| Description |
Crocodylomorph reptiles are an extremely successful clade, having repeatedly radiated into novel ecological roles over the past 230 million years, displayed a remarkable morphological disparity during the Mesozoic, and survived two mass extinctions. Their exceptional fossil record presents an opportunity to investigate macroevolutionary patterns in a single clade and understand the role that ecology plays in mass extinction response. To better reconstruct ecology, I utilized a combination of skull and limb material from extant amniotes. I first investigated the relationship between diet and skull shape in living mammals, lepidosaurs, and crocodylians using a combination of geometric and linear morphometrics. Following this, I used linear morphometrics to investigate the relationship between habitat and humerus, femur, and tibia shape for extant amniotes. I then combined both datasets with those of fossil crocodylomorphs to more precisely reconstruct dietary and locomotor ecology. Finally, I investigated if ecology played a role in the survival and diversification through mass extinctions. I find that skull shape is related to both phylogeny and feeding ecology. The three taxonomic groups plot in different regions of morphospace, with little overlap. In both mammals and lepidosaurs, there is overlap between dietary groups, but dietary generalists tend to occupy central regions of morphospace, in contrast to specialists (i.e., carnivores and herbivores). Limb bones of extant amniotes exhibit clear patterns, with cursorial and arboreal taxa possessing narrow, gracile bones, whereas semiaquatic specimens have robust elements. When fossil crocodylomorphs are included in skull and limb analyses, they cover a larger morphological range than their living relatives, suggesting a greater ecological range. Early crocodylomorphs appear to be terrestrial generalists, frequently falling into a morphological range not occupied by sampled extant amniotes. By the end of the Mesozoic, this range is no longer occupied, despite a wide ecological range. Fisher's exact tests show that dietary ecology did play a role, with ecological generalists preferentially surviving mass extinction events. Crocodylomorphs resilience to mass extinction events may be related to their repeated occupation of this role, and their failure to ecologically diversify following the end-Cretaceous mass extinction may be attributable to the loss of terrestrial generalists. iv |
