Phylogeny, histology and functional morphology of fossil penguins (Sphenisciformes)

The shift from aerial to underwater flight is one of the most radical transitions in avian evolution. Penguins, archetypical wing-propelled divers, possess a rich fossil record and yield insight into this transition. Here, data from comparative osteology, dissections, CT imaging and bone microstructure are combined in a phylogenetic framework to explore penguin evolution, emphasizing assembly of the underwater flight apparatus.;Taxonomic revision grounded in quantifying intraspecific variation results in invalidation of 6 species and recognition of 2 new species. Phylogenetic analysis of 56 penguin taxa utilizing 225 morphological characters and 5 genes yields a well-resolved consensus tree that exhibits strong fit to stratigraphy, indicates numerous Tertiary dispersal events and supports a recent origin for crown penguins (Spheniscidae). On the basis of the phylogenetic result, the old subfamily taxonomy of fossil penguins is discarded in favor of taxonomic names based on monophyletic groups. Problematic fragmentary taxa are placed to the most exclusive level possible using apomorphies.;Synapomorphies of the forelimb are spread throughout the cladogram. Dissection of 5 penguins guides identification of osteological correlates of muscles and retia mirabilia, illuminating key soft tissue transitions. The basal penguin Waimanu lacks features related to downstroke efficiency (hyper-elongate coracoids) and transfer of thrust from wing to body (paddle-like scapula for expanded scapulohumeralis caudalis) seen in all other penguins. Crownward, proximal displacement of the latissimus dorsi insertion (more efficient thrust transfer), and reduction of intrinsic muscles and joints (immobilizing the wing during the thrust-producing upstroke) occur. The humeral rete appears to have arisen at or near the base of the penguin tree. Histological sections document significantly lower bone density in basal penguins, demonstrating osteosclerosis progressed in the clade via increased remodeling of peripheral medullary trabeculae into compact bone. Reticulate vascular canals and absence of LAGs imply rapid attainment of adult size in giant (>60kg) fossil taxa. CT-rendered endocasts reveal stem penguins possessed a hypertrophied flocculus and sagittal eminence (suggesting complex diving capability) while primitively retaining contralateral communication between the anterior tympanic recesses.