Meeting Abstract
40.5 Tuesday, Jan. 5 Aquatic locomotion in fossil birds and early avian transitions from aquatic to terrestrial environments HINIC-FRLOG, S*; MOTANI, R; Carleton University, Ottawa; Univ. of California, Davis shinic@earthsci.carleton.ca
Evolutionary transitions of secondarily aquatic organisms to terrestrial habitats are rare; however, birds represent a notable exception. A study of such transitions in birds requires incorporation of extinct ornithurans which may have made the earliest shift from aquatic to terrestrial realm. Here, we quantitatively evaluate whether basal birds were truly aquatic and identify possible adaptive reasons for their transition back to terrestrial environments. We sampled predominant mode of locomotion and measured 32 osteological characters in 245 species of extant birds in order to examine whether mode of locomotion is statistically detectable based on morphology. Regularized discriminant analyses shows that osteological measurements successfully separate diving birds from surface swimmers and also discriminate among different underwater modes of swimming. Using statistical predictions of the multivariate analyses, we confirm qualitative assessments that Hesperornithiformes were foot-propelled underwater swimmers. We predict that Gansus yumenensis was not an underwater diver. We provide strong support that all living birds had a common aquatic ancestor in the Cretaceous. Hesperornithiformes had a reduced range of motion around the knee and a poor alignment of the hip with the center of mass. These attributes made walking and transition back to land very difficult. On the contrary, Gansus was not as limited in locomotion: at least skimming or surface swimming was possible. Both of these locomotor modes are associated with easier exploration of terrestrial environments. Among aquatic birds, it is only the birds which are restricted to the surface of the water with morphologies overlapping with those of non-swimmers that are more likely to transition back to terrestrial habitats.
