Antarctic Eocene loon (Gaviiformes): last refuge of survivor of a long lineage typically Holarctic?

TAMBUSSI, C.P.; DEGRANGE, F.J.; REGUERO. M.A.; MARENSSI, S.A.; SANTILLANA, S.N.

Porthland

Congreso; XXXII SCAR Scientific Committee on Antarctic Research; 2012

SCAR

Extant loons (four species of the genus Gavia) are foot propelled divers found in North America and northern Eurasia. They breed at northern freshwater sites, but winter along sea costs in temperate areas. Loons had a more southerly distribution than today, and their fossils have been found in California, Florida, Italy, Austria, Chile and Antarctica. The earliest fossil gaviiform that resembles the highly derived bone of modern loons had been described from the Upper Cretaceous of Chile (Quiriquina Formation) and Antarctica (López de Bertodano Formation). It is likely that both records belong to the same species, Neogaeornis wetzeli. A near complete left coracoid (MLP 95-I-10-14) collected in the Cucullaea I Allomember of the marine La Meseta Formation at Seymour Island (Telm 5, Early-Middle Eocene, Ypresian/Lutetian, ~49-52 Ma) can be assigned to a Gaviiformes. The coracoid has a short and robust shaft; the cotyla scapularis is subtriangular and deep; the facies articularis humeralis is flat, oval, and broad; the procoracoid process is broken but the base is very broad; the processus acrocoracoideus is partially broken but it was very well developed; the foramen n. supracoracoidei is incospicuous; the facies articularis sternalis is broad at the level of the angulus medialis; the impressio m. stercoracoidei is shallow and the sulcus m. supracoracoidei is broad and deep; the impressio lig. acrocoracohumeralis is conspicuous, deep and situated proximad to the facies articularis humeralis. Although this fossil cannot be distinguished in size and morphology from the living taxa (e.g Gaviaimmer), the features of the only available specimen is insufficient to determine the specific level at the moment. Several mammalian terrestrial groups, mostly small-sized marsupials (e.g. polydolopids) of likely insectivorous to frugivorous habits and larger-sized placental herbivores (tardigrade edentate, litopterns, astrapotheres and sudamericids) are recovered from Cucullaea I Allomember. Co-occurring penguin bones and mammals in the same level indicates complex depositional environments. By the time of deposition of this high productive mammalian locality, a Nothofagus-dominated megaflora is associated. The presence of a loon in the Eocene of La Meseta Formation does not contradict this scenario. The fossil presented here constitutes the earliest record of Gaviiformes in the southern hemisphere and, also extends the permanence of this Holarctic lineage to the Eocene in the Southern Hemisphere.