FOSSIL VERTEBRATES/Reptiles Other Than Dinosaurs 485 may support the existence of a mass extinction event at that time Perhaps the most spectacular feature of the archosaurian skull is the presence of a new opening in the skull in front of the orbit, the antorbital fenestra This was probably just a pneumatic structure, as in modern birds, making the skull lighter, but this feature readily distinguishes these forms from other diapsids With the exception of a few other basal members of this large group, like Proterosuchus and Euparkeria, all other archosauromorphs belong to either the Crurotarsi, which includes crocodiles and their fossil relatives, and the Ornithodira, which includes flying reptiles, dinosaurs, and their closest relatives, and birds The Crurotarsi include some very spectacular relatives of crocodiles, but the earliest crocodile-like forms belong to the Sphenosuchidae Sphenosuchids are known primarily from the Upper Triassic of Europe, North and South America, and the Lower Jurassic of South Africa In contrast to true crocodiles, Sphenosuchids were relatively lightly built and had an erect, bipedal posture True crocodiles (crocodyliforms) first appeared in the Upper Triassic They are divided into three major groups, each representing an important adaptive radiation These three groups are the Protosuchia, Mesosuchia, and Eusuchia The Protosuchia and Mesosuchia are probably paraphyletic, so they are not real taxa The skull of true crocodiles is large, massively built, and low The skull roof is flat and has heavy pitting on the surface The antorbital fenestra is small in primitive crocodiles and absent in living crocodiles Living crocodiles have a long secondary palate to separate the passage of air from the mouth, as in mammals Palaeozoic Synapsids, the Mammal-Like Reptiles No evolutionary history of reptiles is complete without some consideration of the so-called mammal-like reptiles, the Palaeozoic synapsids Palaeozoic synapsids (also called pelycosaurs and early therapsids) occupy a central position in early amniote evolution Palaeozoic synapsids include some of the oldest known amniotes, and their fossil remains record the earliest successful adaptations of terrestrial vertebrates to herbivorous and active predatory modes of life During the Palaeozoic, synapsids diversified greatly and became the most conspicuous terrestrial vertebrates of their time Although both pelycosaurs and early therapsids are paraphyletic taxa at the base of the clade that includes advanced therapsids and mammals, these Palaeozoic reptiles have played a pivotal role in considerations of synapsid evolution, including the origin of mammals The oldest known mammal-like reptiles have been found in sediments of Early and Middle Pennsylvanian (Late Carboniferous) age in Nova Scotia The remains of Protoclepsydrops haplous, retrieved from upright Sigillaria stumps from Joggins, Nova Scotia are too fragmentary to provide any useful information; even the identification of these remains as a pelycosaur has been questioned By the Late Pennsylvanian, pelycosaurs become the most common amniotes in Laurasian sediments The fossil record of this group is most extensive during the Early Permian, representing nearly 70% of all amniote finds of this time; by the Late Permian, however, pelycosaurs disappear from the fossil record Instead, the early therapsids become the most diverse and common amniotes Only during the Triassic true reptiles become the most diverse and common amniotes As primitive synapsids, pelycosaurs are characterized by the presence of an anteriorly inclined platelike occiput, and a lateral temporal fenestra that is bordered by jugal, squamosal, and postorbital bones This is in contrast to the condition in diapsids, where the lateral temporal fenestra is bordered by a fourth bone, the quadratojugal primitively, and the occiput has large postemporal fenestrae Recent phylogenetic studies of pelycosaurs have proposed that within this group, six major clades (families) can be recognized: small insectivores (Eothyrididae), two different types of large, bulky herbivores (Caseidae and Edaphosauridae), and three different types of medium to largesized carnivores (Varanopidae, Ophiacodontidae, and Sphenacodontidae) Eothyrididae This family was erected as a provisional group for the small pelycosaur Eothyris, known from a single skull from the Lower Permian of Texas, and fragmentary remains of three large pelycosaurs Other fossils were subsequently placed within this family, but at present only two genera, Eothyris, from the Early Permian of Texas, and Oedaleops, from the Early Permian of New Mexico, can be assigned with confidence into the Eothyrididae Eothyris is known from a single skull, whereas Oedaleops is represented by three partial skulls and some limb elements These specimens, although fragmentary, have revealed that eothyridids were small carnivorous amniotes Caseidae The casseids, closely related to the eothyridids, were a widespread and diverse group of herbivorous pelycosaurs, with a fossil record that extended from the Early Permian of North America and western Europe