth and wiped out all the dinosaurs. Except, well, not so much. Most species of dinosaurs went extinct in the aftermath of the impact, but some survived. Those survivors continued on, breeding and evolving over the epochs. Today, there are at least 10,000 species of dinosaur that roam the planet, and they come in a huge diversity of sizes, shapes, and colors. We call them — you guessed it — birds.

Birds aren’t like dinosaurs. They are dinosaurs. And the easiest way to get your head around this fact is to understand what being a dinosaur entails.

So, what makes a dinosaur a dinosaur, anyway?

Phylogenetic is a fancy word for how we classify animals based on genetic relationships between species. The goal of phylogenetics is to produce a phylogeny, a family tree of life. Scientists are finding that the most useful way to group organisms is by clade, which includes all descendants of a given common ancestor. The principles of phylogenetic analysis suggest this is the only correct way of classifying species.

The alternative is to group by class — that is, related organisms that display certain characteristics. The problem with this method is that an organism could get kicked out of a group if it evolves to no longer exhibit a distinguishing feature of its ancestors. This approach gets messy pretty quick — it would be like redrawing your whole family tree because you have blue eyes but none of your ancestors did.

So, back to dinosaurs: Dinosaurs are organisms that belong to the clade Dinosauria. Hundreds of millions of years ago, the first dinosaur was born from two not-quite-dinosaur parents. All of the descendants of this original dino are dinosaurs, no matter how varied or diverse. And birds, as it turns out, are among them.

So birds descended from pterodactyls, then?

Oh, heavens, no! Pterodactyls weren’t even dinosaurs. They were flying reptile cousins of the dinosaurs. They belong to the clade Pterosauria, which branched off from the phylogeny before that first dinosaur that we talked about earlier was born. Unlike the dinosaurs, the pterosaurs actually did go extinct during the great Cretaceous–Paleogene extinction event.

Land creatures have evolved to fly at many points in evolutionary history, and it doesn’t mean they come from the same branch. Bats, for example, are more closely related to humans than they are to birds.

Birds evolved from bird-like dinosaurs like the Archaeopteryx. Its close cousins include the Tyrannosaurs rex and the velociraptors, which were a lot more bird-like than Jurassic Park led you to believe — they were very likely covered in feathers (or protofeathers) and warm-blooded (or somewhat warm-blooded).

Wait, are you trying to tell me that birds are reptiles?

Yes! Consider this: A crocodile is more closely related to a bird than to a lizard. So you want to create a group that includes crocodiles and lizards, but not birds. This might seem like a logical classification based on your understanding and perception of the world, but it is scientifically meaningless. You can call sharks and dolphins similar because they both swim in the ocean, but it’s simply wrong to therefore conclude that a dolphin is a fish.

So either a bird is a reptile, or the concept of “reptile” is an amorphous grouping of animals that share traits that we happen to find reptile-ish, regardless of their actual genetic relationship to each other.

Say it with me: All birds are dinosaurs, but not all dinosaurs are birds. All dinosaurs are reptiles, but not all reptiles are dinosaurs.


Early birds

The earliest known bird is Archaeopteryx ("ancient wing"), which lived around 150 million years ago in what is now southern Germany. The creature weighed around 2 pounds(1 kilogram) and measured about 20 inches (50 centimeters) in length fossil evidence shows that it sported plumage on its tail and body. The shape of its forelimbs and feathers also suggests that Archaeopteryx was capable of powered flight, a trait associated with most modern birds. However, unlike birds today, Archaeopteryx retained individual, clawlike fingers at the tips of its wings.

Fossils of birds from the early Cretaceous Period (145.5 million to 65.5 million years ago) have been found in northeastern China, such as Confuciusornis, which lived around 125 million years ago, and had a beak and long tail-feathers. Some Confuciusornis fossils, described in 2013, even included medullary bone, a spongy tissue found in female birds that are sexually mature, Live Science previously reported.

Another piece of fossil evidence links ancient birds to their modern relatives through their digestion, in the form of the earliest known bird pellet — a mass of indigestible fish bones coughed up by a Cretaceous avian in China around 120 million years ago.


ELI5: Chickens are constantly called living dinosaurs. Why aren’t Bats, Snakes, Apes ,Turtles. mostly all animals called living dinosaurs if there were versions of that same animal walking the earth millions of years ago?

