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.