Antarctic Science page 1 of 10 (2011) & Antarctic Science Ltd 2011 doi:10.1017/S0954102011000216 Penguin cranial remains from the Eocene La Meseta Formation, Isla Marambio (Seymour Island), Antarctic Peninsula CAROLINA ACOSTA HOSPITALECHE and NADIA HAIDR Divisio ´n Paleontologı ´a Vertebrados, Museo de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina acostacaro@fcnym.unlp.edu.ar Abstract: Widely accepted ideas about trophic preferences of early penguins suggest that all Eocene sphenisciforms were piscivorous. However, recent findings from the La Meseta Formation (Eocene) of Antarctica, support the presence of at least two different morphotypes that may have evolved as a niche partitioning strategy, which is consistent with the high diversity recorded. The first of them corresponds to a medium–large sized penguin, resembling the Neogene species in configuration. Another morphotype is represented by extremely large penguins with very long and slender bills, clearly linked to fish catching habits. Received 1 November 2010, accepted 4 January 2011 Key words: fossil penguin, morphotypes, skulls, Spheniscidae Introduction The La Meseta Formation (Eocene), Antarctic Peninsula, is a highly fossiliferous unit in terms of the quantity and diversity of remains. The invertebrate taxa include bivalves, gastropods (Stilwell & Zinsmeister 1992, Bitner 1996), bryozoan colonies (Hara 2001), crinoids (Rasmussen 1979, Baumiller & Gaz ´dzicki 1996), ophiuroids (Aronson et al. 1997, Blake & Aronson 1998), and echinoids (McKinney et al. 1988). Within the vertebrates, fishes (Jerzman ´ska 1988, Jerzman ´ska &S ´ widnicki 1992), whales (Wiman 1905, Borsuk-Bia"ynicka 1988, Fordyce 1989), a sparnotheriodontid mammal (Vizcaino et al. 1997), a ratite bird (Tambussi et al. 1994) and penguins (Myrcha et al. 2002, Jadwiszczak 2006a, Hospitaleche & Reguero 2010 and references cited therein) have been described. Penguins (Aves, Sphenisciformes) are particularly abundant in this unit. Even though knowledge of this group has increased greatly in the past few years, there are still many unanswered questions, especially regarding penguin anatomy and functional morphology. Due to the fact that the systematics of Sphenisciformes relies on the morphology and proportions of the tarsometatarsi, and sometimes humeri, few Antarctic studies have been performed regarding other skeletal elements. Only in a few cases elements other than the tarsometatarsi and humeri have been described (Hospitaleche & Reguero 2010), and analysed in detail (Hospitaleche & Di Carlo 2010, see also Jadwiszczak 2006a, 2006b). The first Antarctic penguin skull to be described was a very fragmentary bill assigned by Olson (1985) to ?Palaeeudyptes sp. Other isolated remains including cranial elements have been described (Myrcha et al. 1990) and more recently re-studied (Jadwiszczak 2003, 2006a). Additional remains from the Submeseta Allomember were described and treated as problematic specimens (Jadwiszczak 2006a) due to their poor preservation and lack of comparable material. In spite of this, Jadwiszczak’s valuable contribution has provided the basis for advancing the knowledge of morphological aspects that might be useful in systematic and palaeobiological studies. According to their general morphology and size, some of these remains have been assigned to Palaeeudyptes gunnari (Wiman, 1905) (Jadwiszczak 2006a), although previous studies assigned them to Anthropornis sp. or Palaeeudyptes sp. Jadwiszczak (2000, 2003) and to ?Palaeeudyptes sp., probably P. klekowskii Myrcha, Tatur & del Valle, 1990 (Jadwiszczak 2006a, see also Myrcha et al. 1990). Other specimens studied by Jadwiszczak (2006a) were assigned to P. gunnari or Archaeospheniscus wimani (Marples, 1953) or were too fragmentary for identification according to Jadwiszczak (2006a). Only one of them corresponds to an incomplete and deformed neurocranium and constitutes until today the only reference to a Palaeogene Antarctic cranium (Jadwiszczak 2006a). All of them come from the Submeseta Allomember. More recently, new findings from La Meseta Formation were described and at least three species were identified on the basis of size differences (Ksepka & Bertelli 2006), although the fragmentary state of the material does not allow their systematic assignment. The above seems to indicate that the preservation of craniums and mandibles is improbable. They are easily destroyed and only small fragments can be recovered from the sediments. In this context of patchy information, even minor skull characteristics that can be detected will be useful for: 1) increasing the knowledge of the anatomy of penguin 1