Extreme cranial asymmetry in extinct benthic feeding sperm whales

The fossil record provides critical insights into morphological changes beyond those observable in extant species, revealing evolutionary novelties. Among mammals, toothed cetaceans are notable for pronounced cranial asymmetry associated with sound-emitting forehead structures. Pygmy sperm whales (Kogiidae),display some of the most extreme cranial asymmetries known. Here, we describe Scaphokogia huachafo sp. nov., a new pygmy sperm whale species from the Late Miocene Pisco Formation, Peru. This species is characterised by extreme craniofacial asymmetry that extends beyond the external bony nares to the surrounding cranial structures. Geometric morphometric and symmetry analyses centred on S. huachafo indicate that this genus exhibits the most extreme facial asymmetry documented among cetaceans, and potentially among mammals. Distinct morphological traits include a robust, ventrally oriented, tubular rostrum composed of dense pachyostotic bone and reinforced by ossified cartilage. Morphological reconstruction of the atlanto-occipital joint in closely related congeners suggest enhanced ventral neck flexibility in Scaphokogia , which in turn is consistent with a benthic foraging behaviour. This extreme cranial morphology likely represents a unique adaptation with no direct modern analogues – one that greatly expands the known diversity of cetacean skull architecture. S. huachafo demonstrably occupied extinct ecological niches, revealing pivotal benthic foraging shifts during the Late Miocene.