Evolution of dinosaurs - AbsoluteAstronomy.com

This article gives an outline and examples of dinosaur evolution. For a detailed list of interrelationships see Dinosaur classification
Dinosaur classification
Dinosaur classification began in 1842 when Sir Richard Owen placed Iguanodon, Megalosaurus, and Hylaeosaurus in "a distinct tribe or suborder of Saurian Reptiles, for which I would propose the name of Dinosauria." In 1887 and 1888 Harry Seeley divided dinosaurs into the two orders Saurischia and...

.

Dinosaur

Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

s evolved from the archosaurs 232-234 Ma (million years ago) in the Ladinian

Ladinian

The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between 237 ± 2 Ma and 228 ± 2 Ma...

age, the latter part of the middle Triassic

Triassic

The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

. Dinosauria is a well-supported clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

, present in 98% of bootstraps

Bootstrapping

Bootstrapping or booting refers to a group of metaphors that share a common meaning: a self-sustaining process that proceeds without external help....

. It is diagnosed by many features including loss of the postfrontal on the skull and an elongate deltopectoral crest on the humerus

Humerus

The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....

From archosaurs to dinosaurs

The process leading up the first dinosaurs can be followed through fossils of the early Archosaurs such as the Proterosuchidae

Proterosuchidae

Proterosuchidae is an early, possibly paraphyletic, assemblage of basal archosauriformes whose fossils are known from the Latest Permian of Russia and the Early Triassic of southern Africa, Russia, China, Australia, and Antarctica...

, Erythrosuchidae
Erythrosuchidae
Erythrosuchidae are a family of large basal archosauromorph carnivores that lived from the later Early Triassic to the early Middle Triassic . Their fossil remains are known so far from South Africa , the Perm region of Russia, and China...

and Euparkeria
Euparkeria
Euparkeria was a small African reptile of the early Triassic period between 248-245 million years ago, close to the ancestry of the archosaurs.- Palaeobiology :...

which have fossils dating back to 250 Ma, through mid-Triassic archosaurs such as Ticinosuchus
Ticinosuchus
Ticinosuchus is an extinct genus of rauisuchian archosaur from the Middle Triassic of Switzerland and Italy.Ticinosuchus was about long, and its whole body, even the belly, was covered in thick, armoured scutes. The structure of the hips shows that its legs were placed under the body almost...

232-236 Ma. Crocodiles

Crocodylomorpha

The Crocodylomorpha are an important group of archosaurs that include the crocodilians and their extinct relatives.During Mesozoic and early Tertiary times the Crocodylomorpha were far more diverse than they are now. Triassic forms were small, lightly built, active terrestrial animals. These were...

are also descendants of mid-Triassic archosaurs.

Dinosaurs can be defined as the last common ancestor of birds (Saurischia

Saurischia

Saurischia meaning 'lizard' and ischion meaning 'hip joint') is one of the two orders, or basic divisions, of dinosaurs. In 1888, Harry Seeley classified dinosaurs into two orders, based on their hip structure...

) and Triceratops
Triceratops
Triceratops is a genus of herbivorous ceratopsid dinosaur which lived during the late Maastrichtian stage of the Late Cretaceous Period, around 68 to 65 million years ago in what is now North America. It was one of the last dinosaur genera to appear before the great Cretaceous–Paleogene...

(Ornithischia

Ornithischia

Ornithischia or Predentata is an extinct order of beaked, herbivorous dinosaurs. The name ornithischia is derived from the Greek ornitheos meaning 'of a bird' and ischion meaning 'hip joint'...

) and all the descendants of that ancestor. With that definition, the pterosaurs and several species of archosaurs narrowly miss out on being classified as dinosaurs. The pterosaurs are famous for flying through the Mesozoic skies on leathery wings and reaching the largest sizes of any flying animal. Archosaur genera

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

that narrowly miss out on being classified as dinosaurs include Schleromochlus 220-225 Ma, Lagerpeton
Lagerpeton
Lagerpeton is a genus of basal dinosauromorph from the Ladinian . Lagerpeton is known from several specimens of hindlimbs, hips, vertebrae, and feet. It was about 0.7 metres long and was found in the Chañares Formation of Argentina. It has unique feet, with an unusually long fourth toe...

230-232 Ma and Marasuchus
Marasuchus
Marasuchus is a genus of dinosaur-like ornithodiran from the middle Triassic Chañares Formation of Argentina. The species Marasuchus lilloensis was originally described as a second species of Lagosuchus, L. lilloensis...

230-232 Ma.

Earliest dinosaurs

The first known dinosaurs were bipedal predators

Carnivore

A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...

that were one to two metres long.

Spondylosoma
Spondylosoma
Spondylosoma is a genus of archosaur of uncertain affinities from the late Ladinian-age Middle Triassic Lower Santa Maria Formation in Geopark of Paleorrota, Brazil. Recent studies have gone back and forth on its identity, suggesting rauisuchian or basal saurischian dinosaur...

may or may not be a dinosaur; the fossils (all postcranial) are tentatively dated at 230-232 Ma.

The earliest confirmed dinosaur fossils include saurischia

Saurischia

n ('lizard-hipped') dinosaurs Saturnalia
Saturnalia (dinosaur)
Saturnalia is an extinct genus of basal sauropodomorph dinosaur known from the Triassic of Rio Grande do Sul, southern Brazil.-Discovery:Saturnalia was originally named on the basis of three partial skeletons...

225-232 Ma, Herrerasaurus
Herrerasaurus
Herrerasaurus was one of the earliest dinosaurs. All known specimens of this carnivore have been discovered in rocks of late Ladinian age in northwestern Argentina...

220-230 Ma, Staurikosaurus
Staurikosaurus
Staurikosaurus is a genus of herrerasaurid dinosaur from the Late Triassic of Brazil.-Discovery:The first known specimen of Staurikosaurus was recovered from the Paleontological Site Jazigo Cinco of the Santa Maria Formation in the geopark of paleorrota , Rio Grande do Sul, southern Brazil...

possibly 225-230 Ma, Eoraptor
Eoraptor
Eoraptor was one of the world's earliest dinosaurs. It was a two-legged saurischian, close to the ancestry of theropods and sauropodomorphs. It lived ca. 231.4 million years ago, in what is now the northwestern region of Argentina...

220-230 Ma and Alwalkeria
Alwalkeria
Alwalkeria is a genus of basal saurischian dinosaur from the Late Triassic of India. It was a small bipedal omnivore.This dinosaur was originally named Walkeria maleriensis by Sankar Chatterjee in 1987, in honor of famous British paleontologist Alick Walker and the Maleri Formation, in which its...

