Not dinosaurs! 🚫🦖
Flying reptiles, sea monsters and sail-backed beasts lived alongside (or before) dinosaurs — but they belong to different branches of the family tree.
So what makes a dinosaur a dinosaur?
Dinosaurs stood with their legs straight underneath their bodies, like you do. Lizards and crocodiles have legs that stick out to the side. Dinosaurs also lived on land — not in the sea or (except birds) in the air.
Dinosaurs are defined by ancestry: all descendants of the most recent common ancestor of birds and Triceratops. Shared features include an open (perforated) hip socket, a shelf-like ridge on the upper thigh bone, and legs held erect beneath the body. Pterosaurs are close relatives within the archosaurs (Ornithodira) but sit outside Dinosauria; marine reptiles and synapsids are much more distant.
Flying reptiles
Pterosaurs were the first vertebrates to fly with flapping wings — a skin membrane stretched from a super-long fourth finger.
Pteranodon
🪁 PterosaursWhy it's not a dinosaur: Pteranodon was a pterosaur, a flying reptile. Pterosaurs were close cousins of dinosaurs, but their legs did not sit straight under the body in the dinosaur way and they evolved on a separate branch — so they are not dinosaurs and not the ancestors of birds.
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Quetzalcoatlus
🪁 PterosaursWhy it's not a dinosaur: Quetzalcoatlus was a pterosaur, a flying reptile. Pterosaurs were close cousins of dinosaurs, but their legs did not sit straight under the body in the dinosaur way and they evolved on a separate branch.
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Rhamphorhynchus
🪁 PterosaursWhy it's not a dinosaur: Rhamphorhynchus was a pterosaur, a flying reptile. Pterosaurs were close cousins of dinosaurs, but their legs did not sit straight under the body in the dinosaur way and they evolved on a separate branch.
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Sea reptiles
While dinosaurs ruled the land, these reptiles ruled the oceans. None of them were dinosaurs.
Elasmosaurus
🐢 PlesiosaursWhy it's not a dinosaur: Elasmosaurus was a plesiosaur, a sea reptile that swam with four flippers. Dinosaurs lived on land with legs held straight under the body, and plesiosaurs belong to a completely separate branch of reptiles.
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Ichthyosaurus
🐬 IchthyosaursWhy it's not a dinosaur: Ichthyosaurus was an ichthyosaur, a dolphin-shaped reptile that spent its whole life in the sea. Dinosaurs were land animals with legs held straight under the body, and ichthyosaurs evolved on a separate branch of reptiles.
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Liopleurodon
🐢 PlesiosaursWhy it's not a dinosaur: Liopleurodon was a pliosaur, a short-necked kind of plesiosaur that lived in the sea. Dinosaurs were land animals with legs held straight under the body, and plesiosaurs are a separate branch of reptiles.
Explore Liopleurodon →![Scientific life reconstruction of Mosasaurus: Life reconstruction of Mosasaurus missouriensis Sources: General body plan is consistent with Lindgren et al. (2010)[1]](/images/mosasaurus-life.webp)
Mosasaurus
🐊 MosasaursWhy it's not a dinosaur: Mosasaurus was a giant sea-going lizard, related to today's monitor lizards and snakes. Dinosaurs belong to a different branch of reptiles, together with crocodiles and birds.
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Mammals and their ancestors
Dimetrodon lived before the first dinosaur and is closer to YOU than to any dinosaur!
Dimetrodon
⛵ SynapsidsWhy it's not a dinosaur: Dimetrodon was a synapsid — part of the group that later gave rise to mammals, so it is more closely related to you than to any dinosaur. It also lived more than 30 million years before the first dinosaurs appeared.
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Repenomamus
🦣 MammalsWhy it's not a dinosaur: Repenomamus was a mammal with fur that fed its babies milk, like badgers or dogs today. Mammals lived at the same time as dinosaurs, but they are on a completely different branch of the animal family tree.
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