Body Size Evolution in Crocodylians and Their Extinct Relatives

Abstract

Crocodylians are currently facing evolutionary decline. This is evinced by the rich fossil record of their extinct relatives, crocodylomorphs, which show not only significantly higher levels of biodiversity in the past but also remarkable morphological disparity and higher ecological diversity. In terms of body size, crocodylians are mostly large animals (>2 m), especially when compared to other extant reptiles. In contrast, extinct crocodylomorphs exhibited a 10‐fold range in body sizes, with early terrestrial forms often quite small. Recent research has shed new light on the tempo and mode of crocodylomorph body size evolution, demonstrating a close relationship with ecology, in which physiological constraints contribute to the larger sizes of marine species. Abiotic environmental factors can also play an important role within individual subgroups. Crocodylians, for instance, have been experiencing an average size increase during Cenozoic, which seems to be related to a long‐term process of global cooling. Key Concepts Although Crocodylia is currently a depauperate group, the fossil record of its closest extinct relatives, crocodylomorphs, can provide important evidence to answer major evolutionary questions, such as on extinction and diversity loss. Crocodylomorph body size has varied significantly over time, as well as between subgroups, ranging from relatively small (<1 m) to gigantic (>10 m) species. Crocodylomorph body size evolution is not consistent with an overall trend towards large or smaller sizes through time; instead, multiple shifts to different evolutionary regimes can explain the observed body size values. Climate alone cannot explain the evolution of body size in all crocodylomorphs, but some environmental factors had stronger influence on individual subgroups. The usually larger sizes of aquatic and marine crocodylomorphs can be explained by physiological constraints associated with thermoregulation and lung capacity when under the water. A strong correlation between temperature and body size found for members of the crown‐group (Crocodylia) indicates that species became larger on average as the world became cooler during the Cenozoic.

Biotic and abiotic factors and the phylogenetic structure of extinction in the evolution of Tethysuchia

“…This niche partitioning is shown by the co-occurrences of multiple tethysuchians associated with diverse diets. Finally, unlike other studies (Godoy et al 2019;Godoy and Turner 2020;Lakin et al 2020), we found a positive correlation between body length (using the log-transformed SL as proxy) and temperature. These results may be explained by the difference in a preferential season of growth duration.…”

“…However, these analyses excluded PNC and used ordinary least squares and GLS instead of phylogenetic comparative methods. A significant relationship has been found for the crocodilian crown-group using the same methodology (Godoy et al 2019; Godoy and Turner 2020), suggesting that larger body sizes are associated with cooler climates. Similar results were found when only extant crocodylomorphs were analyzed (Lakin et al 2020).…”

“…With a longer growth season, they may have grown larger, thus explaining the results shown by the PGLS. However, extant crocodilians, which are also ectothermic, have been noted to follow the opposite pattern (Godoy and Turner 2020; Lakin et al 2020). Both groups share an overall similar thermophysiology; hence, this difference in body-size distribution cannot be explained by the thermometabolism.…”