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.