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Dinosaur

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Dinosaurs
Temporal range: Late TriassicPresent, 233.23 – 0 Mya (Range includes birds (Aves)) (Possible Middle Triassic record)
Dinosauria montage 2.jpg
A compilation of dinosaur skeletons. Clockwise from top left: Microraptor gui (a winged theropod), Apatosaurus louisae (a giant sauropod), Edmontosaurus regalis (a duck-billed ornithopod), Triceratops horridus (a horned ceratopsian), Stegosaurus stenops (a plated stegosaur), Pinacosaurus grangeri (an armored ankylosaur)
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade:
Dracohors
Clade: Dinosauria
Owen, 1842
Major groups
Possible dinosaurs of uncertain affinity

Dinosaurs are a diverse group of

million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is the subject of active research. They became the dominant terrestrial vertebrates after the Triassic–Jurassic extinction event 201.3 mya; their dominance continued throughout the Jurassic and Cretaceous periods. The fossil record shows that birds are feathered dinosaurs, having evolved from earlier theropods during the Late Jurassic epoch, and are the only dinosaur lineage known to have survived the Cretaceous–Paleogene extinction event
approximately 66 mya. Dinosaurs can therefore be divided into avian dinosaurs—birds—and the extinct non-avian dinosaurs, which are all dinosaurs other than birds.

Dinosaurs are varied from

egg-laying, and that nest
-building was a trait shared by many dinosaurs, both avian and non-avian.

While dinosaurs were ancestrally

skeletal modifications such as bony armor and spines. While the dinosaurs' modern-day surviving avian lineage (birds) are generally small due to the constraints of flight, many prehistoric dinosaurs (non-avian and avian) were large-bodied—the largest sauropod dinosaurs are estimated to have reached lengths of 39.7 meters (130 feet) and heights of 18 m (59 ft) and were the largest land animals of all time. The misconception that non-avian dinosaurs were uniformly gigantic is based in part on preservation bias, as large, sturdy bones
are more likely to last until they are fossilized. Many dinosaurs were quite small, some measuring about 50 centimeters (20 inches) in length.

The first dinosaur fossils were recognized in the early 19th century, with the name "dinosaur" (meaning "terrible lizard") being coined by Sir Richard Owen in 1841 to refer to these "great fossil lizards". Since then, mounted fossil dinosaur skeletons have been major attractions at museums worldwide, and dinosaurs have become an enduring part of popular culture. The large sizes of some dinosaurs, as well as their seemingly monstrous and fantastic nature, have ensured their regular appearance in best-selling books and films, such as Jurassic Park. Persistent public enthusiasm for the animals has resulted in significant funding for dinosaur science, and new discoveries are regularly covered by the media.

Definition

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

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

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]

Labeled diagram of a typical archosaur skull, the skull of Dromaeosaurus

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
  • Epipophyses, 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)
  • Radius, a lower arm bone, shorter than 80% of humerus length
  • Fourth trochanter (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)
  • Cnemial crest 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]

Hip joints and hindlimb postures of: (left to right) typical reptiles (sprawling), dinosaurs and mammals (erect), and rauisuchians
(pillar-erect)

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]

History of study

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,

Rutellum impicatum",[44][45] that had been found in Caswell, near Witney, Oxfordshire.[46]

Sir Richard Owen's coining of the word dinosaur, at a meeting of the British Association for the Advancement of Science
in 1841

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.

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 in New York City, while Marsh's is at the Peabody Museum of Natural History at Yale University.[56]

"Dinosaur renaissance" and beyond

Paleontologist Robert T. Bakker with mounted skeleton of a tyrannosaurid (Gorgosaurus libratus
)

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

Pietraroja 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]

Evolutionary history

Origins and early evolution

Full skeleton of an early carnivorous dinosaur, displayed in a glass case in a museum
The early dinosaurs Herrerasaurus (large), Eoraptor (small) and a Plateosaurus skull, from the Triassic

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

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

The supercontinent Pangaea in the early Mesozoic
(around 200 million years ago)

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

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, including crocodilians, dyrosaurs, sebecosuchians, turtles, lizards, snakes, sphenodontians, and choristoderans, also survived the event.[92]

The surviving lineages of neornithine birds, including the ancestors of modern

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]

Classification

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 those of the main dinosaur groups Theropoda, Sauropodomorpha and Ornithischia, compiled by Justin Tweet.[101] Further details and other hypotheses of classification may be found on individual articles.