220-230 Ma. Saturnalia may be a basal saurischian or a prosauropod. The others are basal saurischians.

Among the earliest ornithischia

Ornithischia

Ornithischia or Predentata is an extinct order of beaked, herbivorous dinosaurs. The name ornithischia is derived from the Greek ornitheos meaning 'of a bird' and ischion meaning 'hip joint'...

n ('bird-hipped') dinosaurs is Pisanosaurus
Pisanosaurus
Pisanosaurus is a genus of primitive ornithischian dinosaur from the Late Triassic of what is now South America. It was a bipedal herbivore described by Argentine paleontologist Rodolfo Casamiquela in 1967. Only one species, the type, Pisanosaurus mertii, is known, based on a single partial skeleton...

220-230 Ma. Although Lesothosaurus
Lesothosaurus
Lesothosaurus is an herbivorous ornithischian dinosaur. It was named by paleontologist Peter Galton in 1978, the name meaning "lizard from Lesotho". The genus is monotypic, having only one valid species, Lesothosaurus diagnosticus, within the genus....

comes from 195-206 Ma, skeletal features suggest that it branched from the main Ornithischia line at least as early as Pisanosaurus.
A. Eoraptor
Eoraptor
Eoraptor was one of the world's earliest dinosaurs. It was a two-legged saurischian, close to the ancestry of theropods and sauropodomorphs. It lived ca. 231.4 million years ago, in what is now the northwestern region of Argentina...

, an early saurischia

Saurischia

n, B Lesothosaurus
Lesothosaurus
Lesothosaurus is an herbivorous ornithischian dinosaur. It was named by paleontologist Peter Galton in 1978, the name meaning "lizard from Lesotho". The genus is monotypic, having only one valid species, Lesothosaurus diagnosticus, within the genus....

, a primitive ornithischian, C Staurikosaurus
Staurikosaurus
Staurikosaurus is a genus of herrerasaurid dinosaur from the Late Triassic of Brazil.-Discovery:The first known specimen of Staurikosaurus was recovered from the Paleontological Site Jazigo Cinco of the Santa Maria Formation in the geopark of paleorrota , Rio Grande do Sul, southern Brazil...

(Saurischia) pelvis, D Lesothosaurus
Lesothosaurus
Lesothosaurus is an herbivorous ornithischian dinosaur. It was named by paleontologist Peter Galton in 1978, the name meaning "lizard from Lesotho". The genus is monotypic, having only one valid species, Lesothosaurus diagnosticus, within the genus....

pelvis

It is clear from this figure that early saurischians resembled early ornithischians, but not modern crocodiles. Saurischians are distinguished from the ornithischians by retaining the ancestral configuration of bones in the pelvis. Another difference is in the skull, the upper skull of the Ornithischia is more solid and the joint connecting the lower jaw is more flexible; both are adaptations to herbivory and both can already be seen in Lesothosaurus.

Saurischia

Setting aside the basal Saurischia, the rest of the Saurischia are split into the Sauropodomorpha

Sauropodomorpha

Sauropodomorpha is an extinct clade of long-necked, herbivorous, saurischian dinosaurs which includes the sauropods and their ancestral relatives. Sauropods generally grew to very large sizes, had long necks and tails, were quadrupedal, and became the largest animals to ever walk the Earth. The...

and Theropoda

Theropoda

Theropoda is both a suborder of bipedal saurischian dinosaurs, and a clade consisting of that suborder and its descendants . Dinosaurs belonging to the suborder theropoda were primarily carnivorous, although a number of theropod groups evolved herbivory, omnivory, and insectivory...

. The Sauropodomorpha is split into Prosauropoda and Sauropoda. The evolutionary paths taken by the Theropoda are very complicated. The Dinosauria (2004), a major reference work on dinosaurs, splits the Theropoda into groups Ceratosauria, Basal Tetanurae, Tyrannosauroidea, Ornithomimosauria, Therizinosauroidea, Oviraptorosauria, Troodontidae, Dromaeosauridae and Basal Avialae in turn. Each group branches off the main trunk at a later date. See Dinosaur classification for the detailed interrelationships between these.

Sauropodomorpha

The first sauropodomorphs were prosauropods. Prosauropod fossils are known from the late Triassic to early Jurassic 227-180 Ma. They could be bipedal or quadrupedal and had developed long necks and tails and relatively small heads. They had lengths of 2.5 to 10 m and were primarily herbivorous. The earliest prosauropods, such as Thecodontosaurus
Thecodontosaurus
Thecodontosaurus is a genus of herbivorous basal sauropodomorph dinosaur which lived during the late Triassic period ....

from 205-220 Ma, still retained the ancestral bipedal stance and large head to body ratio.

These evolved into the sauropods which became gigantic quadrupedal herbivores, some of which reached lengths of at least 26 m. Features defining this clade include a ratio of forelimb length to hindlimb length greater than 0.6. Most sauropods still had hindlimbs larger than forelimbs; one notable exception is Brachiosaurus
Brachiosaurus
Brachiosaurus is a genus of sauropod dinosaur from the Jurassic Morrison Formation of North America. It was first described by Elmer S. Riggs in 1903 from fossils found in the Grand River Canyon of western Colorado, in the United States. Riggs named the dinosaur Brachiosaurus altithorax,...

whose long forelimbs suggest that it had evolved to feed from tall trees like a modern-day giraffe.

Sauropod fossils are found from the times of the earliest dinosaurs right up to the K-T extinction event, from 227 to 65 Ma. Most sauropods are known from the Jurassic

Jurassic

The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

, to be more precise between 227 and 121 Ma.

The Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

sauropods form two groups. The Diplodocoidea

Diplodocoidea

Diplodocoidea was a superfamily of sauropod dinosaurs, which included some of the longest animals of all time, including slender giants like Supersaurus, Diplodocus, Apatosaurus, and Amphicoelias...

lived from 121 to 65 Ma. The Titanosauriformes lived from 132 to 65 Ma. The latter clade consists of series of nested subgoups, the Titanosauria, the Titanosauridae and Saltasauridae

Saltasauridae

Saltasauridae is a family of titanosaurian dinosaurs known from the late Cretaceous period of South America, Spain, Madagascar and Mongolia....

. Both the Diplodocoidea and Titanosauriformes are descended from the Neosauropoda

Neosauropoda

Neosauropoda is a division-level clade of sauropods within Dinosauria, and consists of the group leading to Diplodocoidea and Macronaria. Haplocanthosaurus was a typical basal neosauropod from around 150 million years ago, in the Late Jurassic...

, the earliest of which lived in about 169 Ma.

