Not every big prehistoric reptile was a dinosaur. Flying pterosaurs, sea reptiles such as plesiosaurs and ichthyosaurs, and early sail-backed animals like Dimetrodon all lived in the distant past, and none of them were dinosaurs. Here is how scientists decide.
The scientific definition
Palaeontologists define groups by family tree. Dinosaurs are the group made up of the last common ancestor of Triceratops and modern birds, and all of its descendants. Any animal that falls inside that family tree is a dinosaur; anything outside it is not, however big or scary it was.
Features that most dinosaurs share
- Legs held straight under the body, like pillars, instead of sprawling out to the sides like a lizard’s or a crocodile’s.
- A hole right through the hip socket, where the thigh bone fits. This is one of the key features palaeontologists look for in fossils.
- They lived on land. No non-bird dinosaur is known to have been fully marine or to have flown with wings of skin.
Famous animals that are NOT dinosaurs
- Pterosaurs, such as Rhamphorhynchus: close cousins of dinosaurs, but a separate group of flying reptiles.
- Plesiosaurs and pliosaurs, such as Liopleurodon and Pliosaurus: marine reptiles with four flippers.
- Ichthyosaurs, such as Ophthalmosaurus: dolphin-shaped marine reptiles.
- Mosasaurs: giant sea lizards of the Cretaceous, related to today’s monitor lizards and snakes.
- Dimetrodon: a sail-backed animal of the Permian, more closely related to mammals than to dinosaurs, and extinct long before the first dinosaur.
Birds are dinosaurs
Birds evolved from small feathered meat-eating dinosaurs during the Jurassic. Fossils like Archaeopteryx show the transition, with feathers and wings but also teeth and a bony tail. Because birds sit inside the dinosaur family tree, scientists consider them living dinosaurs. When people say the dinosaurs died out 66 million years ago, they mean the non-bird dinosaurs.
Two great groups
Dinosaurs have traditionally been split by the shape of their hips into saurischians (“lizard-hipped”: meat-eaters like Allosaurus, the long-necked sauropods, and birds) and ornithischians (“bird-hipped”: plant-eaters like Stegosaurus and Triceratops). Confusingly, birds belong to the lizard-hipped group. Some recent studies have proposed a different arrangement, and the debate continues.
Key points for your test
- Dinosaurs were land animals with legs directly under the body.
- Pterosaurs, plesiosaurs, ichthyosaurs and mosasaurs were not dinosaurs.
- Birds are dinosaurs; the non-bird dinosaurs died out 66 million years ago.