Eoraptor diagram Click to enlarge image
what makes a dinosaur? Image: Illustration
© Australian Museum


Dinosaurs include extinct animals we know from fossils and the birds we see today. The extinct animals we normally think of as dinosaurs had their heyday in the Mesozoic. 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.

When an animal is called a dinosaur it means that the animal belongs to the group Dinosauria.

The three main groups within Dinosauria are:

  1. Ornithiscians including tricerotops, centrosaurus and stegosaurus and
  2. Sauropods like Brachiosaurus, Argentinosaurus, and the Australian Diamantinasaurus matildae
  3. Theropods which includes Australovenator, Tyrannosaurs and the birds living today

What are the main physical features that all dinosaurs share?

  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

Meet the family

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.

Biological classification means that each group is nested within larger groups.

Dinosauria is nested within a larger group called Archosauria

Archosauria includes the close cousins of the dinosaurs, the pterosaurs, Pterosauria and the crocodile line, Pseudosuchia which includes todays modern crocodiles and alligators as well as several extinct forms so of which were dinosaur mimics. If you compare the skeletons of these extinct crocodilians, you may have thought they were dinosaurs because they look so similar.

The pterosaurs and dinosaurs are separated from the crocodile line based on the arrangement of their ankle bones.

Dinosaurs and pterosaurs separated from each other almost 250 million years ago based on having a hole in their hip socket and a long crest on their upper arm bone.

So, the dinosaurs and pterosaurs may be considered close cousins in an evolutionary sense they are really quite widely separated.

There are flying dinosaurs. We just call them birds today.

Many other prehistoric animals are often confused with dinosaurs like the sail backed mammal relative dimetrodon as well as extinct mammals from the megafauna era, such as diprotodon or the demon duck of doom or a mammoth.

Also mistaken for dinosaurs are large extinct marine reptiles like, plesiosaur, mosasaur and ichthyosaur.

The traits of huge sizes and scales actually evolved much earlier in evolutionary history.

But don’t be disappointed that the pterosaurs aren’t dinosaurs, they ruled the skies for over 150 million years and that’s impressive.

Early dinosaur relatives

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.

The rise of dinosaurs

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.