Abstract The classification of the Pteriomorphia, a major division of the Bivalvia with Ordovician origins, is controversial both among palaeontologists and neontologists. To elucidate phylogenetic relationships new near-complete 18S rDNA sequences of 26 Pteriomorphia, three Protobranchia, three Heterodonta, one Anomalodesmata and three Scaphopoda were obtained, aligned with 71 other published molluscan sequences, and analysed with parsimony, maximum likelihood and spectral analysis. Although Bivalvia appear diphyletic due to heterogeneity of substitution rates among lineages, monophyly of Protobranchia, Heteroconchia and Pteriomorphia is supported. The heteroconch Lucinida, Myoida and Venerida are not monophyletic, and Anomalodesmata arise from within Heteroconchia. The basal nodes of Pteriomorphia have little support but two major clades, [Pinnoidea (Ostreoidea + Pterioidea)] and [(Anomioidea + Plicatuloidea) + (Limoidea + Pectinoidea)], are resolved with more confidence. The strongly supported clade of Anomioidea + Plicatuloidea, the separation of Pinnoidea from Pterioidea and most of the intrafamiliar relationships are not in accordance with morphological classifications. Combining these results with selected morphological characters, a phylogenetic hypothesis is proposed showing Mytiloidea and Arcoidea as the basal pteriomorph groups, the latter giving rise to the clade uniting the pinnoid-ostreoid-pterioid and the anomioid-limoid-pectinoid lines.

Abstract 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).