Dinosaur evolution begins over 230 million years ago with small bipedal archosaurs and unfolds into a dominant terrestrial lineage, one branch of which survives today as birds. Dinosaur evolution marks a dramatic radiation within archosaurs, where skeletal refinement, posture, and metabolism aligned with ecological opportunity.
Dinosaur & Bird Evolution
Archosauria Diverge Within Reptiles
LCA of crocodiles and birds — the larger archosaur branch that later gave rise to crocodilians, pterosaurs, dinosaurs, and birds.
~250 million years ago (+/- 10 million)
LCA of crocodilians and birds (pterosaurs & dinosauria)
Bird-line Archosaurs: Asilisaurus kongwe (within Ornithodira)
245 Million Years Ago
Dinosauromorphs Emerge: Erect hind-limb posture leads to birds and dinosaurs.
Bird-line Archosaur: Nyasasaurus parringtoni
Nyasasaurus is a late bird-line archosaur from just before Dinosauria clearly emerge. It sits on the dinosaur side of Ornithodira, but its exact placement remains uncertain: some analyses place it within Dinosauria, while others place it just outside the group, near other bird-line archosaurs.
243 Million Years Ago
Strengthened hip and shoulder architecture
Dinosauria Emerge: True Dinosaurs!
Dinosauria emerge from a single population of a species about 238 million years ago. This population will lead to all dinosaurs and birds including T.Rex, Brontosaurus, and Triceratops.
238 Million years ago (+/- 5 million)
Fully open hip socket (perforated acetabulum)
Pterosaurs Diverge From Dinosaur Ancestors (within Ornithodira)
Pterosaurs were the first vertebrates to conquer the air, evolving a unique "finger-wing" anatomy that allowed them to dominate the skies for 160 million years.
~237 million years ago (+/- 2 million)
Pterosauria line: Not in dinosauria (split first).
Theropod Line Diverges Within Dinosaria (from Saurischia)
Theropods were the agile, sharp-toothed dinosaur branch that refined the classic predator body plan. They stood fully upright on two legs, balanced with long tails, used grasping hands, and carried specialized skulls and recurved teeth built for active hunting. Over time, this branch produced everything from small early predators to giant hunters—and eventually birds.
~233 million years ago (±2 million years)
Ancestor of T.Rex and bird-line.
Sauropodomorph Line Diverges Within Dinosaria (from Saurischia)
Sauropodomorphs, in their early forms, were lightly built, often partly bipedal, with long necks, small heads, leaf-shaped teeth, and grasping hands.
~232 million years ago (±2 million years)
Ancestor to the sauropods like brontosaurus.
Eoraptor lunensis.
A speculative reconstruction of Eoraptor lunensis. Eoraptor reminds us that classification is not always neat at the beginning of a lineage. Early dinosaurs can be hard to classify because of a mix of traits.
229 Million Years Ago (± 1.5 million)
Ornithischians Diverge Within Dinosauria “Bird Hipped”
Ornithodira is the broader branch that includes dinosaurs, birds, and pterosaurs.
~229 million years ago (±4 million years)
LCA of Pterosaurs and Birds (pterosaurs & dinosauria).
Pterosaurs Emerge
Pterosaurs were not dinosaurs but do share a common ancestor. They are a distinct group of flying reptiles that emerged in the Late Triassic.
215 Million years ago (+/- 5 million)
Proceratosaurus (T.Rex ancestor)
Proceratosaurus had the same general tyrannosaur-style look: a big head, long tail, strong hind legs, short forelimbs, and a built-for-biting predator shape: D-shaped front teeth and a crest on top of the skull.
Lived from 169 to 164 million years ago.
Not a bird ancestor, but part of the theropod mix.
Bashanosaurus primitivus
Bashanosaurus primitivus is one of the earliest known stegosaurs and a strong candidate for representing an early form close to the ancestry of later plated dinosaurs like Stegosaurus.
~168 million years ago.
Stegosaurus ancestor
Avialae: The Bird Line Diverges (Theropoda)
Around 155 million years ago, an early avialan was probably already broadly Archaeopteryx-like, yet still unmistakably dinosaurian: small feathered theropod with teeth, claws, and a long bony tail.