The sauropods are famous for being the largest land animals that ever lived, and for having relatively small skulls. The enlargement of prosauropod and sauropod dinosaurs into these giants and the change in skull length is illustrated in the following charts.
Dinosaurs used in creating these charts are (in date order): Eo Eoraptor
Eoraptor
Eoraptor was one of the world's earliest dinosaurs. It was a two-legged saurischian, close to the ancestry of theropods and sauropodomorphs. It lived ca. 231.4 million years ago, in what is now the northwestern region of Argentina...

; Prosauropods Ri Riojasaurus
Riojasaurus
Riojasaurus was a herbivorous prosauropod dinosaur named after La Rioja Province in Argentina where it was found by José Bonaparte. It lived during the Late Triassic and grew to about long. Riojasaurus is the only known riojasaurid to live in South America.-Description:Riojasaurus had a heavy...

, Pl Plateosaurus
Plateosaurus
Plateosaurus is a genus of plateosaurid dinosaur that lived during the Late Triassic period, around 216 to 199 million years ago, in what is now Central and Northern Europe. Plateosaurus is a basal sauropodomorph dinosaur, a so-called "prosauropod"...

, Yu Yunnanosaurus
Yunnanosaurus
Yunnanosaurus is a genus of prosauropod dinosaur from the Early to Middle Jurassic Period, a position in time that makes it one of the last prosauropods. It is closely related to Lufengosaurus...

, Ms Massospondylus
Massospondylus
Massospondylus and ) is a genus of prosauropod dinosaur from the early Jurassic Period . It was described by Sir Richard Owen in 1854 from remains found in South Africa, and is thus one of the first dinosaurs to have been named...

, Ji Jingshanosaurus
Jingshanosaurus
Jingshanosaurus is a genus of prosauropod dinosaurs from the Early Jurassic. Its fossils, a nearly complete skeleton including the skull, were found near the town of Jingshan , Lufeng County, Yunnan Province, China, from which the name derives. First described in 1995, the type species is J....

, An Anchisaurus
Anchisaurus
Anchisaurus is a genus of basal sauropodomorph, and was an early herbivorous dinosaur. Until recently it was classed as a member of Prosauropoda. The name comes from the Greek αγχι/agkhi...

, Lu Lufengosaurus
Lufengosaurus
Lufengosaurus is a genus of prosauropod dinosaur which lived during the Early Jurassic period in what is now southwestern China.-Discovery and species:...

, Yi Yimenosaurus
Yimenosaurus
Yimenosaurus is a genus of prosauropod dinosaur from the Early Jurassic. Fossils from several individuals were recovered from Yimen, China, and the type species Y. youngi was named by Bai, Yang & Wang in 1990. Reconstructions are based on mostly complete skeletons, missing only parts of the...

, ; and Sauropods Sh Shunosaurus
Shunosaurus
Shunosaurus, meaning "Shu Lizard", is a genus of sauropod dinosaur from Middle Jurassic beds in Sichuan Province in China, 170 million years ago...

, Om Omeisaurus
Omeisaurus
Omeisaurus is a genus of sauropod dinosaur from the Middle Jurassic Period of what is now China. Its name comes from Mount Emei, where it was discovered in the lower Shaximiao Formation of Sichuan Province.Like other sauropods, Omeisaurus was herbivorous and large...

, Mm Mamenchisaurus
Mamenchisaurus
Mamenchisaurus was a plant-eating four-legged dinosaur, known for its remarkably long neck which made up half its total length. It is known from numerous species which ranged in time from 160 to 145 million years ago, from the Oxfordian to Tithonian ages of the late Jurassic Period of...

, Ce Cetiosaurus
Cetiosaurus
Cetiosaurus meaning 'whale lizard', from the Greek cetus/κητος meaning 'sea monster' and saurus/σαυρος meaning 'lizard', was a sauropod dinosaur from the Mid to Late Jurassic Period in what are now Europe and Africa. It is estimated to have been about long and to have weighed roughly...

, Dc Dicraeosaurus
Dicraeosaurus
Dicraeosaurus is a genus of small diplodocoid sauropod dinosaur. It was named for the spines on the back of the neck. The first fossil was described by paleontologist Werner Janensch in 1914.Unlike most diplodocoids, Dicraeosaurus had a large head with a relatively short and wide neck...

, Br Brachiosaurus
Brachiosaurus
Brachiosaurus is a genus of sauropod dinosaur from the Jurassic Morrison Formation of North America. It was first described by Elmer S. Riggs in 1903 from fossils found in the Grand River Canyon of western Colorado, in the United States. Riggs named the dinosaur Brachiosaurus altithorax,...

, Eu Euhelopus
Euhelopus
Euhelopus is a genus of sauropod dinosaur that lived during the Early Cretaceous, between 130 and 112 million years ago . It lived in what is now Shandong Province in China. A large herbivore, it weighed approximately 15-20 tons and attained an adult length of 15m...

, Ap Apatosaurus
Apatosaurus
Apatosaurus , also known by the popular but scientifically deprecated synonym Brontosaurus, is a genus of sauropod dinosaur that lived from about 154 to 150 million years ago, during the Jurassic Period . It was one of the largest land animals that ever existed, with an average length of and a...

, Ca Camarasaurus
Camarasaurus
Camarasaurus meaning 'chambered lizard', referring to the hollow chambers in its vertebrae was a genus of quadrupedal, herbivorous dinosaurs. It was the most common of the giant sauropods to be found in North America...

, Dp Diplodocus
Diplodocus
Diplodocus , or )is a genus of diplodocid sauropod dinosaur whose fossils were first discovered in 1877 by S. W. Williston. The generic name, coined by Othniel Charles Marsh in 1878, is a Neo-Latin term derived from Greek "double" and "beam", in reference to its double-beamed chevron bones...

, Ha Haplocanthosaurus
Haplocanthosaurus
Haplocanthosaurus is a genus of sauropod dinosaur. Two species, H. delfsi and H. priscus, are known from incomplete fossil skeletons. It lived during the late Jurassic period , 155 to 152 million years ago. The type species is H. priscus, and the referred species H...

, Am Amargasaurus
Amargasaurus
Amargasaurus is a genus of dicraeosaurid sauropod dinosaur from the Early Cretaceous Period of what is now South America. It was small for a sauropod, reaching 10 meters length. It would have been a quadrupedal herbivore with a long, low skull on the end of a long neck, much like its relative...

, Ar Argentinosaurus
Argentinosaurus
Argentinosaurus is a genus of titanosaur sauropod dinosaur first discovered by Guillermo Heredia in Argentina. The generic name refers to the country in which it was discovered...

