CONICET | Buscador de Institutos y Recursos Humanos

Madrynornis mirandus, a penguin known from a single nearly complete skeleton dated to 10.0+/-0.3Ma, provides crucial data for reconstructing the evolution of modern penguins. Restudy of the holotype reveals previously overlooked morphological features that support a new phylogenetic placement, allows reconstruction of the muscles of the jaw, and permits generation of a nearly complete and undistorted endocast.The skull of Madrynornis is characterized by widely separated temporal fossae, a weakly bowed jugal, and a long and narrow retroarticular process. Reconstruction of the jaw musculature suggests an overall conformation intermediate between the fish-specialist Spheniscus (banded penguins) and Eudyptes (crested penguins), which consume a higher proportion of invertebrates. The brain endocast shares many derived features with extant penguins including a ?heart-shaped? telencephalon, highly reduced olfactory bulbs, and a lack of cerebellar folds. However, the endocast differs from extant penguins in the less caudally-extended Wulst and stouter flocculus. Originally described as the extinct sister taxon of Eudyptes (crested penguins), Madrynornis has subsequently been considered to be a stem penguin or the sister taxon of Eudyptes + Megadyptes (yelloweyed penguins) by various authors. Our re-analysis suggests Madryornis is certainly a crown penguin (a position supported by six synapomorphies), but occupies a deeper branch basal to the divergence between Eudyptes + Megadyptes and other extant penguins. Following stratigraphic revisions that shift the age estimate for Spheniscus muizoni from ~12Ma to 9.1 Ma, Madrynornis is now recognized as the oldest crown penguin taxon. Our results thus support both a recent replacement of archaic penguins by crown taxa and a South American origin for several extant clades.