Early Eocene (Ypresian) birds from the Okanagan Highlands, British Columbia (Canada) and Washington State (USA)

We survey the known avian fossils from Ypresian (early Eocene) fossil sites of the North American Okanagan Highlands, mainly in British Columbia (Canada). All specimens represent taxa that were previously unknown from the Eocene of far-western North America. Wings from the McAbee site are tentatively referred to the Gaviiformes and would constitute the earliest fossil record of this group of birds. A postcranial skeleton from Driftwood Canyon is tentatively assigned to the Songziidae, a taxon originally established for fossils from the Ypresian of China. Two skeletons from Driftwood Canyon and the McAbee site are tentatively referred to Coliiformes and Zygodactylidae, respectively, whereas three further fossils from McAbee, Blakeburn, and Republic (Washington, USA) are too poorly preserved for even a tentative assignment. The specimens from the Okanagan Highlands inhabited relatively high paleoaltitudes with microthermal climates (except Quilchena: lower mesothermal) and mild winters, whereas most other Ypresian fossil birds are from much warmer lowland paleoenvironments with upper mesothermal to megathermal climates. The putative occurrence of a gaviiform bird is particularly noteworthy because diving birds are unknown from other lacustrine Ypresian fossil sites of the Northern Hemisphere. The bones of the putative zygodactylid show a sulphurous colouration, and we hypothesize that this highly unusual preservation may be due to the metabolic activity of sulphide-oxidizing bacteria. Key words: aves, diagenesis, fossil birds, systematics, taxonomy. Nous avons passe en revue les fossiles aviens connus de localites fossiliferes de l'Ypresien (Eocene precoce) des hautes terres de l'Okanagan nord-americaines, principalement en Colombie-Britannique (Canada). Tous les specimens representent des taxons inconnus jusqu'ici dans l'Eocene de la bordure ouest de l'Amerique du Nord. Des ailes provenant de la localite de McAbee sont provisoirement affectees aux gaviiformes et constitueraient les specimens fossiles les plus anciens de ce groupe d'oiseaux. Un squelette postcranien provenant de Driftwood Canyon est provisoirement affecte aux songziides, un taxon etabli a l'origine sur la base de fossiles de l'Ypresien de Chine. Deux squelettes de Driftwood Canyon et de la localite de McAbee sont provisoirement affectes aux coliiformes et aux zygodactylides, respectivement, alors que trois autres fossiles des localites de McAbee, Blakeburn et Republic (Etat de Washington, Etats-Unis) sont trop mal preserves pour permettre une affectation meme provisoire. Les specimens des hautes terres de l'Okanagan habitaient des paleolatitudes relativement hautes caracterisees par des climats microthermaux (sauf Quilchena : mesothermal inferieur) et des hivers doux, alors que la plupart des autres oiseaux fossiles ypresiens occupaient des paleomilieux de basses terres beaucoup plus chauds caracterises par des climats mesothermaux a megathermaux. La presence putative d'un oiseau gaviiforme est particulierement notable, parce que des oiseaux plongeurs ne sont pas connus d'autres localites fossiliferes ypresiennes lacustres de l'hemisphere nord. Les os du zygodactylide putatif presentent une coloration sulfureuse, et nous postulons que ce mode de preservation tres inhabituel pourrait etre le fait de l'activite metabolique de bacteries oxydant les sulfures. Mots-cles : oiseaux, diagenese, oiseaux fossiles, systematique, taxonomie.
Introduction Ypresian (early Eocene) bird fossils are known from several localities in Europe and North America (Mayr 2009a, 2017a) and some of these sites, such as Messel in Germany and [...]