Archosauria: Fossil Record

Students of archosaur evolution are blessed with a wonderful fossil record for many groups of archosaurs, including some very bizarre extinct taxa. The first archosauromorphs (relatives of the true archosaurs) appear in the fossil record in the Early Triassic; about 245 million years ago, just after the great end-Permian extinction. They include weird hippo-size beaked herbivores called rhynchosaurs, long-necked reptiles called prolacertiforms, evil-looking terrestrial predators like the erythrosuchians and proterosuchians, and close relatives of the true archosaurs, including Euparkeria. Many of these early groups are limited to the Triassic period, not enduring the extinctions in the Late Triassic that the dinosaurs and other taxa survived.

The Late Triassic (about 225 million years ago) witnessed the appearance of several new groups of archosaurs, some of which have living descendants today. Most other archosaurs and archosauromorphs did not make it into the Jurassic, but the crocodilian lineage did, as did the dinosaurs and the tenacious champsosaurs, which all appeared at about the same time in the Late Triassic. Close relatives of the dinosaurs such as the pterosaurs and the possibly dinosaurian Eoraptor and Herrerasaurus also show up in the Late Triassic, too. There have been reports of a putative bird in the Late Triassic, but these seem doubtful and await closer inspection by scientists.

The survivors of the Late Triassic-Early Jurassic extinctions went on to dominate the rest of the Mesozoic Era, but not without hitches. The Late Cretaceous period (around 70 million years ago) was a time of global change and "ecological reshuffling," when many taxa did not make the cut. Pterosaurs, many dinosaurs (including some major bird groups), and some crocodilians vanished around this time.

As the Cenozoic Era dawned 65 million years ago, things were different — crocodiles and champsosaurs were still doing well, but the dinosaurs had taken a one-two punch, and only the neornithine birds persevered to continue the massive diversification that began in the Cretaceous period. Today we are left with only the crocodilians and the birds as extant (living) archosaurs.