Abstract :
[en] he Triassic biotic recovery following the end-Permian mass extinction is marked by a rapid radiation of reptiles secondarily adapted to marine environments. The diversification of Eosauropterygia, the most speciose clade of marine reptiles, is a key part of that rise to dominance of aquatic reptiles. Recent studies of eosauropterygian disparity through the Mesozoic highlighted that the greatest extent of morphological diversity was recorded during the Middle Triassic. This period indeed sees the co-occurrence of numerous species with various body-size ranges, diets, and swimming modes: pachypleurosaurs, nothosaurs and pistosaurs, mostly in the Tethys Ocean. However, these broad-brush studies have focussed their investigations on the shape of mandible and teeth, thus leaving out a significant portion of skeleton. As a result, our understanding of the disparity of Triassic eosauropterygians and how it has fluctuated between groups is limited. To tackle this issue, we reinvestigated and quantified morphological diversification of long-bodied Triassic eosauropterygians. We erected 32 biomechanically-informative traits on teeth, mandible, limbs, and axial skeleton for 142 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 analyses highlight clear ecomorphological differences between these three clades, with no evidence for important whole-body convergent evolution. This morphological distinction is however stronger in cranial rather than in postcranial anatomy: postcranial anatomy morphospace recovers nothosaurs and pistosaurs as distinct but pachypleurosaurs slightly overlaps with the two former groups. This suggests a decoupling in the morphological evolution of these two regions, similar to what has proposed for derived, short-necked plesiosaurians.
