A new fossil loon from the Late Cretaceous of Antarctica and early radiation of foot-propelled diving birds.

S. CHATTERJEE; D.R. MARTINIONI; F. NOVAS; F. J. MUSSEL; R. TEMPLIN

Ottawa

Congreso; Sixty-Sixth Annual Meeting - Society of Vertebrate Paleontology; 2006

Society of Vertebrate Paleontology

ABSTRACT:   Well-preserved skeletons of neognathous birds from the Cretaceous deposits are extremely rare worldwide. A new and gracile species of a fossil loon Polarornis from Upper Cretaceous strata on Vega Island of Antarctica fills a critical gap in the origin and early evolution of neognathous birds. It is about the size of a red-throated loon and is considerably smaller and slender than its sympatric species Polarornis gregorii.  Polarornis was recognized previously as a foot-propelled diving bird but the intact wing material of the new species suggests that it had also developed sophisticated powered flight, indicating dual mode of locomotion. We calculate the flight performance of this new species of Polarornis (using mass = 1.2 kg; wingspan = 0.86 m; aspect ratio = 6.7; wing area = 0.11 m2; and wing loading = 107.3 N/m2) through a computer simulation model, indicating that it was capable of continuous flapping flight. The hindlimbs were less specialized for diving than those of living loons, as indicated by its relatively long and slender femur and short cnemial crest. Apparently foot-propelled adaptations in loons were gradually superimposed on a body plan adapted for powered flight. The new species of Polarornis has striking similarity to the fossil and extant loons, displaying remarkable evolutionary stasis of Gaviidae over 65 million years. The most informative specimens of Late Cretaceous neognaths come from Antarctica including loons, as well fragmentary remains of anseriforms, and charadriiforms. Apparently Antarctica was the cradle of basal neognaths that survived the KT extinction and dispersed to lower latitudes during the early Tertiary. The new fossil loon is the only known example of modern bird that is sufficiently preserved from the age of dinosaurs to permit the study of flight performance. It provides another example of high latitude heterochroneity and favors a Late Cretaceous origin of neognaths rather than an explosive radiation in the early Tertiary.