Scaphokogia huachafo: Peruvian pygmy sperm whale skull shows the most asymmetric face among cetaceans

Aldo Benites-Palomino and ten colleagues described Scaphokogia huachafo sp. nov., a new species of pygmy sperm whale (Kogiidae) from the Late Miocene Pisco Formation of Peru, according to the paper at doi.org nature.comdoi.orgnature.comOpen source ↗. The holotype, catalogued MUSM 972, comes from Aguada de Lomas in the Sacaco basin, Arequipa, from rock laid down between 6.3 and 5.6 million years ago. The animal was about 2.7 m long.

The skull was found in the mid-1990s and stayed unnamed until now. Benites-Palomino examined it in 2015 while working on his bachelor's thesis at the Museo de Historia Natural in Lima. The team checked the bones for post-burial deformation: the sutures are fully closed, and the paper reports no deformation elsewhere on the skull.

Skull structure

Toothed whales already have uneven skulls, because the fatty sound-producing organs sit in the forehead. In S. huachafo the asymmetry extends beyond the external bony nares to the surrounding cranial structures. Both bony nostrils lie left of the midline, and the left one is much larger than the right. The right premaxilla is thick and heavy, and the two sides around the eyes differ in shape and reach.

The rostrum is robust, tubular and directed ventrally, at about 31 degrees downward. It is built of dense pachyostotic bone reinforced by ossified cartilage nature.comdoi.orgnature.comOpen source ↗.

Measurements and feeding

The authors placed 37 landmarks on the facial bones of 3D skull models and ran geometric morphometric and symmetry analyses. The sample held 31 taxa: 11 sperm whales, 4 early toothed whales and 16 members of the group that includes modern dolphins. S. huachafo lay at the extreme among pygmy sperm whales in a whole-face analysis and in a second one that corrected for family relationships.

The authors report that the genus Scaphokogia shows the most extreme facial asymmetry documented among cetaceans, and potentially among mammals. This claim rests on a single skull of this species.

Reconstruction of the atlanto-occipital joint in closely related congeners suggests greater ventral neck flexibility in Scaphokogia. The authors consider this consistent with feeding on or near the seafloor. They describe the cranial form as having no direct modern analogue and as pointing to ecological niches in the Late Miocene that are no longer occupied nature.comdoi.orgnature.comOpen source ↗.