Life › The age of dinosaurs
Dinosaurs
from c. 233 million years ago· Organism or group· Animals
Well establishedStrong fossil, genetic or geological evidence.
Reptiles with upright legs that dominated land for about 135 million years and survive today as birds.
See it on the Life map →What happened
Dinosaurs evolved from small, two-legged archosaurs, the group that also includes crocodiles and pterosaurs. The earliest clear fossils, about 233–230 million years ago, come from Argentina and Brazil. Their legs sat straight under the body like pillars rather than sprawling. For some 30 million years they shared land with crocodile-line rivals, then took over after the end-Triassic extinction. Three main branches: theropods (meat-eaters, including birds), sauropodomorphs (long-necked plant-eaters) and ornithischians (beaked plant-eaters such as Triceratops).
Why it matters
They shaped land ecosystems for most of the Mesozoic and kept mammals small. Their bird descendants number about 11,000 living species.
The evidence (fossils, genes, rocks)
Thousands of skeletons from every continent, plus trackways, eggs, nests, skin and feathers. Richard Owen coined the name, "terrible lizard", in 1842.
Still debated
How fast dinosaurs rose to dominance, how warm-blooded they were, and how their three main branches are related (a 2017 study reshuffled them) are debated.
Inside this (5)
Connections (11)
▸The amniotic egg was inherited by DinosaursWell established
The connection
Dinosaurs come from the sauropsid branch of amniotes and laid amniotic eggs, as birds still do. Many species laid clutches in ground nests or mounds, and some brooded them like birds. A 2020 study argued that the earliest dinosaur eggs were soft-shelled, with hard shells evolving later in several groups.
When and how
From the late Triassic onward; famous nesting sites include Egg Mountain in Montana and Auca Mahuevo in Argentina.
The evidence
Thousands of fossil eggs, some with embryos inside, and adults preserved sitting on their nests, such as Citipati from Mongolia.
Debates and open questions
Whether the first dinosaur eggs were soft or hard is debated, and how many species warmed their eggs with body heat is unknown.
▸Dimetrodon is often mistaken for, but predates, DinosaursWell established
The connection
Dimetrodon often turns up in dinosaur toy sets, but it was a synapsid, closer to mammals than to any dinosaur. It died out around 272 million years ago, about 40 million years before the first dinosaurs appeared and even before the Great Dying.
When and how
Dimetrodon lived about 295–272 million years ago in North America and Europe; the first dinosaurs date to about 233 million years ago.
The evidence
Rock dating separates the two by tens of millions of years, and skull anatomy places Dimetrodon on the mammal side of the amniote family tree.
Debates and open questions
Little is disputed about this link; the open question is why sail-backed forms evolved and what the sail did.
▸The Great Dying (end-Permian extinction) opened the way for DinosaursStrong evidence
The connection
By wiping out most therapsids, which had ruled Permian land, the Great Dying left room for archosaurs, the reptiles that include the ancestors of dinosaurs, crocodiles and pterosaurs. Archosaurs diversified rapidly in the Triassic, and true dinosaurs appeared within about 20 million years of the extinction.
When and how
Pangaea, from about 252 million years ago; early archosaurs and dinosaur relatives are known from Russia, South Africa, Tanzania and Poland.
How sure are we?
Fossils show archosaurs radiating in the Early and Middle Triassic, and footprints from Poland suggest dinosaur cousins within a few million years of the extinction.
Debates and open questions
Whether archosaurs had a built-in advantage, such as upright posture or efficient breathing, or simply got lucky after the extinction is debated.
▸Eoraptor & the first dinosaurs (Argentina) is among the earliest DinosaursWell established
The connection
Eoraptor lived about 231 million years ago, near the start of dinosaur history. It shows the early dinosaur starter kit: small size, two-legged walking, grasping hands and an open hip socket. At the time, dinosaurs were still a minor part of their ecosystems, outnumbered by other reptiles and mammal relatives.
When and how
Ischigualasto Formation, San Juan Province, northwestern Argentina; found by Ricardo Martínez in 1991 and named in 1993 by Paul Sereno and colleagues.
The evidence
Its hip, ankle and skull features match those of later dinosaurs, and volcanic ash in the same rocks gives a radiometric age.
Debates and open questions
Possibly older dinosaurs, such as Nyasasaurus from Tanzania, about 243 million years old, may push the origin back, but their status is debated.
▸Life recovers in the Triassic produced the first DinosaursWell established
The connection
Dinosaurs evolved from small archosaur relatives that diversified during the Triassic recovery. By about 233–230 million years ago, the first true dinosaurs appear in South America. Their early success was modest: they stayed secondary to other reptiles and mammal relatives for some 30 million years.
When and how
Late Triassic South America, in Argentina's Ischigualasto and southern Brazil's Santa Maria formations, about 233–230 million years ago.
The evidence
Fossils show dinosaur-like relatives such as silesaurids appearing first, followed by true dinosaurs, with radiometric dates bracketing both.
