Dinosauria (Dinosaurs)


Dinosaurs are a group of extinct vertebrates that lived from 230 to 65 million years ago (Ma). They originated in southern Pangaea during the Middle Triassic from a small, meter‐long bipedal archosaur (the group that includes crocodiles and pterosaurs). Within a few million years dinosaurs rapidly diversified into two major groups (Ornithischia and Saurischia) based on pelvic structure and three main clades or subgroups (Ornithischia, Sauropodomorpha and Theropoda). Further diversification resulted in the various dinosaur classic types, such as tyrannosaurs, apatosaurs, stegosaurs, ankylosaurs and ceratopsians. These various types reflect the two locomotor styles, bipedal and quadrupedal, as well as the three dietary groups, herbivorous, carnivorous and omnivorous. Dinosaurs spread globally and by the Jurassic appear on all the continents, including ice‐free Antarctica. They also spread into a variety of niches, including flight. The extinction of this seemingly successful group 65 Ma is still the subject of much debate. Hypothetical causes include disease, volcanism, sea level drop, asteroid impact, or a combination of these. The only survivor of the ‘Great Extinction’ are the birds, or avian dinosaurs.

Key Concepts:

  • Dinosaurs appeared in the Late Triassic of South America 230 Ma and survive today as birds.

  • Dinosaurs underwent dramatic diversification within a few million years after their appearance so that most varieties were present by the Middle Jurassic.

  • Dinosaurs also spread globally, occurring on all continents and in a variety of environments.

  • Although an asteroid impact apparently marks the end of the Cretaceous, there is still no uniform agreement that it was the cause for the extinction of nonavian dinosaurs.

  • Modern birds are believed to be descendants of small, feathered theropods.

Keywords: Saurischian; ornithischian; theropod; sauropodomorph; ornithopod; marginocephalian; ankylosaur; stegosaur; brid origin

Figure 1.

Dinosaurs have traditionally been divided into two groups based primarily on the pelvis. Three bones form the pelvis, the horizontal ilium (yellow), the forward projecting pubis (green) and the rearward projecting ischium (purple). In saurischians, the three bones typically form a triangle (a), whereas in ornithischians, they typically form a rectangle (b).

Figure 2.

The major dinosaur groups include the sauropodomorphs (a), represented by the sauropod Brachiosaurus; Theropoda (b) by Tyrannosaurus; Ornithopoda (c) denoted by the hadrosaur Edmontosaurus; the Ceratopsia (d) depicted by Triceratops; the Pachycephalosauria (e) by Pachycephalosaurus; Stegosauria (f) by Stegosaurus and the Ankylosauria (g) by Euoplocephalus. All to scale.

Figure 3.

Cladogram showing the most current thinking about the relationship of the dinosaurs (after Sereno, ). Unlike the traditional evolutionary trees that were based on superficial resemblances of species and the geological age of the fossils, cladograms rely upon shared advanced or derived characters to unite dinosaur species regardless of geological age. The reason for the change in analysis is to concede that the fossil record is very incomplete and that the fossil record may not preserve the first occurrence of an advanced character. For example, the closest dinosaur relative to the Late Jurassic bird Archaeopteryx (∼152 Ma), are the troodontids, typified by Troodon and Saurornitholestes from the Late Cretaceous (∼75 Ma).

Figure 4.

Small theropod Sinosauropteryx showing a coat of fine, hair‐like protofeathers. Note the light and dark bands on the tail. Also, some of the internal organs are visible.

Figure 5.

The sauropod Camarasaurus from the Upper Jurassic Morrsion Formation shows many of the characteristics of sauropods: small head, long neck, long tail and elephant‐like legs.

Figure 6.

The armour plated Stegosaurus is characterised by tall, triangular plates on its back and tail. The purpose of the plates is controversial, with suggestions ranging from radiators to control body heat to sexual and threat display.

Figure 7.

Gastonia shows the characteristic protective body armour. The armour formed within the skin much like that of modern crocodiles.

Figure 8.

Ornithopods, typified by Camptosaurus, lacked protective structures on the head or body. Ornithopods may have travelled in herds for group protection.

Figure 9.

The horned dinosaur Styracosaurus shows the various horns that projected from the head, as well as a frill or collar along the back side of the skull.



Carpenter K (1997) Agonistic behavior in pachycephalosaurs (Ornithischia: Dinosauria): a new look at head‐butting behavior. Contributions to Geology, University of Wyoming 32: 19–25.

Carpenter K, Sanders F, McWhinney LA and Wood L (2005) Evidence for predator–prey relationships: examples for Allosaurus and Stegosaurus. In: Carpenter K (ed) The Carnivorous Dinosaurs, pp. 325–350. Bloomington: Indiana University Press.

Lipkin C and Carpenter K (2008) Looking again at the forelimb of Tyrannosaurus rex. In: Larson P and Carpenter K (eds) Tyrannosaurus rex: The Tyrant King, pp. 166–190. Bloomington: Indiana University Press.

Norell MA and Xu X (2005) Feathered dinosaurs. Annual Review of Earth and Planetary Sciences 33: 277–299.

Sereno P (1999) The evolution of the dinosaurs. Science 284: 2137–2147.

Further Reading

Ackerman J (1998) Dinosaurs take wing. National Geographic 194: 74–99.

Currie PJ and Padian K (1997) Encyclopedia of Dinosaurs. San Diego: Academic Press.

Farlow JO and Brett‐Surman MK (1997) The Complete Dinosaur. Bloomington: Indiana University Press.

Horner JR and Gorman J (1988) Digging Dinosaurs. New York: Workman Publishing.

Lockley MG, Houck K and Prince N (1986) North America's largest dinosaur tracksite: implications for Morrison Formation paleoecology. Geological Society of America Bulletin 57: 1163–1176.

Sereno P (1997) The origin and evolution of dinosaurs. Annual Review of Earth and Planetary Sciences 25: 435–489.

Sloan CP (1999) Feathers for T. rex. National Geographic 196: 98–107.

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How to Cite close
Carpenter, Kenneth(Jun 2011) Dinosauria (Dinosaurs). In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001545.pub2]