Triassic-Jurassic mass extinction as trigger for the Mesozoic radiation of crocodylomorphs

Abstract

Pseudosuchia, one of the two main clades of Archosauria (Reptilia: Diapsida), suffered a major decline in lineage diversity during the Triassic-Jurassic (TJ) mass extinction (approx. 201 Ma). Crocodylomorpha, including living crocodilians and their extinct relatives, is the only group of pseudosuchians that survived into the Jurassic. We reassess changes in pseudosuchian morphological diversity (disparity) across this time interval, using considerably larger sample sizes than in previous analyses. Our results show that metrics of pseudosuchian disparity did not change significantly across the TJ boundary, contrasting with previous work suggesting low pseudosuchian disparity in the Early Jurassic following the TJ mass extinction. However, a significant shift in morphospace occupation between Late Triassic and Early Jurassic taxa is recognized, suggesting that the TJ extinction of many pseudosuchian lineages was followed by a major and geologically rapid adaptive radiation of crocodylomorphs. This marks the onset of the spectacularly successful evolutionary history of crocodylomorphs in Jurassic and Cretaceous ecosystems.

Figure 1.

Two-dimensional morphospace for all 36 pseudosuchian taxa based on principal coordinate axes 1 and 2. Principal coordinate axes 1 and 2 represent cumulatively 26.68% of overall variance (PCo1, 16.69%; PCo2, 9.99%). Black circles, Late Triassic; black stars, Early Jurassic. Numbers for taxa are provided in the electronic supplementary material. Silhouettes taken from Wikipedia and www.phylopic.org.

Figure 2.

Morphological disparity for different subsets of the pseudosuchian dataset. Mean disparity values, based on the sum of ranges and sum of variances are shown with error bars representing 95% CIs, obtained with 1000 bootstrap replicates. Comparisons of mean disparity values are illustrated for: (a,b,c) Late Triassic and Early Jurassic (b, corrected to an equal sample size); (d,e,f) four time intervals (e, corrected to an equal sample size; abbreviations: LC, late Carnian; EN, early Norian; LN, late Norian; R, Rhaetian; H, Hettangian; S, Sinemurian; P, Pliensbachian; T, Toarcian); (g,h,i) Late Triassic and Early Jurassic crocodylomorphs (h corrected to equal sample size).