Early high disparity and rates in the evolution of ichthyosaurs (Reptilia: Diapsida)

Moon, B.C., Stubbs, T.L. 2019 Early high disparity and rates in the evolution of ichthyosaurs (Reptilia: Diapsida). Systematics Association Biennial Meeting (Bristol)

Abstract

Ichthyosaurs were a diverse clade of Mesozoic marine tetrapods; however, the evolution of the group has been understudied. Previous work shows an evolutionary bottleneck across the Triassic–Jurassic boundary, based only on a subset of genera. Here, we present a macroevolutionary analysis of ichthyosaurs using a recently published species‐level dataset. We use an established disparity work‐flow to explore evolutionary rates of discrete skeletal and continuous characters related to body size. Reduction of disparity across the Triassic–Jurassic boundary is less than previously thought, followed by a long‐term decrease. Post‐Triassic ichthyosaurs notably occupy different morphospace compared to Triassic ichthyosaurs, supporting a substantial turnover; however, resolution and preservation is insufficient to be certain on the timing and length of this. Magnitude of early high disparity is dependent on resolution but is accompanied by high initial rates of evolution in discrete characters and in body and skull size predominantly in the Early–Middle Triassic; these trends are agnostic of the time‐scaling used. Our results evidence rapid shifts in morphology associated with changes in ecology early in ichthyosaur evolution, followed by relative stasis as taxa specialize within their niches. They also build the framework for future investigations of the marine incursions of major tetrapod clades.