A cladistic biogeographic analysis of Middle and Late Triassic continental tetrapods: evidence for changing paleobiogeographic patterns and driven sympatric cladogenesis

AGNOLIN, F.L., EZCURRA, M.D., PAIS, D.F.

Jornada; XXV JORNADAS ARGENTINAS DE PALEONTOLOGÍA DE VERTEBRADOS; 2010

Universidad Nacional de San Luis

Triassic tetrapods are of key importance in understanding the evolution of the group, because most modern tetrapod cladesfirst appeared or experienced their initial radiation during this period. In order to test previous paleobiogeographic hypothesesof Triassic tetrapod faunas (e.g. homogeneous and cosmopolitan faunas), tree reconciliation analyses (TRA) were performedwith the aim of recovering biogeographic patterns based on phylogenetic signals provided by a composite tree of209 Middle and Late Triassic continental tetrapods. TRA found significant evidence (p<0.05) for the presence of different paleobiogeographicpatterns during the three time slices analyzed. A Pangean distribution is observed during the MiddleTriassic, in which several cosmopolitan tetrapod groups are found. During the early Late Triassic (Ischigualastian) a stronglypalaeolatitudinally influenced pattern is recovered, with some tetrapod lineages restricted to palaeolatitudinal belts.Optimization of paleolatitude as a continuous character onto the optimal area cladogram revealed a perfect fitting of theIschigualastian paleolatitudinal signal (CI=1.000). During the latest Triassic (Coloradian) Gondwanan territories were moreclosely related to each other than to Laurasian ones, with a distinct tetrapod fauna at low paleolatitudes. Although the paleogeographyduring this time span seems to have allowed biotic interchanges between distant regions, the number ofcladogenetic events resulting in daughter lineages spanning different areas is not very high. Conversely, more than 75% ofthe cladogenetic events recorded in tetrapod phylogeny occurred as sympatric splits or within-area vicariance, indicatingthat evolutionary processes at the regional level were the main drivers in the radiation of Middle and Late Triassic tetrapods.