Nearly complete late Eocene skull from the North Pacific elucidates the cranial morphology and affinities of the penguin-like Plotopteridae.

The extinct Plotopteridae were penguin-like, wing-propelled diving birds of the North Pacific. Recently, the oldest and most complete plotopterid skull has been discovered in the late Eocene lower part of the Lincoln Creek Formation, southern Olympic Peninsula (Washington State, USA), and informs the poorly known cranial morphology of these birds. This skull is somewhat larger than previously described partial skulls from the Oligocene Pysht Formation of the northern Olympic Peninsula, from which it also differs in the shape of the nostrils. It may represent the genus Klallamornis, but a definitive taxonomic assignment is not yet possible. The specimen corroborates a sister group relationship of plotopterids to the suliform Suloidea and exhibits a notable character mosaic. Whereas the long rostrum most closely resembles that of the Fregatidae and some Phalacrocoracidae, the neurocranium is more similar to that of the Sulidae. An arcuate rostral ridge of the basicranium is otherwise only known from the Sphenisciformes, and a pair of prominent longitudinal ridges along the ventral surface of the rostrum is an autapomorphy of plotopterids. The small nostrils are situated at the caudal ends of conspicuous sulci, which are interpreted as vestiges of long, slit-like nostrils and are much less pronounced in extant Suliformes. Long, slit-like nostrils occur in stem group Sphenisciformes and may also have been present in stem group Fregatidae, in which case the nostrils were reduced twice within Suliformes, presumably to prevent salt water influx into the nasal cavity.