It's not that there were chickens walking alongside dinosaurs, it's that modern birds (including chickens) are literally the descendants of dinosaurs. The extinction event at the end of the Cretaceous Period killed most of the species of dinosaurs that lived at the time, but the surviving species of that extinction event were what are called the "avian" or bird-like dinosaurs. Those surviving dinosaurs evolved over the past 65 million years into all of the bird species that we have today.

You take something like apes on the other hand (including humans!) and, sure, we had ancestors at the time of the dinosaurs - early mammals who survived the extinction event and evolved over millions of years into modern apes and humans, alongside other mammalian species. But if you trace the family tree of chickens back, their ancestors 66 million years ago didn't just live alongside dinosaurs, they were actual dinosaurs!


The Secret Weapon that Allowed Dinosaurs to Take Over the Planet Becomes Official TED Talk

A TEDxLSU Talk given by Emma Schachner, Ph.D., Assistant Professor of Cell Biology and Anatomy at LSU Health New Orleans School of Medicine, has now been published as an official TED Talk, available here. In it, Dr. Schachner explains a possible reason why dinosaurs were not only able to survive, but to thrive during the Triassic period more than 200 million years ago. And it’s not what you think.

“When people think about why dinosaurs were so amazing, they usually think about the biggest or the smallest dinosaur, or who was the fastest, or who had the most feathers, the most ridiculous armor, spikes or teeth,” Dr. Schachner notes. “But perhaps the answer had to do with their internal anatomy -- a secret weapon, so to speak. My colleagues and I, we think it was their lungs.”

During the Triassic period, oxygen levels have been estimated to have been as low as 15% compared to today’s 21%, so lungs capable of better handling a low-oxygen environment would confer a significant advantage. But since all that is left of dinosaurs is fossilized skeletons, what kind of information could scientists have about their lungs?

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So what bearing can what happened 200 million years ago to an extinct species have on us?

“Understanding the impacts of atmospheric O2 and CO2 on the evolution, diversification, and extinction of animals is extremely important for humans as we face impending impacts of climate change on our future survival and success,” Schachner concludes.

TED is a nonprofit devoted to spreading ideas, usually in the form of short, powerful talks (18 minutes or less). TED began in 1984 as a conference where Technology, Entertainment and Design converged, and today covers almost all topics — from science to business to global issues — in more than 100 languages. Meanwhile, independently run TEDx events help share ideas in communities around the world.


New dinosaur showed descendants how to dress to impress

Ubirajara jubatus is named after a Tupi Indian name for 'lord of the spear', in reference to the creature's stiffened, elongate integumentary structures, and jubatus from the Latin meaning 'maned' or 'crested'. Image must be credited Credit: Artwork © Bob Nicholls / Paleocreations.com 2020

Scientists have found the most elaborately dressed-to-impress dinosaur ever described and say it sheds new light on how birds such as peacocks inherited their ability to show off.

The new species, Ubirajara jubatus, was chicken-sized with a mane of long fur down its back and stiff ribbons projecting out and back from its shoulders, features never before seen in the fossil record.

It is thought its flamboyant features were used to dazzle mates or intimidate foe.

An international team of scientists co-led by Professor David Martill and researcher Robert Smyth, both at the University of Portsmouth, and Professor Dino Frey at the State Museum of Natural History, Karlsruhe, Germany discovered the new species while examining fossils in Karlsruhe´s collection.

The study is published in the scientific journal Cretaceous Research.

Professor Martill said: "What is especially unusual about the beast is the presence of two very long, probably stiff ribbons on either side of its shoulders that were probably used for display, for mate attraction, inter-male rivalry or to frighten off foe.

"We cannot prove that the specimen is a male, but given the disparity between male and female birds, it appears likely the specimen was a male, and young, too, which is surprising given most complex display abilities are reserved for mature adult males.

"Given its flamboyance, we can imagine that the dinosaur may have indulged in elaborate dancing to show off its display structures."

The ribbons are not scales or fur, nor are they feathers in the modern sense. They appear to be structures unique to this animal.

Mr Smyth said: "These are such extravagant features for such a small animal and not at all what we would predict if we only had the skeleton preserved. Why adorn yourself in a way that makes you more obvious to both your prey and to potential predators?

"The truth is that for many animals, evolutionary success is about more than just surviving, you also have to look good if you want to pass your genes on to the next generation.

