Crocodylomorph cranial shape evolution and its relationship with body size and ecology

• Published in: Journal of evolutionary biology Vol. 33; no. 1; pp. 4 - 21
• Main Author: Godoy, Pedro L.
• Format: Journal Article
• Language: English
• Published: Switzerland Blackwell Publishing Ltd 01-01-2020
• Subjects: adaptive landscape; Animals; Biodiversity; Biological Evolution; Body size; Body Size - physiology; Cenozoic; cranial shape; Cretaceous; Crocodylomorpha; Ecological effects; Ecological monitoring; Ecology; Ecosystem; Evolution; Fossils; Fossils - anatomy & histology; geometric morphometrics; Habitat loss; Jurassic; Lifestyles; macroevolutionary patters; Mathematical morphology; Morphometry; phylogenetic comparative methods; Phylogeny; Reptiles - anatomy & histology; Reptiles - classification; Skull; Skull - anatomy & histology; Terrestrial environments; ecology; adaptive landscape; macroevolutionary patters; body size; phylogenetic comparative methods; cranial shape; geometric morphometrics
• ISSN: 1010-061X; 1420-9101
• DOI: 10.1111/jeb.13540

Crocodylomorpha, which includes living crocodylians and their extinct relatives, has a rich fossil record, extending back for more than 200 million years. Unlike modern semi‐aquatic crocodylians, extinct crocodylomorphs exhibited more varied lifestyles, ranging from marine to fully terrestrial forms. This ecological diversity was mirrored by a remarkable morphological disparity, particularly in terms of cranial morphology, which seems to be closely associated with ecological roles in the group. Here, I use geometric morphometrics to comprehensively investigate cranial shape variation and disparity in Crocodylomorpha. I quantitatively assess the relationship between cranial shape and ecology (i.e. terrestrial, aquatic, and semi‐aquatic lifestyles), as well as possible allometric shape changes. I also characterize patterns of cranial shape evolution and identify regime shifts. I found a strong link between shape and size, and a significant influence of ecology on the observed shape variation. Terrestrial taxa, particularly notosuchians, have significantly higher disparity, and shifts to more longirostrine regimes are associated with large‐bodied aquatic or semi‐aquatic species. This demonstrates an intricate relationship between cranial shape, body size and lifestyle in crocodylomorph evolutionary history. Additionally, disparity‐through‐time analyses were highly sensitive to different phylogenetic hypotheses, suggesting the description of overall patterns among distinct trees. For crocodylomorphs, most results agree in an early peak during the Early Jurassic and another in the middle of the Cretaceous, followed by nearly continuous decline until today. Since only crown‐group members survived through the Cenozoic, this decrease in disparity was likely the result of habitat loss, which narrowed down the range of crocodylomorph lifestyles.
Crocodylomorph cranial shape is comprehensively quantified and its relationship with body size and ecology is investigated. (a) Morphospace occupation of different crocodylomorph subgroups, (b) the strong relationship between size and shape, demonstrating allometric shape changes in crocodylomorph skulls, and (c) morphospace occupation of crocodylomorphs subdivided into three ecological categories: terrestrial, semi‐aquatic and aquatic species.