The phylogenetic position of Sangaia lavinae and the status of Rhytidosteidae (Stereospondyli: Trematosauria)

DIAS-DA-SILVA, SERGIO; MARSICANO, CLAUDIA

Austin, Texas

Congreso; 67th. Annual Meeting Society Vertebrate Paleontology; 2007

SVP

Until recently, rhytidosteids were unknown from South America, a remarkable circumstance as members of this group of temnospondyls are well known from the Lower Triassic of Gondwana and Laurasia.  The new rhytidosteid taxon, Sangaia lavinai, was exhumed from Lower Triassic levels of southern Brazil. As part of a comprehensive re-evaluation of rhytidosteids a new phylogenetic analysis of known representatives of this group and taxa related was performed.  The only previous analysis of the group resulted in a monophyletic Rhytidosteidae evethough the internal relationships of most included taxa remained unresolved. In the new analysis a data-matrix of 95 characters and 24 terminal taxa was evaluated and four most parsimonious trees (MPTs) were found. As a result ,the monophyly of Rhytidosteidae was reassessed and this clade appears as the sister-group of (Brachyopoidea + Laidleria). The sister-group realtionship between rhytidosteids and brachyopoids has been previously suggested in other stereospondyl phylogenies. Nevertheless, the position of the South African Laidleria as the sister-taxon of brachyopoids is unexpected as it has been previously related to a more basal position as the sister-taxon of (Rhytidosteidae + Brachyopoidea).  The Australian Nanolania results as the most basal rhytidosteid.  Regarding Sangaia, its original description claimed that it was a basal taxon close related to Indobrachyops than to any other rhytidosteid due to the presence, in both taxa, of a prominent lachrymal bone, a feature lost in more advanced rhytidosteids. Nevertheless in the new analysis, Sangaia is ally to  Mahavisaurus from Madagascar and they are the sister-group of a clade including all australian rhytidosteids plus Indobrachyops. This results suggest a closer relationship among the eastern Gondwanan rhytidosteids (Australian and Indian taxa) than to the western Gondwanan taxa (South America and Madagascar) which resulted as sister-taxa.