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Archosaurs are a type of diapsid reptile characterized by teeth set in sockets, holes in the head and jaw to reduce skull weight, and a ridge for muscle attachments on the femur. They include dinosaurs, pterosaurs, crocodilians, and birds. Archosaurs first evolved in the Late Permian or Early Triassic era, and their rise may have been due to their ability to stand upright and bypass the Carrier constraint. Mammals and archosaurs evolved around the same time, but archosaurs prevailed, ushering in the age of dinosaurs. Today, mammals have become the dominant life form, but archosaurs still exist as birds.
Archosaurs, or “dominant lizards,” are an infraclass of reptiles, one of three infraclasses of diapsid, or “two-arched” reptiles. Diapsids are one of two classes of reptiles, the other being anapsids, which means “without bows.” The difference between diapsids and anapsids is that the latter have two holes on each side of the skull, near the temples, made to decrease the weight of the skull, while anapsids only have holes for the eyes. Anapsids are today represented by turtles, tortoises and tortoises, while diapsids make up all other reptiles, besides birds. It is not known whether living anapsids are descended from anapsid or diapsid ancestors.
Archosaurs, a type of diapsid, are the group most famous for having dinosaurs as members. Every dinosaur was an archosaur, from the leaf-eating Stegosaurus to the titanic Brachiosaurus and the deadly Tyrannosaurus rex. Pterosaurs, a type of flying reptile that dominated the skies before birds, were also archosaurs, as were living crocodilians (alligators, crocodiles, and gharials) and all birds (descendants of small dinosaurs that survived the KT extinction event). Archosaurs first evolved in the Late Permian (~255 million years ago) or Early Triassic (~250 million years ago), depending on whether one considers the earliest archosaur-like animals to be “archosauriforms” or real archosaurs.
Archosaurs are characterized by teeth set in sockets that make them less likely to be torn out when feeding, and antorbital fenestrae, or holes in the head, in front of the eyes, to reduce skull weight. They also have mandibular fenestrae, which are small holes in the jaw bone, also to decrease weight, and the fourth trochanter, a ridge for muscle attachments on the femur. Some scientists consider the last stretch one of the most important and coincidentally related to the rise of the dinosaurs and the ability of the first archosaurs to survive the most catastrophic mass extinction of all time, the Permian-Triassic extinction, 251 million Years ago .
Archosaurs and the first mammal-like reptiles (therapsids) evolved around the same time, the Permian era. Many believe it could have gone either way, with mammals or reptiles becoming the dominant life form on Earth, but for a variety of reasons, some of which may have been pure coincidence, the archosaurs prevailed, ushering in the 155 million years… long age of the dinosaurs. One reason may have been the femoral crest, which helped archosaurs stand more upright, bypassing the Carrier constraint, a rule that states that an animal with tentacled limbs cannot walk and breathe at the same time. However, some paleontologists are skeptical of this explanation because archosaurs were already ascendants when they had tentacled limbs.
Over the past 65 million years or so, mammals have become the dominant life form on Earth, supplanting archosaurs. However, they are still all around us, in the form of relatively non-threatening birds.
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