(approx), Bo Bonitasaura
Bonitasaura
Bonitasaura is a titanosaurian dinosaur hailing from uppermost layers of the Late Cretaceous Bajo de la Carpa Formation, Neuquen Group, located in Río Negro Province, Northwestern Patagonia, Argentina...

, Q Quaesitosaurus
Quaesitosaurus
Quaesitosaurus is a genus of titanosaurian sauropod found by Kurzanov and Bannikov in 1983. The type species is Quaesitosaurus orientalis. It lived from 85 to 70 million years ago during the Late Cretaceous...

, Al Alamosaurus
Alamosaurus
Alamosaurus is a genus of titanosaurian sauropod dinosaur from the Late Cretaceous Period of what is now North America. It was a large quadrupedal herbivore. Isolated vertebrae and limb bones indicate that it reached sizes comparable to Argentinosaurus and Puertasaurus, which would make it the...

, Sa Saltasaurus
Saltasaurus
Saltasaurus is a genus of titanosaurid sauropod dinosaur of the Late Cretaceous Period. Relatively small among sauropods, though still massive by the standards of modern creatures, Saltasaurus was characterized by a diplodocid-like head...

, Ra Rapetosaurus
Rapetosaurus
Rapetosaurus is a genus of sauropod dinosaur that lived in Madagascar from 70 to 65 million years ago, at the end of the Cretaceous Period. Only one species Rapetosaurus is a genus of sauropod dinosaur that lived in Madagascar from 70 to 65 million years ago, at the end of the Cretaceous Period....

, Op Opisthocoelicaudia
Opisthocoelicaudia
Opisthocoelicaudia was 12-metre-long sauropod dinosaur of the Late Cretaceous Period discovered in Mongolia in 1965 by Polish and Mongolian scientists in what is now the Gobi Desert...

, Ne Nemegtosaurus
Nemegtosaurus
Nemegtosaurus was a sauropod dinosaur from Late Cretaceous Period of what is now Mongolia. It was named after the Nemegt Basin in the Gobi Desert, where the remains — a single skull — were found...

.

With the exception of Argentinosaurus (included to fill a gap in time), these graphs show only the length of sauropods for whom near-complete fossil skeletons are known. It doesn't show other very large sauropods (see Dinosaur size#Sauropods) because these are only known from very incomplete skeletons. The ratio of skull length to body length is much higher in Eoraptor than in sauropods. The longest skull graphed is of Nemegtosaurus, which is not thought be a particularly large sauropod. The skull of Nemegtosaurus was found near the headless skeleton of 11 metre long Opisthocoelicaudia, and it has been suggested that they may be the same species, but see Nemegtosauridae

Nemegtosauridae

Nemegtosauridae is a family of probably titanosaurian sauropod dinosaurs based originally on two late Cretaceous Mongolian species known only from their diplodocid-like skulls: Nemegtosaurus and Quaesitosaurus. Authorities disagree as to the relationship of these two genera with other sauropods,...

.

The relationship between the evolution of large herbivores and large plants remains uncertain. About 50% of the plants over the time of the dinosaurs were conifers, they increased in number in the Triassic until stabilising in about 190 Ma. Cycads formed the second largest group until about 120 Ma. Ferns were present in roughly constant numbers the whole time. Flowering plants began about 120 Ma and by the end of the period had taken over from the cycads. All dinosaur herbivores appear to have been adversely affected by the extinction event at the end of the Jurassic.

Theropoda

By far the earliest fossils of Theropoda (not counting the basal saurischians) are of the Coelophysoidea

Coelophysoidea

Coelophysoids were common dinosaurs of the Late Triassic and Early Jurassic periods. They were widespread geographically, probably living on all continents. Coelophysoids were all slender, carnivorous forms with a superficial similarity to the coelurosaurs, with which they were formerly...

, including Coelophysis
Coelophysis
Coelophysis , meaning "hollow form" in reference to its hollow bones , is one of the earliest known genera of dinosaur...

and others, from late Triassic and early Jurassic 227-180 Ma. Cladistic analysis sometimes connects these to the group called Ceratosauria

Ceratosauria

Ceratosaurs are members of a group of theropod dinosaurs defined as all theropods sharing a more recent common ancestry with Ceratosaurus than with birds. There is presently no universally agreed upon listing of species or diagnostic characters of Ceratosauria, though they were less derived...

. Principal features of both include changes in the pelvic girdle and hind limb that differ between the sexes. Other ceratosauria first appear in the late Jurassic of western North America.

These are followed by the basal Tetanurae

Tetanurae

Tetanurae, or "stiff tails", is a clade that includes most theropod dinosaurs, as well as birds. Tetanurans first appear during the early or middle Jurassic Period.-Definition:...

, of whom fossils have been found from the mid Jurassic to past the end of the early Cretaceous 180 Ma to 94 Ma. They have a relatively short maxillary tooth row. They did not all branch off the evolutionary line leading to coelurosaurs at the same time. Basal tetanurans include Megalosauridae, spinosaurids, a diverse clade of allosaurs, and several genera of less certain affinities, including Compsognathus
Compsognathus
Compsognathus was a small, bipedal, carnivorous theropod dinosaur. The animal was the size of a turkey and lived around 150 million years ago, the early Tithonian stage of the late Jurassic Period, in what is now Europe. Paleontologists have found two well-preserved fossils, one in Germany...

. With the exception of Compsognathus
Compsognathus
Compsognathus was a small, bipedal, carnivorous theropod dinosaur. The animal was the size of a turkey and lived around 150 million years ago, the early Tithonian stage of the late Jurassic Period, in what is now Europe. Paleontologists have found two well-preserved fossils, one in Germany...

they are large-bodied. Allosaurs form a distinct long-lived clade that share some cranial characters. They include the well known Allosaurus
Allosaurus
Allosaurus is a genus of large theropod dinosaur that lived 155 to 150 million years ago during the late Jurassic period . The name Allosaurus means "different lizard". It is derived from the Greek /allos and /sauros...

and Sinraptor
Sinraptor
Sinraptor is a genus of theropod dinosaur from the Late Jurassic. The name Sinraptor comes from the Latin prefix "Sino", meaning Chinese, and "Raptor" meaning thief. The specific name dongi honours Dong Zhiming...

among others.

