The ancestral crocodilian Protosuchus. Bulletin of the AMNH ; v. 97, article 3

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Date
1951
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Publisher
New York : [American Museum of Natural History]
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Abstract
"In this paper there are presented a description in detail and a discussion of Protosuchus richardsoni, from the Dinosaur Canyon beds of northern Arizona. It is shown that the Dinosaur Canyon beds are intermediate in position between the typical Upper Triassic Chinle formation below, and the supposed Lower Jurassic sediments above. Consequently, the relationships of the Dinosaur Canyon beds and its contained fossils to the Triassic below or the Jurassic above are a question open to some differences of opinion. Protosuchus is definitely a primitive crocodilian. It is small as crocodilians go, being about 1 meter in length. The skull is rather short and broad and quite crocodilian in many of its features. For instance, the skull roof shows a characteristic crocodilian arrangement of bones, with the squamosal, the parietals, and the frontals enlarged, and with the supratemporal fenestrae reduced in size. The top of the cranial region is quite flat, as is typical of many crocodilians. There is no antorbital fenestra, which is a character diagnostic for the crocodilians. In some respects, however, Protosuchus shows either primitive characters or special features in the skull. The orbit is directed more laterally than in most crocodilians. The snout is short, yet in the front part of the dental series there is a long diastema between the last premaxillary tooth, which is enlarged, and the first maxillary tooth, a most peculiar specialization for a crocodilian. There seemingly are 24 presacral vertebrae in Protosuchus, a number similar to that for the crocodilians, while there are two sacrals and about 35 caudals. The vertebrae are platycoelous, and in the dorsolumbar and caudal regions they have somewhat elongated centra, while the spines are closely comparable in relative height and strength to the vertebral spines in other crocodilians. Protosuchus shows a definite advance towards the crocodilians in the extension of the transverse processes, to which the ribs are articulated by both head and tubercle in the region behind the neck. The shoulder girdle is strongly crocodilian in that the coracoid is greatly elongated to such a degree that it is almost equal to the scapula in size. The bones of the leg are similar to those of the Crocodilia, and the radiale and ulnare in the carpus are elongated fully as much as they are in typical crocodilians of Mesozoic, Cenozoic, and Recent times. The pelvis is characterized by the elongated, rod-like pubis, a crocodilian character, but proximally this bone participates in the acetabulum, whereas in later crocodilians it is completely excluded from the acetabulum. The ischium is large, and the two ischia are joined by an extensive symphysis. The ilium is elongated anteriorly to form a strong process above and in front of the acetabulum. As in the fore limb, the bones of the hind limb are very much like the same bones in other crocodilians. The astragalus and calcancum on the tarsus are enlarged, and the latter bone is characterized by its large, expanded tuber. The fifth digit is reduced to a short, metatarsal hook, a character that is inherited by the crocodilians from their thecodont ancestors. The body armor is heavy. On the back there is a double row of rectangular scutes running along the midline, while the belly and the ventral and lateral surfaces of the tail are protected by smaller rectangular scutes. This array of characters indicates the strong crocodilian habitus of Protosuchus, for it is evident that in most respects this reptile is a true crocodilian. It does retain certain thecodont heritage characters, but these are quite secondary in importance as compared with the crocodilian features that typify the genus. Such resemblances as are apparent between Protosuchus and the thecodonts, especially genera like Aetosaurus and Stegomus, are clearly due to the persistence in Protosuchus of basic thecodont heritage characters or to parallelism in evolution. On the other hand the resemblances between Protosuchus and the South African genera Notochampsa and Erythrochampsa are close and are based on the comparisons of crocodilian habitus characters. Therefore, Protosuchus, Notochampsa, and Erythrochampsa are without much doubt closely related, and are here considered as members of a single family and suborder of ancestral crocodilians"--P. 181.
Description
p. 147-182, [10] p. of plates (2 folded) : ill., map ; 27 cm.
Includes bibliographical references (p. 182).
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