What are dinosaurs? | Natural History Museum Accept cookies? We use cookies to give you the best online experience. We use them to improve our website and content, and to tailor our digital advertising on third-party platforms. You can change your preferences at any time. Manage my preferences Yes, accept all Skip to content Visit The Museum at South Kensington The Museum at Tring Back The Museum at South Kensington Back Plan your visit What's on Explore the Museum Access The Museum at Tring Back Getting here What's on Explore the Museum Facilities Discover Back News Anthropocene British wildlife Collections Dinosaurs Human evolution Oceans Space What on Earth? Video Take part Join and support Back Membership Make a donation Patrons Corporate partnerships Shop Schools Our science Back Deworm3 About Who we are Objectives Achievements Research tool kit Publications News Search Membership Donate Search Popular search terms: British wildlife Wildlife Photographer of the Year Dinosaurs Space Our Broken Planet Activities to try at home Dinosaurs took on a huge variety of shapes and sizes. Some walked on four legs and some on two. Here are a selection of Late Cretaceous dinosaurs from what is now the USA. From back to front: Ankylosaurus, Tyrannosaurus, Triceratops, Struthiomimus, Pachycephalosaurus, and the more recently named Acheroraptor and Anzu. Adapted from an image by Durbed, via Wikimedia Commons, licensed under CC BY-SA 3.0. Read later Close Beta During Beta testing articles may only be saved for seven days. Create a list of articles to read later. You will be able to access your list from any article in Discover. You don't have any saved articles. + add current article to list DINOSAURS What are dinosaurs? By Sam Rae and Lisa Hendry 0 Our brief introduction to dinosaurs reveals a key feature that gave them an advantage over other prehistoric reptiles. Dinosaurs are a group of reptiles that dominated the land for over 140 million years (more than 160 million years in some parts of the world). They evolved diverse shapes and sizes, from the fearsome giant Spinosaurus to the chicken-sized Microraptor, and were able to survive in a variety of ecosystems. One of the reasons for dinosaurs' success is that they had straight back legs, perpendicular to their bodies. This allowed them to use less energy to move than other reptiles that had a sprawling stance like today's lizards and crocodiles. With their legs positioned under their bodies rather than sticking out to the side, dinosaurs' weight was also better supported. Dinosaurs had an upright stance (shown on the left), while lizards and crocodiles have a sprawling stance (shown on the right). Adapted from an image by Fred the Oyster, via Wikimedia Commons, licensed under CC BY-SA 4.0. Many dinosaur species became extinct around 66 million years ago, but a group of living dinosaurs are still with us today: birds. Other prehistoric reptiles Dinosaurs are archosaurs, a larger group of reptiles that first appeared about 251 million years ago, near the start of the Triassic Period. Some other non-dinosaur reptiles are also archosaurs, including pterosaurs (the now-extinct flying reptiles) and modern crocodiles and their ancestors. These and many other types of ancient reptiles are often wrongly called dinosaurs. Marine reptiles, such as ichthyosaurs, plesiosaurs and mosasaurs are not dinosaurs. Nor is Dimetrodon or other reptiles in the same group (previously called 'mammal-like reptiles' and now called synapsids). None of these other extinct groups shared the characteristic upright stance of dinosaurs. Despite its prehistoric appearance, Dimetrodon was not a dinosaur. It was a different kind of extinct reptile and lived even longer ago than dinosaurs. Test how well you know your dinosaurs: take our quiz Main characteristics dinosaurs share: They had an upright stance, with legs perpendicular to their body. This is the main feature that sets dinosaurs apart from other reptiles. Like other reptiles, they laid eggs. With the exception of some birds, for example penguins, dinosaurs lived on land, not in the sea. Their skull had a hole between the eye socket and nostril. This feature is shared by all archosaurs. Dinosaurs also had two holes behind the eye socket. Large, strong jaw muscles went through the holes to attach directly to the top of the skull. As a result, the jaws were able to open wide and clamp down with more force. Find out what the world was like during the time of the dinosaurs 194 Evolution Feature Prehistoric Dinosaurs Dino Directory Explore more than 300 dinosaurs by name, shape or when and where they lived. Discover dinosaurs Find out what Museum scientists are revealing about how dinosaurs looked, lived and behaved. Dig up dino facts Dinosaurs When did dinosaurs live? Explore the age of the dinosaurs, from the time they first appeared to the mass extinction 66 million years ago. Dinosaurs Beyond Jurassic World: what we really know about dinosaurs and how What's the next best thing to studying prehistoric dinosaurs in the flesh? Dinosaurs What's the coolest dinosaur? Watch four of our dinosaur experts battle it out for the top spot. Watch a pterosaur fly This animal is Anhanguera, a flying reptile that lived with the dinosaurs. Have a look at how it might have flown. Need more dinosaurs in your life? Browse our online shop for all things dinosaur. 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