The great radiation of Theropoda into many different clades of Coelurosauria

Coelurosauria

Coelurosauria is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. In the past, it was used to refer to all small theropods, although this classification has been abolished...

must have happened in the mid to late Jurassic, because Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...

was around in about 152-154 Ma, and cladistic analysis has shown that many other groups of Coelurosauria

Coelurosauria

branched off before that. Fossil evidence from China suggests that the earliest feathers

Feathered dinosaurs

The realization that dinosaurs are closely related to birds raised the obvious possibility of feathered dinosaurs. Fossils of Archaeopteryx include well-preserved feathers, but it was not until the early 1990s that clearly non-avialan dinosaur fossils were discovered with preserved feathers...

were found on the primitive Coelurosauria. The most primitive of these, e.g. on the tyrannosauroid

Tyrannosauroidea

Tyrannosauroidea is a superfamily of coelurosaurian theropod dinosaurs that includes the family Tyrannosauridae as well as more basal relatives. Tyrannosauroids lived on the Laurasian supercontinent beginning in the Jurassic Period...

Dilong
Dilong (dinosaur)
Dilong is a genus of small tyrannosauroid dinosaur. The only species is Dilong paradoxus. It is from the Lower Cretaceous Yixian Formation near Lujiatun, Beipiao, in the western Liaoning province of China. It lived about 130 million years ago...

, were simply hollow-cored fibres that would have been useful for insulation but useless for flying.

Occasional bones and cladistic analyses point to the Tyrannosauroidea branching off from the other Theropoda early, in the middle Jurassic, although nearly complete skeletons haven't yet appeared before Eotyrannus
Eotyrannus
Eotyrannus was a genus of tyrannosauroid theropod dinosaur hailing from the Early Cretaceous Wessex Formation beds, included in Wealden Group, located in the southwest coast of the Isle of Wight, United Kingdom...

from 121-127 Ma, and the many close relatives of Tyrannosaurus
Tyrannosaurus
Tyrannosaurus meaning "tyrant," and sauros meaning "lizard") is a genus of coelurosaurian theropod dinosaur. The species Tyrannosaurus rex , commonly abbreviated to T. rex, is a fixture in popular culture. It lived throughout what is now western North America, with a much wider range than other...

itself don't appear before 84 Ma, near the end of the late Cretaceous.

Ornithomimosauria

Ornithomimosauria

The Ornithomimosauria, ornithomimosaurs or ostrich dinosaurs were theropod dinosaurs which bore a superficial resemblance to modern ostriches. They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia...

fossils are known from 127 to 65 Ma. The earliest branch from the main line of Ornithomimosauria is believed to be Harpymimus
Harpymimus
Harpymimus is a basal ornithomimosaurian theropod dinosaur from the Early Cretaceous Period of what is now Mongolia. Unlike later, more derived ornithomimosaurs, Harpymimus still possessed teeth, although they appear to have been restricted to the dentary of the lower jaw.-Discovery and naming:In...

.

The Therizinosauroidea are unusual theropods in being almost all vegetarian. Fossil Therizinosauroidea are known from 127 to 65 Ma.

Maniraptorans include Oviraptorosauria

Oviraptorosauria

Oviraptorosaurs are a group of feathered maniraptoran dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot - like skulls, with or without bony crests atop the head...

, Deinonychosaurs and birds

Fossil birds

Birds are generally believed to have evolved from certain feathered theropod dinosaurs, and there is no real dividing line between birds and dinosaurs, except of course that some of the former survived the Cretaceous-Tertiary extinction event while the latter did not. For the purposes of this...

. They are characterized by an ulna with a curved shaft.

Oviraptorosauria

Oviraptorosauria

n fossils are known from 127 to 65 Ma. They have a toothless skull that is extremely modified. The skeleton has an unusually short tail.

Deinonychosaurs, named after the enlarged sickle-shaped second digit of the foot, are closely related to birds

Fossil birds

. They have two distinct families, Troodontidae

Troodontidae

Troodontidae is a family of bird-like theropod dinosaurs. During most of the 20th century, troodontid fossils were few and scrappy and they have therefore been allied, at various times, with many dinosaurian lineages...

and Dromaeosauridae

Dromaeosauridae

Dromaeosauridae is a family of bird-like theropod dinosaurs. They were small- to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek dromeus meaning 'runner' and sauros meaning 'lizard'...

. Troodontid fossils are known from 127 to 65 Ma. They have a more slender build and longer limbs. The earliest named troodontid fossil known is Sinornithoides
Sinornithoides
Sinornithoides is a genus of troodontid theropod dinosaur. It lived during the Early Cretaceous . It measured approximately 1 meter long...

. Dromaeosaurid fossils are known from about 127 to 65 Ma with the exception of Utahraptor
Utahraptor
Utahraptor is a genus of theropod dinosaurs, including the largest known members of the family Dromaeosauridae. Fossil specimens date to the upper Barremian stage of the early Cretaceous period...

. The skeletal remains of Utahraptor are about 127-144 Ma. This is interesting because according to a recent cladistic analysis, Utahraptor is about as far from the ancestral Theropoda as it is possible to get, further than Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...

. Dromaeosaurids have a larger second digit; this family includes the well known dinosaurs Dromaeosaurus
Dromaeosaurus
Dromaeosaurus was a genus of theropod dinosaur which lived during the Late Cretaceous period , sometime between 76.5 and 74.8 million years ago, in the western United States and Alberta, Canada. The name means 'running lizard'....

, Deinonychus
Deinonychus
Deinonychus was a genus of carnivorous dromaeosaurid dinosaur. There is one described species, Deinonychus antirrhopus. This 3.4 meter long dinosaur lived during the early Cretaceous Period, about 115–108 million years ago . Fossils have been recovered from the U.S...

and Velociraptor
Velociraptor
Velociraptor is a genus of dromaeosaurid theropod dinosaur that existed approximately 75 to 71 million years ago during the later part of the Cretaceous Period. Two species are currently recognized, although others have been assigned in the past. The type species is V. mongoliensis; fossils...

.

Ancient birds (Avialae

Avialae

Avialae is a clade of dinosaurs containing their only living representatives, birds , and the most immediate extinct relatives of birds.-Competing definitions:...

) include both the Aves, which are defined as descendants of the common ancestor of modern birds and Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...

, and the more primitive Epidendrosaurus. Fossil birds

Fossil birds

stretch down from 154 Ma through the K-T extinction event at 65 Ma to the present day. Scores of complete skeletons have now been found of the more recent Confuciusornis
Confuciusornis
Confuciusornis is a genus of primitive crow-sized birds from the Early Cretaceous Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago...

, which is an early representative of the Ornithurae

Ornithurae

Ornithurae is the name of a natural group which includes all modern birds as well as their extinct relatives with plough-shaped pygostyles, a bone at the end of the tail which allows the tail feathers to fan and retract....