  • Dinosauria
Restoration of six ornithopods; far left: Camptosaurus, left: Iguanodon, center background: Shantungosaurus, center foreground: Dryosaurus, right: Corythosaurus, far right (large) Tenontosaurus
.
  • Ornithischia ("bird-hipped"; diverse bipedal and quadrupedal herbivores)
  • Thyreophora (armored dinosaurs; bipeds and quadrupeds)
  • Eurypoda
    (heavy, quadrupedal thyreophorans)
Restoration of four ceratopsids: top left - Triceratops, top right - Styracosaurus, bottom left - Anchiceratops, bottom right - Chasmosaurus
.
  • Chaoyangsauridae (small, frill-less basal ceratopsians)
  • Neoceratopsia
    ("new ceratopsians")
  • Ceratopsoidea
    (large-horned ceratopsians)
  • Ceratopsidae (large, elaborately ornamented ceratopsians)
  • Centrosaurinae (ceratopsids mostly characterized by frill and nasal ornamentation)
  • Nasutoceratopsini
    (centrosaurines with enlarged nasal cavities)
  • Centrosaurini
    (centrosaurines with enlarged nasal horns)
  • Pachyrhinosaurini (mostly had nasal bosses instead of horns)
  • Ornithopoda (various sizes; bipeds and quadrupeds; evolved a method of chewing using skull flexibility and numerous teeth)
  • Elasmaria (mostly southern ornithopods with mineralized plates along the ribs; may be thescelosaurids)
  • Rhabdodontomorpha (with distinctive dentition)
  • Dryosauridae (mid-sized, small headed)
  • Camptosauridae
    (mid-sized, stocky)
  • Hadrosauriformes
    (ancestrally had a thumb spike)
  • Hadrosauroidea (large quadrupedal herbivores, with teeth merged into dental batteries)
Restoration of four macronarian sauropods: from left to right Camarasaurus, Brachiosaurus, Giraffatitan, and Euhelopus
  • Sauropodiformes
    (heavy, bipeds and quadrupeds)
  • Sauropoda (very large and heavy; quadrupedal)
  • Diplodocoidea (skulls and tails elongated; teeth typically narrow and pencil-like)
  • Dicraeosauridae (small, short-necked diplodocoids with enlarged cervical and dorsal vertebrae)
  • Diplodocidae (extremely long-necked)
  • Macronaria (boxy skulls; spoon- or pencil-shaped teeth)
  • Titanosauriformes
    ("titan lizard forms")
  • Euhelopodidae (stocky, mostly Asian)
  • Titanosauria (diverse; stocky, with wide hips; most common in the Late Cretaceous of southern continents)
  • Coelophysoidea (early theropods; includes Coelophysis and close relatives)
  • †"Dilophosaur-grade neotheropods" (larger kink-snouted dinosaurs)
  • Averostra ("bird snouts")
  • Ceratosauria (generally elaborately horned carnivores that existed from the Jurassic to Cretaceous periods, originally included Coelophysoidea)
  • Abelisauridae (large abelisauroids with short arms and oftentimes elaborate facial ornamentation)
  • Noasauridae (diverse, generally light theropods; may include several obscure taxa)
  • Elaphrosaurinae
    (bird-like; omnivorous as juveniles but herbivorous as adults)
  • Noasaurinae
    (small carnivores)
  • Megaraptora (theropods with large hand claws; either carnosaurs or coelurosaurs, potentially tyrannosauroids)
  • Carnosauria (large meat-eating dinosaurs; megalosauroids sometimes included)
  • Coelurosauria (feathered theropods, with a range of body sizes and niches)
  • †"Nexus of basal coelurosaurs" (used by Tweet to denote well-known taxa with unstable positions at the base of Coelurosauria)
  • Tyrannoraptora ("tyrant thieves")
  • Ornithomimosauria (small-headed, mostly toothless, omnivorous or possible herbivores)
  • Therizinosauroidea
    (larger therizinosaurs)
  • Caenagnathidae (toothless oviraptorosaurs known from North America and Asia)
  • Oviraptoridae (characterized by two bony projections at the back of the mouth; exclusive to Asia)
  • Paraves (avialans and their closest relatives)
  • Microraptoria (characterized by large wings on both the arms and legs; may have been capable of powered flight)
  • Eudromaeosauria (hunters with greatly enlarged sickle claws)
  • Avialae (modern birds and extinct relatives)

Timeline of major groups

Timeline of major dinosaur groups per Holtz (2007).

QuaternaryNeogenePaleogeneCretaceousJurassicTriassicHolocenePleistocenePlioceneMioceneOligoceneEocenePaleoceneLate CretaceousEarly CretaceousLate JurassicMiddle JurassicEarly JurassicLate TriassicMiddle TriassicEarly TriassicOrnithopodaCeratopsiaPachycephalosauriaAnkylosauriaStegosauriaHeterodontosauridaeAvialaeDeinonychosauriaOviraptorosauriaTherizinosauriaAlvarezsauriaOrnithomimosauriaCompsognathidaeTyrannosauroideaMegaraptoraCarnosauriaMegalosauroideaCeratosauriaCoelophysoideaTitanosauriaBrachiosauridaeDiplodocoideaCetiosauridaeTuriasauriaVulcanodontidaeMassospondyildaeRiojasauridaePlateosauridaeGuaibasauridaeHerrerasauridaeQuaternaryNeogenePaleogeneCretaceousJurassicTriassicHolocenePleistocenePlioceneMioceneOligoceneEocenePaleoceneLate CretaceousEarly CretaceousLate JurassicMiddle JurassicEarly JurassicLate TriassicMiddle TriassicEarly Triassic

Paleobiology

Knowledge about dinosaurs is derived from a variety of fossil and non-fossil records, including fossilized bones,

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]

Size

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 1000 kg (220 to 2200 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]

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.[110]

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;[111] this mount is 12 meters (39 ft) tall and 21.8 to 22.5 meters (72 to 74 ft) long,[112][113] and would have belonged to an animal that weighed between 30000 and 60000 kilograms (70000 and 130000 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.[114] 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.[115]

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 80000 to 100000 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,[116] 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 122400 kg (270000 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.[117]

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).[118][119] Other large carnivorous theropods included Giganotosaurus, Carcharodontosaurus and Tyrannosaurus.[119] 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).[120] The largest individuals may have weighed as much as 16 metric tons (18 short tons).[121]

The smallest dinosaur known is the

Anchiornis huxleyi is currently the smallest non-avialan dinosaur described from an adult specimen, with an estimated weight of 110 g (3.9 oz)[125] and a total skeletal length of 34 centimeters (1.12 ft).[124][125] The smallest herbivorous non-avialan dinosaurs included Microceratus and Wannanosaurus, at about 60 centimeters (2.0 ft) long each.[126][127]

Behavior

A nesting ground of the hadrosaur Maiasaura peeblesorum
was discovered in 1978

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

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]

intra-specific combat