bioRxiv2018
Background Little is known about the long-term patterns of body size evolution in Crocodylomorpha, the > 200-million-year-old group that includes living crocodylians and their extinct relatives. Extant crocodylians are mostly large-bodied (3–7 m) predators. However, extinct crocodylomorphs exhibit a wider range of phenotypes, and many of the earliest taxa were much smaller (< 1.2 m). This suggests a pattern of size increase through time that could be caused by multi-lineage evolutionary trends of size increase or by selective extinction of small-bodied species. In this study, we characterise patterns of crocodylomorph body size evolution using a model fitting-approach (with cranial measurements serving as proxies). We also estimate body size disparity through time and quantitatively test hypotheses of biotic and abiotic factors as potential drivers of crocodylomorph body size evolution.Results Crocodylomorphs reached an early peak in body size disparity during the Late Jurassic, and underwent essentially continually decreases in disparity since then. A multi-peak Ornstein-Uhlenbeck model outperforms all other evolutionary models fitted to our data (including both uniform and non-uniform), indicating that the macroevolutionary dynamics of crocodylomorph body size are better described within the concept of an adaptive landscape, with most body size variation emerging after shifts to new macroevolutionary regimes (analogous to adaptive zones). We did not find support for a consistent evolutionary trend towards larger sizes among lineages (i.e., Cope’s rule), or strong correlations of body size with climate. Instead, the intermediate to large body sizes of some crocodylomorphs are better explained by group-specific adaptations. In particular, the evolution of a more aquatic lifestyle (especially marine) correlates with increases in average body size, though not without exceptions.Conclusions Shifts between macroevolutionary regimes provide a better explanation of crocodylomorph body size evolution than do climatic factors, suggesting a central role for lineage-specific adaptations rather than climatic forcing. Shifts leading to larger body sizes occurred in most aquatic and semi-aquatic groups. This, combined with extinctions of groups occupying smaller body size regimes (particularly during the Late Cretaceous and Cenozoic), gave rise to the upward-shifted body size distribution of extant crocodylomorphs compared to their smaller-bodied terrestrial ancestors.AbbreviationsCranial measurementsDCLdorsal cranial lengthODCLorbito-cranial lengthEvolutionary modelsBMBrownian motionEBEarly burstOUOrnstein-UhlenbeckBMSmulti-regime BM model that allows parameter σ2 to varyOUMVmulti-regime OU model that allows θ and σ2 to varyOUMAmulti-regime OU model in which θ and α can varyOUMVAOU model in which all three parameters (θ, α and σ2) can varyModel parametersθtrait optimum of OU-based modelsαattraction parameter of OU-based modelsσ2Brownian variance or rate parameter of BM or OU-based modelsμevolutionary trend parameter of BM-based modelsZ0estimated trait value at the root of the tree of OU-based modelsOptimality criteriaAICAkaike’s information criterionAICcAkaike’s information criterion for finite sample sizesBICBayesian information criterionpBICphylogenetic Bayesian information criterionView Full Text