Early croc-like reptiles
A reconstruction of Proterosuchus fergusi.
It is not clear if proterosuchids are a natural group — if all of the members are more closely related to each other than to other archosaurs — or not. It is possible that they are a sort of wastebasket for large primitive archosauriforms, a collection of animals that are classified together more for convenience than because there is strong evidence that they are all related. Nevertheless, proterosuchids are the first animals on the line to birds and crocs that have all of the features traditionally associated with archosaurs: a pair of holes in the skull between the eyes and the nostrils called antorbital fenestrae that held air sacs similar to our sinuses; a hole in each lower jaw called a mandibular fenestra that contained jaw muscles; teeth that are flattened from side to side and serrated, like a steak knife; and certain features of the braincase. Because they have all of these features, proterosuchids give us a picture of what the earliest archosauriforms were like.
Text by Matt Wedel, 5/2007; Proterosuchus image in the public domain