What is a dinosaur?

Dinosaurs are classified as a group of reptiles, although some of their features are found in mammals and birds living today rather than in reptiles. It appears that some dinosaurs were warm-blooded and walked with an upright gait.

The word ‘dinosaur’ means ‘terrible lizard’ in Greek. It was coined in 1842 by Sir Richard Owen, an English Professor of Comparative Anatomy and Physiology.

The extinct animals we normally think of as dinosaurs lived during the Mesozoic era.

Just like a family tree shows your ancestors and present-day relatives, an evolutionary tree shows how dinosaurs are related to other groups of animals and to each other. Dinosaurs (those that are extinct and living birds) are part of the evolutionary tree of amniotes.

The first amniotes were primitive four-legged reptiles from which all other reptiles and mammals evolved. One branch of early reptiles led to the archosaurs – the group that includes dinosaurs, crocodiles and pterosaurs. Dinosaurs themselves are split into two main groups based on their hip structure, and then smaller branches based on other shared features.

The oldest known relative of the dinosaurs is a dog-sized four-legged animal called Asilisaurus kongwe, that lived about 240 million years ago. The fossil bones of at least 14 individuals were discovered in Tanzania.

Asilisaurus belonged to the group of animals called silesaurs, which were closely related to dinosaurs. Silesaurs continued to live alongside dinosaurs until near the end of the Triassic, about 200 million years ago. These fossils show that the dinosaur lineage had already split from dinosaur-like relatives by at least 240 million years.

When dinosaurs first appeared in the Middle Triassic, about 230 million years ago, they were simply another group of small reptiles in a world full of reptiles. By the end of the Triassic they dominated life on land and would for another 140 million years, till the close of the Mesozoic.

Was their rise due to chance, superiority or something else? The answer probably includes all of these. A ‘well-timed’ extinction event early in the Late Triassic wiped out most of their competition, clearing the way for dinosaurs. We are not sure why they then thrived. Perhaps they were better adapted to the arid, dry conditions than other animals were or their more efficient, upright way of running gave them an advantage.

The main physical features that all dinosaurs share are:

  1. Hole in skull between eye socket and nostril
  2. Two holes in skull behind eye socket
  3. Ankle that bends in single plane like a hinge
  4. Hip socket with hole in centre
  5. Limbs held directly under the body
  6. Three or more sacral (located near pelvis) vertebrae

Eoraptor diagram

Diagram of Eoraptor skeleton showing the main physical features that all dinosaurs share.

Image: Illustration
© Australian Museum

Learn about how dinosaurs lived

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