Skeletal morphology and evidence for swimming in a fossil stem pinniped, Puuila darwini, from the Canadian High Arctic

Puijila darwini is a new genus of fossil carnivore discovered in sediments of the Haughton Formation within the Haughton impact crater on Devon Island, NU (75°N). A previous phylogenetic analysis identified Puijila and the Oligocene fossil Potamotherium vallentoni as stem pinnipeds (seals, sea lions, walrus). Unlike extant pinnipeds, Puijila did not have flippers, but shows evidence of webbed manus and pes. Puijila has limb proportions and a long tail which are more reminiscent of terrestrial carnivores. Puijila is an important morphological intermediate between modern pinnipeds and their terrestrial ancestors and can provide insight into the early land-to-sea transition of the group. A comparative description of the forelimb indicated Puijila was more postcranially specialized for swimming than modern otters (lutrines), but less specialized than its fossil relative Potamotherium. The forelimb specializations of Puijila provide evidence that the limb may have been used differently during swimming than those of otters. Tetrapod lineages that become secondarily aquatic often display specializations in their internal bone structure that accompany this transition. These specializations can be understood in ancestral pinnipeds, such as Puijila, by examining the relationship between bone microstructure and swimming behaviour in their extant relatives, Arctoidea (bears, weasels, pinnipeds etc.). Bone microstructure parameters and swimming behaviour and body mass were compared using phylogenetically independent contrasts. Swimming behaviour and body mass were positively correlated. Bone microstructure showed evidence of correlated evolution only with body mass, indicating that body mass is the best predictor of swimming behaviour in arctoids. Linear discriminant function analysis supported the observations made from the forelimb description that Puijila was semi-aquatic.