Chigutisaurid amphibians from the Upper Triassic of Argentina and their phylogenetic relationships

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. tenaxandP. 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. SE V E R A L temnospondyl amphibians have been described from the Upper Triassic of Argentina in the past 50 years (Cabrera 1944; Rusconi 1948 a, 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 limensisRusconi, 1948 a (Lydekkerinidae);Icanosaurus rectifronsRusconi, 1951 (Trematosauridae); Promastodonsaurus bellmanniBonaparte, 1963 (Capitosauridae); and Chigutisaurus tunuyanensis Rusconi, 1948 b, Chigutisaurus tenaxRusconi, 1949,Chigutisaurus cacheutensis Rusconi, 1953 andPelorocephalus ischigualastensis Bonaparte, 1975, all included in Rusconi’s Chigutisauridae. Recently, most of this temnospondyl material was restudied and its taxonomic status re-evaluated (Marsicano 1993 a, 1 93b, 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 reevaluation of the phylogenetic relationships of chigutisaurids (Warren and Hutchinson 1993; Sengupta 1995; Damiani and Warren 1996). Institutional abbreviations . MCNA, Museo de Ciencias Naturales y Antropolo ́gicas, Mendoza; MLP, Museo de Ciencias Naturales de La Plata, Buenos Aires; PVL, Paleontologı ́a de Vertebrados Instituto Miguel Lillo, Tucuma ́n. [Palaeontology, Vol. 42, Part 3, 1999, pp. 545–565] q The Palaeontological Association S Y S T E M A T I C P A L A E O N T O L O G Y ‘TEMNOSPONDYLI’ sensuMilner, 1993 Family CHIGUTISAURIDAE Rusconi, 1949 GenusPELOROCEPHALUSCabrera, 1944 Type species. Pelorocephalus mendozensis Cabrera, 1944. Diagnosis. Chigutisaurids with a raised longitudinal keel along the ventral surface of the cultriform process of the parasphenoid, a condition not present in other members of the family. In the mandible, there is a well-developed anterior Meckelian fenestra on the suture between splenial and postsplenial, a condition considered derived in relation to the location of the fenestra wholly within the postsplenial, as occurs in brachyopids. A contact between the surangular and prearticular excludes the articular from the postglenoid area (PGA); a prearticular exposure on the PGA is consider plesiomorphic as it is present in brachyopids and also in the basal chigutisaurid Keratobrachyops . The chordatympanic foramen is located lingually on the suture between the angular and the prearticular; thus the angular is well exposed on the lingual surface of the mandible, in contrast with other members of Chigutisauridae in which the chordatympanic foramen is located wholly within the prearticular and the angular is not well exposed on the lingual surface of the mandible. Pelorocephalus mendozensis Cabrera, 1944