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Penguins are by far the most dominant group of marine vertebrates in the Eocene La Meseta Formation (Seymour Island, Antarctica). We analysed the penguin fauna recovered there from both a systematic and a biostratigraphic point of view. We have added two new species (Tonniornis mesetaensis and T. minimum) and have defined a biostratigraphic unit, the Anthropornis nordenskjoeldi Biozone. This interval of strata, easily distinguishable by the numerous occurrence of penguin bones and the phosphatic brachiopod Lingula, is located nearly 30–35 m below the top of the 145 m-thick Submeseta Allomember. The highest morphological and taxonomic penguin diversity living sympatrically (organisms that live simultaneously in the same place), including giant and tiny species, is documented in this interval. Fossil penguins bones studied in this paper, recovered from rocks interpreted as shallow-marine deposits, accumulated between 34.2 and 36.13 Ma (late Late Eocene).

Two biogeographical units are generally recognized in the present shelf area of Argentina: the Magellanian and Argentinian Provinces. The two provinces differ in their fossil record. The evolution of these provinces has been characterized by migrations, extinctions, pseudoextinctions and, perhaps, even speciation events. Marine vertebrate assemblages with some similarities to the Argentinian fauna were already present in the Miocene, whereas no associations similar to those of the Magellanian fauna have been found in South America before the Pleistocene. Two successive major marine transgressions flooded northern Patagonia during the Miocene: the ‘Patagoniense’ (Early Miocene) and the ‘Entrerriense’ (Middle to Late Miocene). We analyse three rich fossil assemblages that were formed during these transgressions. The absence of Magellanian Miocene vertebrate assemblages is consistent with the hypothesis of a more southern distribution of the cold‐temperate fauna at that time. In Patagonia, as in other regions, an increased number of living groups appeared from the Lower to Upper Miocene. The Late Miocene aquatic mammals had a modern aspect, and some of the fish species are still living in the South Atlantic Ocean. In this contribution, we stress that warm‐temperate fishes and a high diversity of penguins are found together at the base of the Gaiman Formation. We hypothesize that penguins were adapted to live in warmer waters than those of the latest Cenozoic and the Recent. Finally, we recall that many taxa became extirpated because of the global temperature drops of the late Cenozoic. However, some fishes and pinnipeds which were extirpated only in the Atlantic Ocean are discussed here. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103, 423–440. Actualmente se reconocen dos provincias biogeográficas en la plataforma marina argentina actual: la Magallánica y la Argentina. Las dos unidades difieren en su registro fósil. Se sugiere aquí que la evolución de las provincias fue compleja y caracterizada por migraciones, eventos de especiación, extinciones, pseudoextinciones. Durante el Mioceno ya había asociaciones de vertebrados marinos con similitudes taxonómicas con los de la Provincia Argentina pero no se detectaron hasta ahora asociaciones fósiles similares a las de la Magallánica hasta el Pleistoceno. Aquí describimos tres ricas asociaciones faunísticas marinas del Mioceno que se depositaron durante dos grandes transgresiones atlánticas: el ‘Patagoniense’ (Mioceno Temprano) y el ‘Entrerriense’ (Mioceno Medio a Tardío). La ausencia de asociaciones de tipo magallánicas en el Mioceno confirma la hipótesis de que la fauna templado‐fría se distribuía más al sur en ese tiempo. En Patagonia, tal como en otras áreas, se registra un número mayor de formas actuales desde el Mioceno Temprano al Tardío. Los mamíferos acuáticos del Mioceno Tardío tenían un aspecto moderno; incluso alguna de las especies de peces perdura en la actualidad. Asimismo, en este trabajo llamamos la atención sobr...

The Antarctic pelagornithid record is restricted to few isolated remains from the Eocene of Seymour Island in the Antarctic Peninsula. Here we report the oldest Antarctic pseudo-toothed bird. It is represented by an incomplete humerus lacking its proximal end, which comes from the lower Eocene levels of the La Meseta Formation (Seymour Island). This new specimen facilitates a review of all known pelagornithids from this continent. Antarctic pelagornithids were classified into two morphotypes that exhibit a mix of putative plesiomorphic and derived characters. Considering the worldwide pelagornithid record and according to estimated wingspans, four approximate size-types were identified. The oldest Antarctic specimens (two fragmentary humeri, middle Ypresian) were assigned to morphotype 1 and correspond to the large size-type. The younger materials (Bartonian/?Priabonian) here assigned to morphotype 2 (some cranial remains, fragmentary tarsometatarsus and humerus) correspond to the giant size-type and represent one of the largest known pseudo-toothed birds. Even though species level phylogenetic affinities of Pelagornithidae remain poorly resolved, three key evolutionary events can be recognized: (1) the disappearance ofDasornisin the Early Eocene and the appearance of more advanced forms with a trend to the specialization of large soaring capacity, (2) the origin ofPelagornissensu lato species in the early Oligocene, and (3) the appearance and dominance of a highly specialized terminal group at Mio/Pliocene time span.

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Abstract:The first articulated skeleton of a penguin from the late Eocene of Antarctica is described. MLP 96-I-6-13 comes from the upper Submeseta Allomember (La Meseta Formation) of Isla Marambio (locality DPV 10/84). The significance of this finding in the context of the Anthropornis nordenskjoeldi biozone is discussed. An osteologic description of the recovered elements and a brief discussion of its systematic determination are provided. MLP 96-I-6-13 is the first articulated skeleton with sure specific assignment to Palaeeudyptes gunnari (Wiman, 1905), a species previously known only through isolated tarsometatarsi and included in the groups of Wiman.

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