"Modern birds are famed for their elaborate plumage and displays that are used to attract mates—the peacock's tail and male birds-of-paradise are textbook examples of this. Ubirajara shows us that this tendency to show off is not a uniquely avian characteristic, but something that birds inherited from their dinosaur ancestors."

Ubirajara jubatus lived about 110 million years ago, during the Aptian stage of the Cretaceous period, and is closely related to the European Jurassic dinosaur Compsognathus.

A section of the long, thick mane running down the animal´s back is preserved nearly intact. The arms were also covered in fur-like filaments down to the hands.

The mane is thought to have been controlled by muscles allowing it to be raised, in a similar way a dog raises its hackles or a porcupine raises its spines when threatened.

Ubirajara could lower its mane close to the skin when not in a display mode allowing the creature to move fast without getting tangled in vegetation.

Professor Martill said: "Any creature with movable hair or feathers as a body coverage has a great advantage in streamlining the body contour for faster hunts or escapes but also to capture or release heat."

The mane isn't the only extraordinary feature.

The researchers describe as 'enigmatic' the creature's long, flat, stiff shoulder ribbons of keratin, each with a small sharp ridge running along the middle. These ribbons were positioned to not impede freedom of movement in its arms and legs, so wouldn't have limited the animal's ability to hunt, preen and send signals.

Mr Smyth argues the elaborate plumage of Ubirajara might have improved its chances of survival.

He said: "We know lots of dinosaurs had bony crests, spines and frills that were probably used for display but we don't see these very often in living birds. In birds, crests are made of feathers.

"This little dinosaur provides some insight into why this might be the case.

"Bone requires a lot of energy for a body to grow and maintain, it's also heavy and can cause serious injury if broken.

"Keratin—the material that makes up hair, feathers and scales—is a much better display alternative for a small animal like this one. Keratin is less costly for a body to produce, it's also lightweight, flexible and can be regularly replaced if damaged.

"Ubirajara is the most primitive known dinosaur to possess integumentary display structures. It represents a revolution in dinosaur communication, the effects of which we can still see today in living birds."

Professor Frey excavated the specimen from the two slabs of stone in which it lay and, using X-ray, found previously hidden skeletal elements and soft tissue, allowing the researchers to build a clear picture of its features.

Ubirajara jubatus is the first non-avian dinosaur to be described from Brazil's Crato Formation, a shallow inland sea laid down about 110 million years ago. It is also the first non-avian dinosaur found on the ancient supercontinent of Gondwana with preserved skin.

Another of the researchers on the team, Hector Rivera Sylva, from Museo del Desierto, Mexico, said as well as the discovery being a watershed in this field, it was also important for the Americas.

He said: "The Ubirajara jubatus is not only important because of the integumentary structures present for the first time in a non-avian dinosaur, completely changing the way of seeing the behaviour of certain dinosaurs. Rather, the scientific value transcends, forming a watershed, since it is the first evidence for this group in Latin America, as well as one of the few reported for the subcontinent of Gondwana, expanding the knowledge about non-avian feathered dinosaurs for America, whose evidence is very scarce."


History

The idea of possible intelligent descendants of dinosaurs had been described in different types of media, including Car Sagan's book The Dragons of Eden: Speculations on the Evolution of Human Intelligence, where he mention a possible descendant of Saurornithoides if these never became extinct, having a "base 8" arithmetic system.

The most familiar concept of all is probably the one created by Dale Russell around 1982, with the candidate species in this case being the Troodon. This was established under certain criteria, including the postulation he put forward that there was a steady increase in the encephalitic quotient, or 'EQ', among dinosaurs (the relative brain weight when compared to other species with the same body weight) and observed that Troodon's EQ was nearly six times greater than that of other dinosaurs, though it was still well below that of humans. Under these calculations, he inferred that the rate of growth over the millions of years would have culminated in the development of a brain with the same capacities as ours. Other features speculated on this model were also the development of opposable articulated digits in the forelimbs, widely considered to be extremely advantageous for tool use. As various reptiles and birds their genitals would not be visible, the way they would feed the young would be regurgitated and their mode of communication would sound like birds.

After a few decades with this model, with new discoveries about dinosaurs and with new interpretations and speculations, new theories had arrived to reinterpret the conclusions about how sapience in dinosaurs could evolve and how these could look.


Watch the video: Dinosaurs LIVING Descendants (November 2021).