Ecological opportunity and the rise and fall of crocodylomorph evolutionary innovation

Understanding the origin, expansion and loss of biodiversity is fundamental to evolutionary biology. The approximately 26 living species of crocodylomorphs (crocodiles, caimans, alligators and gharials) represent just a snapshot of the group’s rich 230-million-year history, whereas the fossil record reveals a hidden past of great diversity and innovation, including ocean and land-dwelling forms, herbivores, omnivores and apex predators. In this macroevolutionary study of skull and jaw shape disparity, we show that crocodylomorph ecomorphological variation peaked in the Cretaceous, before declining in the Cenozoic, and the rise and fall of disparity was associated with great heterogeneity in evolutionary rates. Taxonomically diverse and ecologically divergent Mesozoic crocodylomorphs, like marine thalattosuchians and terrestrial notosuchians, rapidly evolved novel skull and jaw morphologies to fill specialized adaptive zones. Disparity in semi-aquatic predatory crocodylians, the only living crocodylomorph representatives, accumulated steadily, and they evolved more slowly for most of the last 80 million years, but despite their conservatism there is no evidence for long-term evolutionary stagnation. These complex evolutionary dynamics reflect ecological opportunities, that were readily exploited by some Mesozoic crocodylomorphs but more limited in Cenozoic crocodylians.

Methods
Sampling. We sample 240 crocodylomorph skulls and 204 lower jaws 

Shape analyses. Disparity was quantified using 2-D geometric morphometrics, with a mixed landmark/semi-landmarks approach.

Phylogeny. A composite crocodylomorph supertree was assembled .

Disparity. Disparity was quantified using custom code and the R package dispRity.

Evolutionary rates. Rates of evolution were analysed in a Bayesian framework based on the multivariate variable-rates model in BayesTraits.