Investigating patterns of crocodyliform cranial disparity through the Mesozoic and Cenozoic

Crocodyliforms are traditionally considered a morphologically conservative group, retaining a similar body plan and small range of cranial morphologies throughout their evolutionary history. This qualitative assessment ignores many extinct highly divergent groups. Here the author employs geometric morphometric methods to characterize the crania of 131 extant and extinct crocodyliforms and track changes in disparity and morphospace occupation through time (Early Jurassic-Recent). The data were phylogenetically corrected using a novel method based on squared change parsimony. Cranial disparity peaked in the Late Cretaceous followed by a dramatic decline into the earliest Paleogene. This decline matches in neither timing nor magnitude with changes in diversity, and is not directly related to the end-Cretaceous mass extinction. The decline was partly driven by the evacuation of the region of morphospace exemplified by a short, narrow snout - a morphotype never explored by crown-group crocodylians. The Cretaceous peak in range-based disparity metrics is largely driven by the radiation of the bizarre notosuchians and their exclusion from the data set brings Cretaceous disparity closer in-line with Jurassic and Cenozoic levels. However, the peak remains in variance-based metrics, suggesting the high average dissimilarity between forms during this time period is not driven by this clade alone. Modern crocodylian disparity is low relative to Cretaceous levels, but is similar to Jurassic levels.