Biogeography of Triassic tetrapods: evidence for provincialism and driven sympatric cladogenesis in the early evolution of modern tetrapod lineages

Abstract

Triassic tetrapods are of key importance in understanding their evolutionary history, because several tetrapod clades, including most of their modern lineages, first appeared or experienced their initial evolutionary radiation during this Period. In order to test previous palaeobiogeographical hypotheses of Triassic tetrapod faunas, tree reconciliation analyses (TRA) were performed with the aim of recovering biogeographical patterns based on phylogenetic signals provided by a composite tree of Middle and Late Triassic tetrapods. The TRA found significant evidence for the presence of different palaeobiogeographical patterns during the analysed time spans. First, a Pangaean distribution is observed during the Middle Triassic, in which several cosmopolitan tetrapod groups are found. During the early Late Triassic a strongly palaeolatitudinally influenced pattern is recovered, with some tetrapod lineages restricted to palaeolatitudinal belts. During the latest Triassic, Gondwanan territories were more closely related to each other than to Laurasian ones, with a distinct tetrapod fauna at low palaeolatitudes. Finally, more than 75 per cent of the cladogenetic events recorded in the tetrapod phylogeny occurred as sympatric splits or within-area vicariance, indicating that evolutionary processes at the regional level were the main drivers in the radiation of Middle and Late Triassic tetrapods and the early evolution of several modern tetrapod lineages.

Figure 1.

Composite trees of Middle (grey) and Late Triassic (black) tetrapods employed in the TRA analyses. The geographical procedence and reptile age (Late Triassic) are detailed in each terminal. For abbreviations see the electronic supplementary material, S1.

Figure 2.

Recovered cladograms of optimized geographical areas (left), events reconstructed by the TRA (center) and histograms depicting the frequency of the randomized trees (right) for the (a) Middle Triassic, (b) Ischigualastian and (c) Coloradian time slices. In the events reconstructed by the TRA (center) the dark grey areas represent the actual number of recovered events and the light green areas the ratio of biogeographical/cladogenetic events multiplied by 100. The arrows in the histograms represent the number of codivergences recovered for this time slice. Abbreviations: DI, ‘dispersals’ (taxon shift, probably owing to sampling bias in the fossil record); EX, extinctions; Ischi, Ischigualasto; SS, sympatric splits; VI, vicariances.

Figure 3.

Palaeogeographical reconstructions of the (a) Middle Triassic and (b) Late Triassic indicating the geographical areas analysed here. (c) OACs calibrated to the palaeolatitude ((a,b) redrawn from Blakey 2006) and PltCIs. Abbreviations as in figure 2.