A high latitude hesperornithiform (Aves) from Devon Island: palaeobiogeography and size distribution of North American hesperornithiforms

摘要

A new hesperornithiform bird specimen from Devon Island in the Canadian High Arctic is represented by three cervical vertebrae and is assigned to Canadaga arctica. The new specimen is only the second occurrence of C. arctica and corresponds in morphology and size to the type specimen from Bylot Island, also in the High Arctic. This new fossil adds to the record of North American hesperornithiforms, which had a well-documented Arctic presence. Body size comparisons of all North American specimens from the Campanian reveal that the largest known hesperornithiforms were from high latitudes, but otherwise no clear correlation between body size and latitude is apparent. The largest hesperornithiforms (Canadaga arctica and Heperornis regalis) are found at the highest latitudes, while the smallest forms (Baptornis advenus and Parahesperornis alexi) are found at the southern extent of the birds' range. Coniornis (a medium body-sized genus) is only found in the middle of the range. No size trends are discemable within the genus Hesperornis or within the species H. regalis. The presence of large hesperornithiforms at high latitudes may indicate that either strong seasonal distribution of resources contributed to larger body sizes at higher latitudes, or Campanian thermal gradients along the Western Interior Seaway were significant enough to affect body size for thermoregulatory reasons (sensu Bergmann's rule). The absence of body size trends within mid-latitude Hesperornis specimens suggests that the climatic gradient in the southern portion of the Seaway was not strong enough to force morphological evolutionary responses, or that character displacement, migration and/or other factors affected body size. Sample size and the inherent problems of an incomplete fossil record must also be considered.