Debates and open questions
When dinosaurs first appeared, perhaps as early as 243 million years ago, and whether a wet climate spell, the Carnian Pluvial Episode, boosted them are debated.
▸Pterosaurs were flying cousins of DinosaursWell established
The connection
Pterosaurs and dinosaurs both belong to the ornithodirans, the bird-side branch of archosaurs, and share a Triassic ancestor. Pterosaurs are not dinosaurs: they took to the air on their own, with skin wings, while birds later evolved feathered flight within the theropod dinosaurs.
When and how
Their shared ancestor lived in the Early to Middle Triassic, probably over 240 million years ago; both groups appear in late Triassic rocks.
The evidence
Shared ankle and leg features place pterosaurs next to dinosaurs, and lagerpetid fossils described in 2020 help link the two.
Debates and open questions
Pterosaurs' exact ancestors remain unclear, and their earliest fossil record is thin, so how flight evolved in them is unknown.
▸End-Triassic extinction cleared rivals for DinosaursStrong evidence
The connection
In the late Triassic, dinosaurs shared land with crocodile-line archosaurs such as phytosaurs, aetosaurs and rauisuchians, which were often larger and more varied. The end-Triassic extinction wiped out nearly all of these rivals while dinosaurs came through. In the early Jurassic, dinosaurs moved into the vacated roles and became the dominant large land animals.
When and how
About 201 million years ago, worldwide; well recorded in eastern North America's Newark Basin and in East Greenland.
How sure are we?
Fossils and footprints show crocodile-line groups vanishing at the boundary, while dinosaur footprints become larger and more common just above it.
Debates and open questions
Why dinosaurs survived and rivals did not is unclear: better breathing, insulation against volcanic winters, or simple luck have all been proposed.
▸Dinosaurs include Tyrannosaurus rexWell established
The connection
T. rex belongs to the tyrannosaurs, a branch of the theropods, the meat-eating dinosaurs. For most of their history, from about 165 million years ago, tyrannosaurs were small and lightly built; only in the last 15–20 million years of the Cretaceous did they become giant top predators in Asia and North America.
When and how
Early tyrannosaur relatives such as Guanlong lived in China about 160 million years ago; T. rex lived in western North America about 68–66 million years ago.
The evidence
Shared skull, tooth and hip features place T. rex in the tyrannosaur family on dinosaur family trees, which agree across many independent analyses.
Debates and open questions
How tyrannosaurs grew so huge so late, and whether T. rex's direct ancestors came from Asia across a land bridge, are still studied.
▸Dinosaurs include the theropod branch where Birds evolve from dinosaursWell established
The connection
Birds evolved within the theropods, from small feathered maniraptorans related to Velociraptor and Troodon. Step by step, their ancestors shrank and gained longer arms, feathered wings and bird-like breathing, until true fliers emerged in the Jurassic. Birds are therefore not just descended from dinosaurs; they are dinosaurs.
When and how
Jurassic, about 165–150 million years ago; key fossils include Anchiornis from China, about 160 million years old, and Archaeopteryx from Germany.
The evidence
Hundreds of shared skeletal features, feathered dinosaur fossils, and egg and growth studies; family-tree analyses consistently place birds inside theropods.
Debates and open questions
Which maniraptoran group is closest to birds, and how flight began, are debated.
▸Dinosaurs were ended, except birds, by K–Pg extinction: end of the non-bird dinosaursWell established
The connection
After more than 160 million years, every non-bird dinosaur, from long-necked sauropods to Triceratops and T. rex, vanished at the K–Pg boundary. Large bodies, big food needs and slow-growing young left them exposed when the impact cut off plant growth, while some small, seed-eating ground birds survived.
When and how
Worldwide, 66 million years ago; the best record of the final dinosaurs comes from the Hell Creek Formation of Montana and the Dakotas.
The evidence
Dinosaur fossils occur close below the boundary clay in Montana and elsewhere, and none above it, apart from a few disputed reworked bones.
Debates and open questions
Whether dinosaur diversity was already falling in the last few million years is debated; recent studies suggest any apparent decline reflects gaps in the fossil record.
▸First mammals lived in the shadow of DinosaursStrong evidence
The connection
For more than 130 million years, most mammals stayed small, typically mouse- to cat-sized, while dinosaurs filled the large-animal roles. Even so, mammals spread into many niches as swimmers, gliders, diggers and climbers. A few were bigger: Repenomamus reached about a meter long, and one smaller species was found with a baby dinosaur in its stomach.
When and how
Worldwide, about 200–66 million years ago; many important finds come from China's Jurassic and Cretaceous rocks.
How sure are we?
Fossil mammal sizes stay small throughout the Mesozoic, then increase rapidly after the dinosaurs disappear.
Debates and open questions
Whether dinosaurs actively held mammals back, or other factors such as plants and climate limited them, is debated.