Cretaceous-Cenozoic Evolution of the Crocodylia, and the Role of Environmental Change in Driving Diversity

Abstract

The Cretaceous-Paleogene (K-Pg) mass extinction, 66 Ma, was one of five major extinctions in Earth history. Crocodylomorpha, originating in the Late Triassic, were affected with only three lineages surviving into the Cenozoic. However, the severity of the mass extinction on crown crocodilians remains unexplored. The primary aim of this thesis is to examine the impact of the K-Pg event on crown crocodilians, and how environmental changes across the boundary influenced their diversity, disparity, and biogeographical spread. A case study is made of the phosphate deposits of Morocco which span the K-Pg boundary and multiple new crown crocodilians are described from the Paleocene-Ypresian beds. The first four new species described are diagnosed as members of Gavialoidea and Tomistominae, highly specialised slender-snouted crocodilians which range from the Cretaceous to the present day. The phylogenetic position of these groups within the crown group is debated. Therefore, both morphological and combined datasets were examined in a time-calibrated framework to examine how the conflict influences our understanding of macroevolutionary patterns across the K-Pg extinction. The morphology and size of the new material prompted additional investigations into disparity, using linear and geometric morphometrics. Results show a distinct peak in disparity in the aftermath of the K-Pg. The second set of specimens described are diagnosed as a new species of Alligatoroidea. This species represents the first diagnostic material of Alligatoroidea in Africa. Using a time-calibrated phylogenetic framework, the results from this study suggest a pattern of rapid biogeographic dispersal for alligatoroids following the K-Pg. The results presented in this thesis find that the K-Pg was a strong driver for macroevolutionary patterns amongst the crown crocodilians. A thorough understanding of patterns of survival and extinction of crocodilians will ultimately help us to more fully understand the modern biota and how global environmental changes threaten this group.

Date of Award	13 Feb 2019
Original language	English
Awarding Institution	University of Bath
Supervisor	Matthew Wills (Supervisor) & Nick Longrich (Supervisor)