Abstract
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 include 24 species and span a narrow range of phenotypes, being mostly large-bodied (3–7 m) semi-aquatic 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 during the evolutionary history of Crocodylomorpha. Furthermore, the influence of abiotic and biotic factors on crocodylomorph body size evolution is currently poorly understood. In this study, we quantify patterns of body size disparity through the evolutionary history of crocodylomorphs. We use phylogenetic comparative methods to characterise the macroevolutionary patterns that gave rise to this disparity, and to quantitatively test hypotheses of abiotic (i.e., palaeotemperature and palaeolatitude) and intrinsic (habitat) factors as potential drivers.
Results Crocodylomorphs reached an early peak in body size disparity during the Late Jurassic, and underwent essentially continual decline since then. A multi-peak Ornstein-Uhlenbeck model outperforms all other evolutionary models fitted to our data (either single- or multi-regime models), indicating that the macroevolutionary dynamics of crocodylomorph body size evolution are better described within the concept of adaptive landscape, with most body size variation emerging after shifts to new adaptive zones (macroevolutionary regimes). 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, in several clades, with increases in average body size, though not without exceptions.
Conclusions On large phylogenetic scales, shifts between macroevolutionary regimes (representing lineage-specific innovations) are more important than isolated climatic factors. Shifts leading to larger body sizes are commonly associated with predominantly aquatic or semi-aquatic groups. This, combined with the extinction of smaller-sized regimes (particularly during the Late Cretaceous and Cenozoic), gave rise to the upward-shifted body size distribution of extant crocodylomorphs compared to their terrestrial ancestors.
Abbreviations
- Cranial measurements
- DCL
- dorsal cranial length
- ODCL
- orbito-cranial length
- Evolutionary models
- BM
- Brownian motion
- EB
- Early burst
- OU
- Ornstein-Uhlenbeck
- BMS
- multi-regime BM model that allows parameter σ2 to vary
- OUMV
- multi-regime OU model that allows θ and σ2 to vary
- OUMA
- multi-regime OU model in which θ and α can vary
- OUMVA
- OU model in which all three parameters (θ, α and σ2) can vary
- Model parameters
- θ
- trait optimum of OU-based models
- α
- attraction parameter of OU-based models;
- σ2
- Brownian variance or rate parameter of BM or OU-based models
- μ
- evolutionary trend parameter of BM-based models
- Z0
- estimated trait value at the root of the tree of OU-based models
- Optimality criteria
- AIC
- Akaike’s information criterion
- AICc
- Akaike’s information criterion for finite sample sizes
- BIC
- Bayesian information criterion
- pBIC
- phylogenetic Bayesian information criterion