Dinosaurs

See also: Dinosaur classification.

**Dinosaurs** are a diverse group of reptiles of the clade **Dinosauria**. They first appeared during the Triassic period, between 243 and 233.23 million years ago, 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 million years ago; their dominance continued through the Jurassic and Cretaceous periods. The fossil record demonstrates that birds are modern feathered dinosaurs, having evolved from earlier theropods during the late Jurassic Period. As such, birds were the only dinosaur lineage to survive the Cretaceous–Paleogene extinction event 66 million years ago. Dinosaurs can therefore be divided into *avian dinosaurs*, or birds; and *non-avian dinosaurs*, which are all dinosaurs other than birds. This article deals primarily with non-avian dinosaurs.

Dinosaurs are a varied group of animals from taxonomic, morphological and ecological standpoints. Birds, at over 10,000 living species, are the most diverse group of vertebrates besides perciform fish. Using fossil evidence, paleontologists have identified over 500 distinct genera and more than 1,000 different species of non-avian dinosaurs. Dinosaurs are represented on every continent by both extant species (birds) and fossil remains. Through the first half of the 20th century, before birds were recognized to be dinosaurs, most of the scientific community believed dinosaurs to have been sluggish and cold-blooded. Most research conducted since the 1970s, however, has indicated that all dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction. Some were herbivorous, others carnivorous. Evidence suggests that egg-laying and nest-building are additional traits shared by all dinosaurs, avian and non-avian alike.

While dinosaurs were ancestrally bipedal, many extinct groups included quadrupedal species, and some were able to shift between these stances. Elaborate display structures such as horns or crests are common to all dinosaur groups, and some extinct groups developed 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 meters (59 feet) and were the largest land animals of all time. Still, the idea that non-avian dinosaurs were uniformly gigantic is a misconception based in part on preservation bias, as large, sturdy bones are more likely to last until they are fossilized. Many dinosaurs were quite small: *Xixianykus*, for example, was only about 50 cm (20 in) long.

Since the first dinosaur fossils were recognized in the early 19th century, mounted fossil dinosaur skeletons have been major attractions at museums around the world, and dinosaurs have become an enduring part of world culture. The large sizes of some dinosaur groups, as well as their seemingly monstrous and fantastic nature, have ensured dinosaurs' 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.

From Wikipedia: Dinosaur classification

Dinosauria

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

## Notes

1. ↑ Although dinosaurs are reptiles, views on them as being cold-blooded animals have changed in recent years; see physiology for more details.

## References

1. ↑ Agnolín, Federico L.; Rozadilla, Sebastián (2017). "Phylogenetic reassessment of *Pisanosaurus mertii* Casamiquela, 1967, a basal dinosauriform from the Late Triassic of Argentina". *Journal of Systematic Palaeontology* **16** (10): 853–879. .
2. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `cau2018`