Endosseous labyrinth anatomy of Antarctic stem penguins (Aves, Sphenisciformes)

DEGRANGE, F.J.; TAMBUSSI, C.P.

Jornada; XXVII Jornadas Argentinas de Paleontología de Vertebrados; 2013

For the first time, the endosseous labyrinth of the inner ear of an Eocene Antarctic penguin(MLP 12-I-20-1) could be reconstructed and studied in a comparative context (Paraptenodytesantarcticus, four living penguins, and a loon). At the present time, the Antarctic specimen could notbe assigned to anamed taxa, but there is a strong possibility it may belong to a previously known La MesetaFormation penguin. Semicircular canals of MLP 12-I-20-1are grossly enlarged, showing a relatively marked lower development than the crown penguins and loons, although the endosseous labyrinth volume is considerably higher (either relative to body size or brain volume). Although skewed, canals are aligned in approximately orthogonal planes in three-dimensional space, contrary to extant penguins and loons. The shape of the anterior and posterior cross-sectional canals iscircular whereas it is elliptical in the horizontal. Strongly elliptical canals were also observed in a variety of birds and their functional meaning is still obscure. In comparison with sampled taxa, the anterior semicircular canal is conspicuously shorter and enters cranially in its ampulla. The origin of the horizontal canal lies on the lateral surface of the common crus in all the compared taxa. This is a notorious difference with aerial fliers which have a caudal origin on the medial surface of the crus. The horizontal canal is tilted upwards from the basicranial plane (~20°); its expansion, which is markedly lower in the Antarctic fossil, could be suggesting a relatively reduced sensitive ability to fast movements involved during pursuit diving.