. Ornithurans all have a bony pygostyle

Pygostyle

Pygostyle refers to a number of the final few caudal vertebrae fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these....

, to which tail feathers are anchored. For more details on the evolution of birds, see Evolution of birds.

Ornithischia

Ornithischia or Predentata is an extinct order of beaked, herbivorous dinosaurs. The name ornithischia is derived from the Greek ornitheos meaning 'of a bird' and ischion meaning 'hip joint'...

, as the name indicates, was coined for the birdlike pelvic girdle, although they are not the ancestors of birds.

The ornithischian skull and dentition was modified very early by a herbivorous diet. Lesothosaurus separated early, but the skull of Lesothosaurus already shows such adaptations, with broad proportions, a less flexible upper jaw, and a more mobile connection for the lower jaw.

The major clades were already established by the early Jurassic. The ornithischians divided into armoured thyreophorans and unarmoured ornithopods and marginocephalia

Marginocephalia

Marginocephalia is a clade of ornithischian dinosaurs that includes the thick-skulled pachycephalosaurids, and horned ceratopsians. They were all herbivores, walking on two or four legs, and are characterized by a bony ridge or frill the back of the skull...

ns.

Thyreophorans

Surface body armour (scutes) is the most striking feature of the thyreophora

Thyreophora

The Thyreophora were a subgroup of the ornithischian dinosaurs...

ns. Scutellosaurus
Scutellosaurus
Scutellosaurus is a genus of herbivorous dinosaur that lived in North America around 200 to 196 million years ago, in the Early Jurassic times....

has these but otherwise differs little from Lesothosaurus. It has a long tail and combined bipedal-quadrupedal posture that separates it from all later thyreophorans including Stegosauria

Stegosauria

Known colloquially as stegosaurs, the Stegosauria are a group of herbivorous dinosaurs of the Jurassic and Early Cretaceous Periods, being found mostly in the Northern Hemisphere, predominantly in what is now North America and China....

and Ankylosauria

Ankylosauria

Ankylosauria is a group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. They are first known to have appeared in the early Jurassic Period of...

. These two clades, although quite different in overall appearance, share many unusual features in the skull and skeleton.

Stegosaurs

Stegosauria

are easily recognised by the prominent row of plates above the spine and ling spines on the tail. Most stegosaurs, but not Stegosaurus
Stegosaurus
Stegosaurus is a genus of armored stegosaurid dinosaur. They lived during the Late Jurassic period , some 155 to 150 million years ago in what is now western North America. In 2006, a specimen of Stegosaurus was announced from Portugal, showing that they were present in Europe as well...

, also have a spine over each shoulder. These spines and plates have evolved from the earlier surface scutes. Huayangosaurus
Huayangosaurus
Huayangosaurus is a genus of stegosaurian dinosaur from the Middle Jurassic of China. The name derives from "Huayang", an alternate name for Sichuan , and "saurus", meaning "lizard"...

is the oldest and most primitive known stegosaur.

Ankylosaurs

Ankylosauria

are easily recognised by their extensive body armour. The skull is heavily ossified. Early in their evolution, ankylosaurs split into the Nodosauridae

Nodosauridae

Nodosauridae is a family of ankylosaurian dinosaurs, from the Late Jurassic to the Late Cretaceous Period of what are now North America, Asia, Antarctica and Europe.-Characteristics:...

and Ankylosauridae

Ankylosauridae

An ankylosaurid is a member of the Ankylosauridae family of armored dinosaurs that evolved 125 million years ago and became extinct 65 million years ago during the Cretaceous-Tertiary extinction event...

, distinguished by features of the skull.

Ornithopoda

Ornithopods fall into one of three distinct clades - Heterodontosauridae

Heterodontosauridae

Heterodontosauridae is a family of early ornithischian dinosaurs that were likely among the most basal members of the group...

, Hypsilophodontidae, and Iguanodontia.

Heterodontosaurids are very small (body length < 1 m) and lived in the early to late Jurassic. Apart from Abrictosaurus
Abrictosaurus
Abrictosaurus is a genus of heterodontosaurid dinosaur from the Early Jurassic Period of what is now southern Africa. It was a small bipedal herbivore or omnivore, approximately 1.2 meters long, and weighing less than 45 kilograms .This dinosaur is known from the fossil remains of only...

all have a short upper canine and longer lower canine. The forelimbs in known fossils are unusually long.

Hypsilophodontids more closely resemble their ancestors than the heterodontosaurids do. The most distinctive features are short scapula and rod-shaped pre-pubic process. The earliest is Agilisaurus
Agilisaurus
Agilisaurus is a genus of ornithischian dinosaur from the Middle Jurassic Period of what is now eastern Asia. The name is derived from the Latin agilis meaning 'agile' and the Greek sauros meaning 'lizard', and refers to the agility suggested by its lightweight skeleton and long legs...

from the middle Jurassic of China.

Iguanodontians are a diverse but morphologically tight knit array of genera known from fossils of the late Cretaceous. Significant modifications include the evolution of tooth batteries, a ligament-bound metacarpus and a digitigrade hand posture. Tenontosaurus
Tenontosaurus
Tenontosaurus is a genus of medium- to large-sized ornithopod dinosaur. The genus is known from the late Aptian to Albian ages of the middle Cretaceous period sediments of western North America, dating between 115 to 108 million years ago...

is the most basal iguanodontian. Others include Iguanodon
Iguanodon
Iguanodon is a genus of ornithopod dinosaur that lived roughly halfway between the first of the swift bipedal hypsilophodontids and the ornithopods' culmination in the duck-billed dinosaurs...

, Camptosaurus
Camptosaurus
Camptosaurus is a genus of plant-eating, beaked ornithischian dinosaurs of the Late Jurassic period of western North America. The name means 'flexible lizard', ....

and Muttaburrasaurus
Muttaburrasaurus
Muttaburrasaurus is a genus of herbivorous ornithopod dinosaur, living in what is now northeastern Australia between 100 and 98 million years ago during the early Cretaceous Period. It has been recovered in some analyses as a member of the iguanodontian family Rhabdodontidae...

.

Marginocephalia

are named for a shelf that projects over the back of the skull. They include the pachycephalosauria

Pachycephalosauria

Pachycephalosauria is a clade of ornithischian dinosaurs. Well-known genera include Pachycephalosaurus, Stegoceras, Stygimoloch, and Dracorex. Most lived during the Late Cretaceous Period, in what is now North America and Asia. They were all bipedal, herbivorous/omnivorous animals with thick skulls...

ns and ceratopsia

Ceratopsia

Ceratopsia or Ceratopia is a group of herbivorous, beaked dinosaurs which thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. The earliest known ceratopsian, Yinlong downsi, lived between 161.2 and 155.7...

ns.

