TST Evolution Timeline: Dinosaurs & Birds

The bilaterian branch gave rise to today's arthropods, mollusks, fish, amphibians, reptiles, birds, and mammals. The significant idea is directionality. From a radial (circle) to a bilateral (line) symmetry, life transitioned from a passive "being" to an active "doing."

The Ordovician–Silurian extinction shows how climate change can reshape evolution by collapsing old ecosystems and opening space for new life.

The amniotic egg evolved in the first amniotes, which evolved into today's reptiles, birds, and mammals.

The Mesozoic era starts with the end-Permian mass extinction 252 million years ago. It ends the reign of dinosaurs with the K–Pg extinction 66 million years ago.

Asilisaurus shows that the bird-line archosaurs were already evolving dinosaur-like bodies before true dinosaurs appeared.

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.

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.

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.

Pterosaurs were not dinosaurs but do share a common ancestor. They are a distinct group of flying reptiles that emerged in the Late Triassic.

The common ancestor of the diplodocids is still unknown, but it gave rise to several distinct giant-necked forms. In Diplodocus, notice the long, narrow skull. In Apatosaurus, note the deeper, more robust skull and heavier build. Finally, in the slimmer Brontosaurus, notice the similar shape but somewhat lighter, less massive form.

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.

Brontosaurus, in the revived interpretation, looks broadly similar to Apatosaurus but is argued to be less massive and less robust. A bit lighter-built overall.

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.

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.

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.

Argentinosaurus shows how far the sauropod body plan could go. By the Late Cretaceous, some titanosaurs had become the largest land animals known, turning the long-necked dinosaur design into one of evolution’s most extreme achievements.

Hesperornithiformes were extinct Cretaceous diving birds that lived outside modern bird crown-group Neornithes. They show that highly specialized aquatic birds had already evolved by about 90 million years ago.

Pteranodon was one of the great soaring pterosaurs of the Late Cretaceous, but it was only one branch in a much larger pterosaur story.

Galloanserae is the living bird branch that includes landfowl and waterfowl: chickens, turkeys, pheasants, ducks, geese, and swans.

Styracosaurus was a striking horned ancient cousin of Triceratops, showing that ceratopsids branched into different styles long before the dinosaurs came to an end.

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.

At the K–Pg boundary, birds were already diverse, but most of that Late Cretaceous variety died out, leaving only a small toothless slice of the bird world to continue.

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.

The K–Pg extinction was a sudden global catastrophe that ended the long dominance of non-avian dinosaurs and opened the way for mammals and modern birds to expand into a transformed world.