We describe a new medium-sized penguin, Madrynornis mirandus gen. et sp. nov., from the early late Miocene Puerto Madryn Formation, Chubut Province, Argentina. Although it is evident that extant and fossil penguins form a remarkably homogeneous family of birds, Spheniscidae, their within-group phylogenetic relationships are less obvious. In order to identify the phylogenetic position of the new taxon, we conducted a phylogenetic analysis using 44 osteological characters sampled from 14 representative species of all living genera and five fossil species of Spheniscidae. The family is clearly monophyletic and Madrynornis mirandus is closely related to living taxa. Our phylogenetic interpretation is congruent with biostratigraphic data, with Paraptenodytes from the early Miocene (about 20 Ma) located at the base of the Spheniscidae. Classically, two basic tarsometatarsal types were recognized (one for pre-Miocene and the other for the post-Miocene penguins) based on the pattern of the proximal foramina and the hypotarsus. Madrynornis mirandus exhibits an arrangement of the proximal foramina and a degree of metatarsals fusion similar to that in the living forms, although its elongation index (total length/proximal width) is reminiscent of the extinct Paraptenodytes (a penguin historically recognized as a pre-Miocene form, coming from the early Miocene of Argentina). Madrynornis reveals that the two basic tarsometatarsal types co-existed among Miocene penguins.

Key words: Aves, Spheniscidae, penguins, Miocene, Puerto Madryn Formation, Chubut Province, Argentina.

Carolina Acosta Hospitaleche acostacaro@museo.fcnym.unlp.edu.ar and Claudia P. Tambussi tambussi@museo.fcnym.unlp.edu.ar, Conicet, División Paleontología Vertebrados, Museo de La Plata, Paseo del Bosque s/n, 1900 Ciudad de La Plata, Argentina; Mariano Donato mdonato@museo.fcnym.unlp.edu.ar, Conicet, Laboratorio de Sistemática y Biología Evolutiva (LASBE), Museo de La Plata, Paseo del Bosque s/n, 1900 Ciudad de La Plata, Argentina; Mario Cozzuol cozzuol@icb.ufmg.br, Departamento de Zoologia, Instituto de Cięncias Biológicas, Av. Antônio Carlos, 6627 Pampulha, 31270-910, Belo Horizonte, RS, Brazil.