Early diversification of avian limb morphology and the role of modularity in the locomotor evolution of crown birds.

High disparity among avian forelimb and hind limb segments in crown birds relative to non-avialan theropod dinosaurs, potentially driven by the origin of separate forelimb and hind limb locomotor modules, has been linked to the evolution of diverse avian locomotor behaviors. However, this hypothesized relationship has rarely been quantitatively investigated in a phylogenetic framework. We assessed the relationship between the evolution of limb morphology and locomotor behavior by comparing a numerical proxy for locomotor disparity to morphospace sizes derived from a dataset of 1,241 extant species. We then estimated how limb disparity accumulated during the crown avian radiation. Lastly, we tested whether limb segments evolved independently between each limb module using phylogenetically informed regressions. Hind limb disparity increased significantly with locomotor disparity after accounting for clade age and species richness. We found that forelimb disparity accumulated rapidly early in avian evolution, whereas hind limb disparity accumulated later, in more recent divergences. We recovered little support for strong correlations between forelimb and hind limb morphology. We posit that these findings support independent evolution of locomotor modules that enabled the striking morphological and behavioral disparity of extant birds.

New mammalian and avian records from the late Eocene La Meseta and Submeseta formations of Seymour Island, Antarctica.

The middle-late Eocene of Antarctica was characterized by dramatic change as the continent became isolated from the other southern landmasses and the Antarctic Circumpolar Current formed. These events were crucial to the formation of the permanent Antarctic ice cap, affecting both regional and global climate change. Our best insight into how life in the high latitudes responded to this climatic shift is provided by the fossil record from Seymour Island, near the eastern coast of the Antarctic Peninsula. While extensive collections have been made from the La Meseta and Submeseta formations of this island, few avian taxa other than penguins have been described and mammalian postcranial remains have been scarce. Here, we report new fossils from Seymour Island collected by the Antarctic Peninsula Paleontology Project. These include a mammalian metapodial referred to Xenarthra and avian material including a partial tarsometatarsus referred to Gruiformes (cranes, rails, and allies). Penguin fossils (Sphenisciformes) continue to be most abundant in new collections from these deposits. We report several penguin remains including a large spear-like mandible preserving the symphysis, a nearly complete tarsometatarsus with similarities to the large penguin clade Palaeeudyptes but possibly representing a new species, and two small partial tarsometatarsi belonging to the genus Delphinornis. These findings expand our view of Eocene vertebrate faunas on Antarctica. Specifically, the new remains referred to Gruiformes and Xenarthra provide support for previously proposed, but contentious, earliest occurrence records of these clades on the continent.