Terrible lizards that never vanished—they became birds
Dinosaurs dominated Mesozoic land after the Triassic-Jurassic extinction, yet only the avian lineage survived the end-Cretaceous catastrophe. Richard Owen coined dinosaur in 1842. Over 900 non-avian genera are known, while more than 11,000 living bird species carry the clade forward.
Dinosaurs first appeared early in the Triassic and took charge of life on land after the extinction 201.3 million years ago, staying dominant through the Jurassic and Cretaceous. Birds evolved from theropods in the Late Jurassic and are the only branch to come through the catastrophe of about 66 million years ago, so biologists split the group into avian dinosaurs and the extinct rest. Birds nest inside Maniraptora, within the coelurosaur theropods, within the saurischians. Remains of more than 1,000 non-avian species turn up on every continent, and the group had spread worldwide by the Early Jurassic at the latest.
Through most of the twentieth century scientists imagined sluggish, cold-blooded beasts, but research since the 1970s shows active animals with high metabolisms and complex social lives, all standing on upright legs like birds rather than sprawling like lizards. The ancestors walked on two legs; many later groups went on four, and some, such as Iguanodon and Anchisaurus, managed both. Horns, crests, armour and spines recur across the family, and although many Mesozoic species were human-sized or smaller, the biggest sauropods reached an estimated 39.7 metres long and 18 metres tall, the largest land animals ever.
Formally, Dinosauria is the latest common ancestor of Triceratops and modern birds plus all its descendants, which works out as Ornithischia plus Saurischia: armoured ankylosaurs, plated stegosaurs, frilled ceratopsians, thick-skulled pachycephalosaurs, duck-billed ornithopods, theropods and sauropodomorphs. In 2017 Matthew Baron, David Norman and Paul Barrett argued that ornithischians sit closer to theropods than to sauropodomorphs, and redefined the group using Triceratops, the house sparrow and Diplodocus so that sauropods stayed inside. Sterling Nesbitt identified twelve unambiguous skeletal hallmarks, among them a forearm bone shorter than 80% of the upper arm bone.
Pterosaurs, mosasaurs, ichthyosaurs, plesiosaurs and Dimetrodon are often lumped in, yet none held their hind limbs upright beneath the body, and Dimetrodon is a synapsid rather than a member of the reptile-and-bird clade. Owen built the name from Greek words for terrible and lizard, intending to evoke size and majesty as much as teeth and claws. Mounted skeletons in museums and films such as Jurassic Park have kept public enthusiasm high enough to bring the science substantial funding.
Source: Dinosaur
