Initial ecomorphological radiation of Eosauroptergyia

The Triassic biotic recovery following the Permian–Triassic boundary mass extinction is marked by a rapid radiation of reptiles secondarily adapted to marine environments. The initial radiation of Eosauropterygia, the most speciose clade, is a key part of that rise to dominance of marine reptiles. Recent studies on Mesozoic marine reptile disparity highlighted that eosauropterygians had greatest morphological diversity during the Middle Triassic, with the co-occurrence of pachypleurosauroids, nothosauroids and pistosauroids, mostly along the margins of the Tethys Ocean. However, these previous studies quantitatively analysed the disparity of Eosauropterygia as a whole without focussing on Triassic taxa. As a result, our understanding of the diversification of Middle Triassic eosauropterygians along the Tethys and the pattern of their morphospace occupation is highly limited. To tackle this issue, we reinvestigated and quantified morphological diversification of long-bodied Triassic eosauropterygians. We erected 31 biomechanically informative traits on teeth, mandible, limbs, and axial skeleton for 185 specimens spread over 36 species belonging to Pachypleurosauroidea, Nothosauroidea and Pistosauroidea. We submitted our trait data to ordination methods to recreate the evolution of morphospace occupation by Triassic eosauropterygians. Our multivariate morphometric analyses highlight clear ecomorphological distinction between the three clades, with no evidence of important whole-body convergent evolution. This pattern is mostly due to craniodental differences and strongly linked to feeding specializations. We also highlight a clear regional disparity pattern evolution differing between nothosauroids and pachypleurosauroids of which the latter likely experienced a remarkable diversification in the eastern Tethys during the Pelsonian.