Pachycephalosaurs are best known for their thick upper fronts to their skull. The oldest known is Stenopelix
Stenopelix
Stenopelix is a genus of small ornithischian dinosaur from the Early Cretaceous of Germany. It was perhaps a basal ceratopsian from the Barremian Stage of the Cretaceous period, sometime between 130 and 125 million years ago...

, from the early Cretaceous of Europe.

Ceratopsia

Ceratopsia

ns, famous for Protoceratops
Protoceratops
Protoceratops is a genus of sheep-sized herbivorous ceratopsian dinosaur, from the Upper Cretaceous Period of what is now Mongolia. It was a member of the Protoceratopsidae, a group of early horned dinosaurs...

, Triceratops
Triceratops
Triceratops is a genus of herbivorous ceratopsid dinosaur which lived during the late Maastrichtian stage of the Late Cretaceous Period, around 68 to 65 million years ago in what is now North America. It was one of the last dinosaur genera to appear before the great Cretaceous–Paleogene...

and Styracosaurus
Styracosaurus
Styracosaurus was a genus of herbivorous ceratopsian dinosaur from the Cretaceous Period , about 76.5 to 75.0 million years ago...

illustrate the evolution of frilled and horned skulls. The frills evolved from the shelf common to all Marginocephalia. Ceratopsians are separated into basal ceratopsians, including the parrot-beaked Psittacosaurus
Psittacosaurus
Psittacosaurus is a genus of psittacosaurid ceratopsian dinosaur from the Early Cretaceous Period of what is now Asia, about 130 to 100 million years ago. It is notable for being the most species-rich dinosaur genus...

, and neoceratopsians.
Diversity of ceratopsia

Ceratopsia

n skulls. A) Skeleton of Protoceratops
Protoceratops
Protoceratops is a genus of sheep-sized herbivorous ceratopsian dinosaur, from the Upper Cretaceous Period of what is now Mongolia. It was a member of the Protoceratopsidae, a group of early horned dinosaurs...

. B) to I) Skulls. B) & C) Psittacosaurus
Psittacosaurus
Psittacosaurus is a genus of psittacosaurid ceratopsian dinosaur from the Early Cretaceous Period of what is now Asia, about 130 to 100 million years ago. It is notable for being the most species-rich dinosaur genus...

side & top. D) & E) Protoceratops side & top. F) & G) Triceratops
Triceratops
Triceratops is a genus of herbivorous ceratopsid dinosaur which lived during the late Maastrichtian stage of the Late Cretaceous Period, around 68 to 65 million years ago in what is now North America. It was one of the last dinosaur genera to appear before the great Cretaceous–Paleogene...

side & top. H) & I) Styracosaurus
Styracosaurus
Styracosaurus was a genus of herbivorous ceratopsian dinosaur from the Cretaceous Period , about 76.5 to 75.0 million years ago...

side (without lower jaw) & top.

The evolution of ceratopsid dinosaurs shares characteristics with the evolution of some mammal groups, both were "geologically brief" events precipitating the simultaneous evolution of large body size, derived feeding structures, and "varied hornlike organs."

The sequence of ceratopsian evolution in the Cretaceous is roughly from Psittacosaurus (121 -99 Ma) to Protoceratops
Protoceratops
Protoceratops is a genus of sheep-sized herbivorous ceratopsian dinosaur, from the Upper Cretaceous Period of what is now Mongolia. It was a member of the Protoceratopsidae, a group of early horned dinosaurs...

(83 Ma) to (Triceratops 67 Ma and Styracosaurus 72 Ma). In side view the skull of Psittacosaurus bears very little resemblance to that of Styracosaurus but in top view a similar pentagonal arrangement can be seen.

Fossil record

The first few lines of primitive dinosaurs diversified rapidly through the Triassic

Triassic

period; dinosaur species quickly evolved the specialised features and range of sizes needed to exploit nearly every terrestrial ecological niche

Ecological niche

In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...

. During the period of dinosaur predominance, which encompassed the ensuing Jurassic

Jurassic

and Cretaceous

Cretaceous

periods, nearly every known land animal larger than 1 meter in length was a dinosaur.

One measure of the quality of the fossil record is obtained by comparing the date of first appearance with the order of branching of a cladogram

Cladogram

A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...

based on the shape of fossil elements. Close correspondence exists for ornithiscians, saurischians and subgroups. The cladogram link between coelophysids

Coelophysidae

The Coelophysidae are a family of primitive carnivorous theropod dinosaurs. Most species were relatively small in size. The family flourished in the Late Triassic and Early Jurassic periods....

and ceratosaurs

Ceratosauria

is an exception, it would place the origin of coelophysids much too late. The simplest explanation is convergent evolution - ceratosaur bones evolved independently into a shape that resembles that of the earlier coelophysids. The other possibility is that ceratosaurs evolved much earlier than the fossil record suggests.

Most dinosaur fossils have been found in the Norian

Norian

The Norian is a division of the Triassic geological period. It has the rank of an age or stage . The Norian lasted from 216.5 ± 2.0 to 203.6 ± 1.5 million years ago. It was preceded by the Carnian and succeeded by the Rhaetian.-Stratigraphic definitions:The Norian was named after the Noric Alps in...

-Sinemurian

Sinemurian

In the geologic timescale, the Sinemurian is an age or stage in the Early or Lower Jurassic epoch or series. It spans the time between 196.5 ± 2 Ma and 189.6 ± 1.5 Ma...

, Kimmeridgian

Kimmeridgian

In the geologic timescale, the Kimmeridgian is an age or stage in the Late or Upper Jurassic epoch or series. It spans the time between 155.7 ± 4 Ma and 150.8 ± 4 Ma . The Kimmeridgian follows the Oxfordian and precedes the Tithonian....

-Tithonian

Tithonian

In the geologic timescale the Tithonian is the latest age of the Late Jurassic epoch or the uppermost stage of the Upper Jurassic series. It spans the time between 150.8 ± 4 Ma and 145.5 ± 4 Ma...

, and Campanian

Campanian

The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...

-Maastrichtian

Maastrichtian

The Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...

periods. Continuity of lineages across the intervening gaps shows that those gaps are artifacts of preservation rather than any reduction in diversity or abundance.

In many instances, cladistic analysis shows that ancestral lineages of varying durations fall in those gaps. The length of missing ancestral lineages in 1997 range from 25 Ma (Lesothosaurus
Lesothosaurus
Lesothosaurus is an herbivorous ornithischian dinosaur. It was named by paleontologist Peter Galton in 1978, the name meaning "lizard from Lesotho". The genus is monotypic, having only one valid species, Lesothosaurus diagnosticus, within the genus....

