Birds as well as the extinct dinosaurs, pterosaurs (flying contemporaries of the dinosaurs) and the ‘thecodontians’, a ragbag group that includes the ancestors of all the other archosaurs.
The archosaurs arose some 250 million years ago, as far as we can tell. The first group, the proterosuchids, spread nearly worldwide. Their fossils are known from the Soviet Union, southern Africa, Antarctica, Australia, India, China and South America. They show the archosaurs, but in no other animals: an antorbital fenestra (a particular hole in the skull), recurved flat -sided teeth and a fourth trochanter on the femur (a specific ridge on the thigh bone).
During the Triassic period, some 245-208 million years ago, the archosaurs radiated (evolved and diversified) as moderately successful carnivores and gave rise to one herbivorous group. The Triassic ‘thecodontians’ split into two main lineages. One included the superficially crocodile-like phytosaurs, the herbivorous aetosaurs (which also looked rather like crocodiles, but had snub noses for rooting up plant food, and narrow leaf-like teeth) and the often massive, carnivorous rauisuchians. Finally, in the Late Triassic, this Lineage sprouted some lightweight bipedal (two-legged); animals that probably fed on insects and small lizard-like animals. These were, perhaps surprisingly, the first crocodilians. The group adopted its amphibious, quadrupedal (four-legged) fish-eating existing only some 20 million years later, after the extinction of the phytosaurs.
The second archosaur lineage included active carnivores such as Ornithosuchus, which could walk quadrupedally or bipedally, and the lightweight Lagosuchus, which was a biped. These animals are so close to being dinosaurs in many features, it now seems remarkable that many scientists had denied it until recently. Lagosuchus, in particular, shows a long list of ‘dinosaur’ characters: its bipedal posture; the long limbs with the shin bones (tibia and fibula) longer than the femur; the perforated acetabulum (the bowl-like depression in the hip bone that receives the ball-shaped end of the femur).
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