The genera of phytosaurs

Abstract

The long-snouted, crocodile-like, amphibious reptiles of the Late Triassic known as phytosaurs are assigned to 5 genera on the basis of cranial characters. The most primitive genus, Paleorhinus, has external nares well forward of the antorbital fenestrae, the posttemporal arch at level of the skull roof, a slender rostrum without crests, homodont dentition, a low quadrate, and no trace of a posterior squamosal process. Mystriosuchus, which has highly specialized posterior nares, tall quadrate, and depressed posttemporal bar, retains the primitive form of the squamosal, homodont dentition, and slender (much longer) rostrum of Paleorhinus from which it may be derived through Francosuchus (the European subgenus of Paleorhinus). The remaining phytosaur genera are characterized by rounded processes of the squamosal bone projecting well behind the occipital surface of the skull, and by the development of heterodont dentition and generally more robust skulls. Angistorhinus, the most primitive member of this lineage, retains a primitive posttemporal arch at the level of the skull deck, but is advanced over Paleorhinus in its posteriorly placed external nares. Its rostrum is slender and lacks crests; the dentition is only slightly differentiated. Phytosaurus and Rutiodon, the more specialized members of this group, have depressed posttemporal arches and tall quadrates. Rostra of Rutiodon vary from slender and uncrested in small skulls to moderately robust with high dorsal crests in the posterior half in the largest individuals; teeth vary from moderately to strongly heterodont. The squamosal processes are slender and show progressive phyletic elongation. The ratio of rostral to postrostral length varies inversely with size. Phytosaurus skulls are characterized by massive rostra with continuous dorsal crests and have a relatively constant rostral ratio independent of size; the squamosal processes are short and deep; dentition is strongly heterodont; considerable evidence suggests that the dorsal armor plates differed from other phytosaurs.