A new small-sized penguin from the late Eocene of Seymour Island with additional material of Mesetaornis polaris

ABSTRACT Here, we report on two tarsometatarsi assignable to relatively small-sized Eocene Antarctic penguins, housed in the palaeozoological collections of Naturhistoriska riksmuseet, Stockholm. The Priabonian fossils were collected by museum staff during two joined Argentinean and Swedish expeditions from the Submeseta Formation on Seymour Island, Antarctic Peninsula. One specimen represents a new early sphenisciform, Marambiornopsis sobrali gen. et sp. nov., the sixth small-sized tarsometatarsus-based penguin species known from the Antarctic Eocene. Micro-CT scanning revealed the presence of quite large and essentially empty metatarsal medullary cavities. The second fossil can unequivocally be assigned to Mesetaornis polaris. The specimen represents only the second record of this species and supposedly a relatively young bird. Micro-CT scanning showed that in M. polaris the metatarsal medullary cavities are less developed than in M. sobrali – the cortical and trabecular bone tissues left rather little room for significant hollow spaces. Both specimens also differ in overall density of their trabecular networks.Genus name: LSIDurn:lsid:zoobank.org:act:604F02AA-DD0C-4789-B4D6-85EA68A39A68 Species name: LSIDurn:lsid:zoobank.org:act:477F0FC5-D07B-4A92-97FD-3C4805BAF503