155 Million years ago (+/- 5 million)
Stegosaurus
Stegosaurus is the classic plated dinosaur most people picture: large back plates, a small head, and a spiked tail used for defense. It lived late in the Jurassic
Lived 152 to 145 million years ago.
First True Bird: Archaeopteryx
Archaeopteryx reminds us that major transformations often happen gradually, with old traits and new traits living side by side for a long time. The story of birds began with their dinosaur past.
149 Million years ago (+/- 1 million)
Confuciusornithiformes Birds Emerge (Now Extinct)
Weird carryovers and side experiments: clawed wings, elaborate ribbon-like tail feathers, and a mix of advanced beak features with a still primitive dinosaurian body.
~131 Million years ago.
Extinct bird line (clawed wings, elaborate ribbon-like tail feathers)
Enantiornithes Birds Emerge (Now Extinct)
Enantiornithes were one of the most successful early bird branches of the Cretaceous, but unlike Confuciusornithiformes, they generally kept their teeth and often looked a bit more like small, sharp-faced bird-dinosaurs than beaked proto-birds.
~131 Million years ago.
Extinct bird line (clawed wings, teeth)
The Last Stegosaurus: Wuerhosaurus
Wuerhosaurus was one of the last known stegosaurs, carrying the classic plated-and-spiked body plan into the Early Cretaceous of what is now China.
110 Million years ago (+/- 10 million)
Hesperornithiformes Birds Emerge (Now Extinct)
~90 Million years ago.
Neornithes (Crown Birds) emerge
Three branches of modern birds evolved from within neornithes: Struthio camelus (the ostrich), Gallus gallus (chickens), and Passer domesticus (the house sparrow is a good one).
~90 Million years ago (+/- 10 million).
Zuniceratops
A smaller, earlier horned dinosaur that helps show the transition toward the larger, more elaborate ceratopsids.
Lived from about 90 to 89 million years ago.
Triceratops ancestor
Palaeognathae Birds Emerge
Palaeognathae is the living bird branch that includes ostriches, emus, cassowaries, rheas, kiwis, and tinamous.
~85 Million years ago (+/- 10 million).
Ground birds: ostriches, emus, tinamous, etc.
Galloanserae Birds Emerge (from Neognathae)
Galloanserae is the living bird branch that includes landfowl and waterfowl: chickens, turkeys, pheasants, ducks, geese, and swans.
~80 Million years ago (+/- 8 million).
Fowl: chickens, turkeys, pheasants, ducks, etc.
Neoaves Birds Emerge (from Neognathae)
~78 Million years ago (+/- 4 million).
Led to common birds: crows, sparrows, robins, hawks, owls, hummingbirds, etc.
Tyrannosaurus Rex
T. rex lived in western North America about 69 to 66 million years ago. All dinosaurs, except potentially three lines of bires, went extinct 66 million years ago, when the Chicxulub asteroid hit.
Lived from 69 to 66 million years ago.
Triceratops
Three facial horns, broad frill, and powerful four-legged body. It was one of the last great non-avian dinosaurs and is the classic fully developed ceratopsid most people picture when they think of horned dinosaurs. Lived from about 68 to 66 million years ago.
Lived from 68 to 66 million years ago.
The Last Pterosaurs
By the end of the Cretaceous, the surviving pterosaurs were mostly advanced, toothless pterodactyloids.
66 Million years ago (K–Pg extinction)
The Last Ornithischians
The last ornithischians still displayed four striking body plans at the end of the Cretaceous: horned ceratopsians, duck-billed hadrosaurs, armored ankylosaurs, and dome-headed pachycephalosaurs.
66 Million years ago (K–Pg extinction)
The Last Sauropods
By the end, sauropods had narrowed to one last great branch—titanosaurs—but that branch still held real variety.
66 Million years ago (K–Pg extinction)
The Last Theropods
The last theropods still ranged from giant apex predators to smaller runners and hunters, while birds overhead carried the theropod branch beyond the extinction event.
66 Million years ago (K–Pg extinction)