Morphology of flightlessness in Chendytes, fossil seaducks (Anatidae: Mergini) of coastal California

ABSTRACT

The anatid genus Chendytes comprises two species from the Pleistocene and early Holocene of California: the comparatively large C. lawi and the smaller (poorly represented) C. milleri. Osteological comparisons confirm that Chendytes is a member of the Mergini, probably most closely related to the eiders (Somateria). Peculiarities of the pectoral skeleton of C. lawi include features of the humerus, ulna, and carpometacarpus. C. lawi exceeded the largest extant seaduck, the Common Eider (Somateria mollissima), in most dimensions of the skull and pelvic limb. Pectoral reduction in C. lawi was among the most extensive known for Anseriformes; reductions in C. milleri were of lesser magnitude. Distal wing elements of C. lawi had undergone the greatest shortening, whereas the elongate pelvic limb of C. lawi was characterized by disproportionately short femora and long tibiotarsi. Based on femur lengths and body masses of modern Mergini, estimated body masses of C. lawi and C. milleri are 2,550–3,700 g and 1,820–2,650 g, respectively. Based on mean wing areas and skeletal wing lengths of modern Mergini, and shortening of remiges in flightless Anas, an estimate for the wing area of C. lawi is 190 cm². The ‘minimal’ estimate of body mass and approximated wing area yield an estimated wing-loading of 13.4 g cm⁻² for C. lawi, a figure which exceeds those of extant Anseriformes (including flightless species) and the theoretical maximum wing-loading permitting flight (2.5 g cm⁻²). K-means clustering of 44 femora of C. lawi indicated that differences in skeletal dimensions between provisional sex groups approximated those of extant Mergini, and led to estimated mean body masses of 2,820–4,055 g and 2,360–3,430 g for males and females, respectively. Multivariate analyses of extant Mergini and C. lawi demonstrate the importance of the statistics employed to quantify body size in fossil species. Flightlessness of Chendytes is compared to those of other foot-propelled dividing birds and considered with respect to ontogeny, probable paleoecology, and extinction of the genus.