Comparative osteology and adaptations of the extinct spectacled cormorant ( <i>Urile perspicillatus</i> )

The spectacled cormorant (Urile perspicillatus) is a recently extinct species of Phalacrocoracidae that survived into historical times only on the Commander Islands in the Northern Pacific. The species had a broader Pleistocene distribution and an apparently shared pattern of demographic history with the Steller’s sea cow (Hydrodamalis gigas). The ecology and adaptations of the spectacled cormorant remain poorly understood. We report over 450 subfossil bones from Bering Island, including two incomplete skeletons—the largest collection for this species. Comparative osteology shows the spectacled cormorant was morphologically most similar to the extant Red-faced cormorant (U. urile) and Pelagic cormorant (U. pelagicus), but exhibited distinct cranial autapomorphies, particularly in skull morphology. We identified sexual size dimorphism in Urile perspicillatus comparable to that observed in the great cormorant (Phalacrocorax carbo). A detailed analysis of the pectoral girdle and limb bone proportions of U. perspicillatus indicates that this species does not differ substantially from other volant phalacrocoracids. Hindlimb morphology indicates efficient underwater locomotion, suggesting bottom-feeding ecology similar to other Urile species, but its larger size enabled deeper diving. The sympatry of three closely related cormorants with similar feeding ecology suggests local adaptive radiation in a benthic resource-rich ecosystem.