. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Early Archosaurian Evolution • Rei 271 earliest Jurassic of Arizona), commonly grouped in the crocodilian suborder Proto- suchia, have been regarded as transitional between the ancestral sphenosuchids and the later typical crocodiles (Mesosuchia, Sebecosuchia, Eusuchia). According to this conception, the assumption is made that the crocodiles evolved from primi- tively bipedal pseudosuchians, and that they returned to a quadrupedal gait as an adaptation to the amphibious way of life (for broader information on these i
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Early Archosaurian Evolution • Rei 271 earliest Jurassic of Arizona), commonly grouped in the crocodilian suborder Proto- suchia, have been regarded as transitional between the ancestral sphenosuchids and the later typical crocodiles (Mesosuchia, Sebecosuchia, Eusuchia). According to this conception, the assumption is made that the crocodiles evolved from primi- tively bipedal pseudosuchians, and that they returned to a quadrupedal gait as an adaptation to the amphibious way of life (for broader information on these ideas on crocodilian origins, see Haughton, 1924; von Huene, 1925; Colbert and Mook, 1951; Kalin, 1955). Recently, Sill (1967) has made a thorough reappraisal of the question, on the basis of the bearing of Proterochampsa upon crocodilian origins. Proterochampsa (Reig, 1959) (Fig. 11) is an obvious crocodile from the late Middle Triassic Iscbigualasto beds of Argentina, showing a remarkable combination of primitive, transitional, and advanced character-states. It is the earliest crocodile so far known, and it is definitely earlier than the spheno- suchids reported to be the pseudosuchian ancestors of the crocodiles. The crocodilian nature of Protero- champsa is evident from the morphology of the dorsal surface of the skull, the presence of a rudimentary secondary palate built up by the premaxilla and the maxilla, the sculptured bones of the roof of the skull, and the structure of the vertebral apophyses. Besides this, it is noteworthy that the anterior foot shows the typical carpal specializations of modern crocodiles: elongated radiale and ulnare carpal bones. This is demonstrated by a nearly complete anterior leg found in association with the remains of a coelurosaurian dinosaur in the Ischigualasto beds (Reig, 1963a).1 The femur and the humerus, known to the author through undescribed specimens as- sociated with skull remains, are also typi- cally crocodiloid. Unfortunately
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