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Morphometric analysis of cranial morphology in pinnipeds (Mammalia, Carnivora): disparity, dimorphism, ecology and ontogeny
 
SVPCA Conference
 
Platform presentation (20 minutes)
Authors
 
*Anjali Goswami
 
 
Katrina E Jones
 
Abstract
 
Pinnipeds (seals, sea lions and walruses) are fully aquatic carnivorans (Mammalia, Carnivora) that show a wide range of feeding and reproductive strategies and have a global distribution. This study examined pinniped cranial morphology and the factors that have influenced their evolution. 3D morphometric data from 138 specimens (20 genera) were collected using a digitizer and analysed with Procrustes analysis and Principal Components Analysis. Ecological correlates of shape, ontogeny and dimorphism were examined. Data from previous studies of terrestrial carnivorans were also included for comparison.
The three pinniped families occupy distinct areas of morphospace, despite significant ecological overlap. While most species within Phocidae (seals) and Otariidae (sea lions) cluster near the mean shape for their respective family, a few species in both families converge onto Odobenidae (walrus) space. These convergences can be separated into feeding (Erignathus, Hydrurga) and reproduction-related (Cystophora, Mirounga, Otaria) adaptations, demonstrating that different selection pressures can produce similar morphologies. Interestingly, taxa that converge onto Odobenidae space also display longer ontogenetic trajectories. The young for these taxa cluster near the mean shape for their respective families, showing that the cranial convergences develop primarily during the juvenile growth period. Dimorphism is apparent in some of these convergent species, and those (Cystophora, Otaria) showing the most extreme cranial shape dimorphism display less body size dimorphism, possibly due to larger female body size in polar waters. Pinniped cranial disparity is comparable to that of terrestrial carnivorans, despite representing far fewer species. This remarkable discordance between taxonomic and morphological diversity indicates that pinnipeds have undergone strong selective pressures on cranial morphology, probably due to adaptations to a marine lifestyle. We conclude that the extreme specialization of the pinniped postcranial skeleton for swimming has placed increased importance on the cranium for feeding and reproductive specializations, resulting in a wide range of cranial morphology.
London 2020