Bird tracks from the Upper Cretaceous Cantwell Formation of Denali National Park, Alaska, USA: a new perspective on ancient northern polar vertebrate biodiversity

The Upper Cretaceous Cantwell Formation in Denali National Park and Preserve (DENA), Alaska, contains an unparalleled fossil avian biodiversity. The Cantwell Formation, thousands of metres thick, was deposited near its current latitude and is exposed throughout much of DENA and elsewhere in the central Alaska Range. The Formation comprises a lower, dominantly fluvial sedimentary unit and an upper, mostly volcanic unit. Sedimentation of the lower unit was mainly in alluvial fan, braided and meandering stream, and lacustrine environments, with possible marginal-marine influence at times. Pollen data suggest that these sedimentary rocks are late Campanian or early Maastrichtian in age; thus the Cantwell Formation is correlative with other well-known dinosaur localities in Alaska. Bird tracks are preserved in multiple locations along a 40-km transect in DENA in fluvial and lacustrine deposits. Some bird tracks are found in association with dinosaur tracks and others are found on beds interbedded with dinosaur track-bearing layers. The approximate body sizes of the birds based on tracks show a range from sparrow- to heron-sized birds (25-30% larger than the modern Sandhill Crane Grus canadensis). The Cantwell Formation contains footprints assigned to several ichnotaxa found in either Asia or North America based on such morphological criteria as the presence or absence of a hallux, print size and shape, and angle of divarication: Aquatilavipes swiboldae, Ignotornis mcconnelli, Magnoavipes denaliensis sp. nov., Gruipeda vegrandiunus sp. nov. and Uhangrichnus chuni. The presence of a mixed Asian and North American ichnofauna suggests that at least some birds used Alaska as a bridge between Asia and North America. This diverse assemblage of avian traces, combined with the known fossil bone record and invertebrate trace fossil record, demonstrates that the northern Late Cretaceous polar region contained significant biodiversity.