Pelorocephalus

Abstract: A revision of the temnospondyl chigutisaurids from the Upper Triassic of Argentina is presented. The validity of the taxa originally proposed is discussed in the light of new material. The genus Pelorocephalus (Chigutisauridae) is diagnosed by the presence of a longitudinally keeled cultriform process, occurrence of the anterior Meckelian fenestra on the suture between splenial and postsplenial, location of the chordatympanic foramen on the contact between angular and prearticular, and exposure of the angular on the lingual mandibular surface. Three species, Pelorocephalus mendozensis , P. tenax and P. cacheutensis, are distinguished mainly by differences in the structure of the occiput. Phylogenetic relationships among all known representatives of this family, including taxa from India and Australia, are discussed. S EVE R AL temnospondyl amphibians have been described from the Upper Triassic of Argentina in the past 50 years (Cabrera 1944; Rusconi 1948a, 1948b, 1949, 1951, 1953; Bonaparte 1963, 1975). The validity of the different nominal taxa and the families represented has been discussed by various authors (Reig 1961, 1962; Welles and Estes 1969; Bonaparte 1970, 1973, 1978), but without a critical analysis of the specimens involved. Originally, the taxa described, and the families to which they were assigned, were as follows: Pelorocephalus mendozensis Cabrera, 1944 (Brachyopidae); Otuminisaurus limensis Rusconi, 1948a (Lydekkerinidae); Icanosaurus rectifrons Rusconi, 1951 (Trematosauridae); Promastodonsaurus bell- manni Bonaparte, 1963 (Capitosauridae); and Chigutisaurus tunuyanensisRusconi, 1948b, Chigutisaurus tenax Rusconi, 1949, Chigutisaurus cacheutensis Rusconi, 1953 and Pelorocephalus ischigualastensis Bonaparte, 1975, all included in Rusconi's Chigutisauridae. Recently, most of this temnospondyl material was restudied and its taxonomic status re-evaluated (Marsicano 1993a, 1993b, 1994, 1996). As a result, only two temnospondyl families are considered to be represented in the Upper Triassic of southern South America, Chigutisauridae and Capitosauridae, with the chigutisaurids, at present, the most important amphibian component of the tetrapod faunas found in that region. In the last few years, chigutisaurids have also been recognized in other Gondwanan areas, including Australia and India. The Australian chigutisaurid record includes material from the Early Triassic Arcadia Formation (Warren 1981; Damiani and Warren 1996), the Early Jurassic Evergreen Formation (Warren and Hutchinson 1983) and the Early Cretaceous Strzelecki Formation (Warren et al. 1997). Indian chigutisaurids are known only from the Late Triassic Maleri Formation (Sengupta 1995). This restricted spatial distribution of chigutisaurids contrasts with their long temporal range (Scythian (Early Triassic) to Aptian (Early Cretaceous)), apparently the longest among temnospondyl families. In this paper, a redescription of the type specimens of Rusconi's chigutisaurids is presented and their taxonomic assignment is discussed in the light of new material. This new information permits a re- evaluation of the phylogenetic relationships of chigutisaurids (Warren and Hutchinson 1993; Sengupta 1995; Damiani and Warren 1996).