First partial skeleton of Delphinornis larseni Wiman, 1905, a slender-footed penguin from the Eocene of Antarctic Peninsula

摘要

The oldest fossil record of Antarctic penguins comes from Seymour Island (Antarctic Peninsula) and dates to the Paleocene and Eocene. The Paleocene bones are extremely rare, whereas specimens from the latter epoch are numerous. Despite the recent discoveries of incomplete skeletons assignable to the giant penguins from the Eocene of Antarctic Peninsula, the reliable systematics of their smaller contemporaneous relatives, known from isolated bones, have remained dependent on the tarsometatarsus. Here, new data on the skeleton of Delphinornis larseni, the most abundant among non-giant Eocene penguins, are reported. The specimen, collected from the Submeseta Formation on Seymour Island, comprises the incomplete pelvis and numerous bones from the hind-limb skeleton, including a well-preserved (diagnostic) tarsometatarsus. The acetabular foramen is, like in larger fossil penguins, clearly smaller than the elongated ilioischiadic foramen. The area of the latter opening, not occupied by the connective-tissue sheet, supposedly accounted for one-third of the foramen. We propose that the ischiadic artery was, unlike in present-day penguins, the main blood vessel supplying most of the hind limb. The proximal fovea of the femoral head is uniquely preserved, revealing an osteological aspect of the bone-ligament interface. We surmise that the individual was similar, in terms of body size, to extant Pygoscelis papua, but was characterized by more elongate feet. In our opinion, it was probably a young bird, up to several years old.