The macroevolution and macroecology of Mesozoic lepidosaurs

Abstract

I studied with the aid of geometric morphometrics and novel phylogenetic comparative methods patterns of macroevolution in Mesozoic lepidosaurs in order to answer some of the intriguing aspects of their early evolution. The results presented here show that: A) The Late Triassic rhynchocephalian Clevosaurus latidens was not related to the genus Clevosaurus as was previously suggested, and indeed it represents a new genus of an early diverging opisthodontian renamed here as Fraserosphenodon latidens. Additionally, parsimony and Bayesian phylogenetic analysis recovered very similar topologies for the phylogeny of Rhynchocephalia, allowing to formally name two higher clades of derived rhynchocephalians: Eusphenodontia and Neosphenodontia. B) Geometric morphometric analysis of rhynchocephalians confirms the high morphological disparity of the group, while evolutionary rates analysis suggests that rhynchocephalian evolution was driven by heterogenous rates. Both, evolutionary rates analysis and geometric morphometrics shows that the modern “Tuatara” has rather low rates and is morphologically average if compared with other rhynchocephalians, which suggests that it is a morphologically conservative species. C) Dental disparity, body size analysis and geometric morphometrics of Mesozoic squamates suggest that small body size, low diversity and low dental disparity seem to have been the ancestral state of the earliest squamates. However, changes in the biota during the Cretaceous Terrestrial Revolution in the Middle/Late Cretaceous triggered changes in the ecosystem that influenced ecological and morphological adaptations in squamates that resulted in their radiation at the end of the Cretaceous. D) Early lepidosaur evolution was driven by heterogeneous rates; nevertheless, when comparing evolutionary rates of rhynchocephalians, squamates and all Lepidosauria, it is possible to appreciate that rhynchocephalian rates of body size evolution were outstandingly high and sustained over a long period of time, which suggests that rhynchocephalian decline may unexpectedly be linked to their high rates of evolution sustained over time. This fits with Simpson’s tachytelic evolution theory that suggests that a lineage with high evolutionary rates is prone to extinction. The thesis shows how modern computational methods can provide answers to long-running debates in comparative macroevolution.

Date of Award	1 Oct 2019
Original language	English
Awarding Institution	University of Bristol
Supervisor	Michael J Benton (Supervisor) & Tom Stubbs (Supervisor)