, Genasauria

Genasauria

Genasauria is an extinct clade of beaked, herbivorous dinosaurs. The group was first named by paleontologist Paul Sereno in 1986, and is typically split into Thyreophora, a group of armored dinosaurs; and Neornithischia, which included the horned and duck-billed dinosaurs...

, Hadrosauroidea

Hadrosauroidea

Hadrosauroidea is a clade or superfamily of ornithischian dinosaurs that includes the "duck-billed" dinosaurs, or hadrosaurids, and their close relatives. Many primitive hadrosauroids, such as the sail-backed Ouranosaurus, have traditionally been included in a paraphyletic "Iguanodontidae"...

, Sauropoda

Sauropoda

Sauropoda , or the sauropods , are an infraorder of saurischian dinosaurs. They had long necks, long tails, small heads , and thick, pillar-like legs. They are notable for the enormous sizes attained by some species, and the group includes the largest animals to have ever lived on land...

, Neoceratopsia, Coelurosauria

Coelurosauria

) to 85 Ma (Carcharodontosauridae

Carcharodontosauridae

Carcharodontosaurids were a group of carnivorous theropod dinosaurs. In 1931 Ernst Stromer named Carcharodontosauridae as a family, in modern paleontology this name indicates a clade within Carnosauria...

). Because the dinosaurian radiation began at small body size, the unrecorded early history may be due to less reliable fossilization of smaller species. However, some missing lineages, notably of Carcharodontosauridae

Carcharodontosauridae

and Abelisauridae

Abelisauridae

Abelisauridae is a family of ceratosaurian theropod dinosaurs. Abelisaurids thrived during the Cretaceous Period, on the ancient southern supercontinent of Gondwana, and today their fossil remains are found on the modern continents of Africa and South America, as well as on the Indian...

, require alternative explanations because the missing range extends across stages rich in fossil material.

Body size

Body size is important because of its correlation with metabolism, diet, life history, geographic range and extinction rate. The modal body mass of dinosaurs lies between 1 and 10 tons throughout the Mesozoic and across all major continental regions. That said, there was a trend towards increasing body size within many dinosaur clades, including the Thyreophora, Ornithopoda, Pachycephalosauria, Ceratopsia, Sauropomorpha, and basal Theropoda. Marked decreases in body size have also occurred in some lineages, but are more sporadic. The best known example is the decrease in body size leading up to the first birds; Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...

was below 10 kg in weight, and later birds Confuciusornis
Confuciusornis
Confuciusornis is a genus of primitive crow-sized birds from the Early Cretaceous Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago...

and Sinornis are starling- to pigeon-sized. This occurred for easier flight.

Mobility

The ancestral dinosaur was a biped. The evolution of a quadrupedal posture occurred four times, among the ancestors of Euornithopoda, Thyreophora

Thyreophora

The Thyreophora were a subgroup of the ornithischian dinosaurs...

, Ceratopsia

Ceratopsia

and Sauropodomorpha

Sauropodomorpha

. In all four cases this was associated with an increase in body size, and in all four cases the trend is unidirectional without reversal.

Dinosaurs exhibit a pattern of the reduction and loss of fingers on the lateral side of the hand (digits III, IV and V). The primitive function of the dinosaur hand is grasping with a partially opposable thumb, rather than weight-bearing. The reduction of digits is one of the defining features of tyrannosaurids

Tyrannosauridae

Tyrannosauridae is a family of coelurosaurian theropod dinosaurs which comprises two subfamilies containing up to six genera, including the eponymous Tyrannosaurus. The exact number of genera is controversial, with some experts recognizing as few as three...

, only having two functional digits on very short forelimbs.

Effect of food sources

The ancestral dinosaur was a carnivore. Herbivory among dinosaurs arose three times, at the origin of the ornithischian, sauropodomorph, and therizinosaurid clades. Individual therizinosaurids are herbivorous or omnivorous. Herbivory among the ornithischians and sauropodomorphs was never reversed.

The potential co-evolution of plants and herbivorous dinosaurs has been subject to extensive speculation. The appearance of prosauropods in the late Triassic has been tentatively linked either to the demise or diversification of types of flora at that time. The rise of ceratopsids

Ceratopsidae

Ceratopsidae is a speciose group of marginocephalian dinosaurs including Triceratops and Styracosaurus...

and iguanodont and hadrosaurid

Hadrosaurid

Hadrosaurids or duck-billed dinosaurs are members of the family Hadrosauridae, and include ornithopods such as Edmontosaurus and Parasaurolophus. They were common herbivores in the Upper Cretaceous Period of what are now Asia, Europe and North America. They are descendants of the Upper...

ornithopods in the Cretaceous has been tentatively linked to the angiosperm radiation. Unfortunately, there are still no hard data on dietary preferences of herbivorous dinosaurs, apart from data on chewing technique and gastroliths.

Biogeography

Dinosaurian faunas, which were relatively uniform in character when Pangaea

Pangaea

Pangaea, Pangæa, or Pangea is hypothesized as a supercontinent that existed during the Paleozoic and Mesozoic eras about 250 million years ago, before the component continents were separated into their current configuration....

began to break up, became markedly differentiated by the close of the Cretaceous. Biogeography

Biogeography

Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...

is based on the splitting of an ancestral species by the emplacement of a geographic barrier. Interpretation is limited by a lack of fossil evidence for eastern North America, Madagascar, India, Antarctica and Australia. No unequivocal proof of the biogeographical action on Dinosaur species has been obtained, but some authors have outlined centres of origin for many dinosaur groups, multiple dispersal routes, and intervals of geographic isolation.

Dinosaurs that have been given as evidence of biogeography include abelisaurid theropods from South America and possibly else where on Gondwana

Gondwana

In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

.

Relationships between dinosaurs show abundant evidence of dispersal from one region of the globe to another. Tetanuran theropods travelled widely through western North America, Asia, South America, Africa and Antarctica. Pachycephalosaurs and ceratopsians show clear evidence of multiple bidirectional dispersion events across Beringa.

Extinction

The Cretaceous–Tertiary extinction event, which occurred 65.5 million years ago at the end of the Cretaceous period, caused the extinction of all dinosaurs except for the line that had already given rise to the first birds.

From archosaurs to dinosaurs

Earliest dinosaurs

Saurischia

Sauropodomorpha

Theropoda

Ornithischia

Thyreophorans

Ornithopoda

Marginocephalia

Fossil record

Body size

Mobility

Effect of food sources

Biogeography

Extinction

See also