PRELIMINARY REPORT ON THE MICROBIAL, INVERTEBRATE, AND VERTEBRATE TRACE FOSSILS FROM DENALI NATIONAL PARK AND PRESERVE, ALASKA: INSIGHTS INTO THE BIODIVERSITY OF A POLAR ECOSYSTEM

Research in the fluvially dominated lower part of the Upper Cretaceous Cantwell Formation in Denali National Park has produced microbial, invertebrate, and vertebrate ichnofossils, including fish swimming trails, and footprints and trackways of pterosaurs, theropods, ornithopods, birds, and possible mammal burrows. These ichnofossil discoveries are paramount to better understand this Late Cretaceous polar ecosystem because they serve as proxies for: body fossils as none have been found; biodiversity, communities, and ecologic interactions; and climate. Microbial mat structures include wrinkled textures and sheets. Invertebrate ichnofossils are analogous to traces produced by extant: nematodes (Animalia: Nematoda) = thin-diameter Cochlichnus and Unisulcus; aquatic oligochaetes (Annelida: Oligochaeta) = large-diameter Cochlichnus; mud-loving beetles (Coleoptera: Heteroceridae) = Steinichnus; midge fly larvae (Diptera: Chironomidae) = short U-shaped, small-diameter burrows rarely with a bottom; tubificid worms (Oligochaeta: Tubificidae) = short, thin vertical burrows similar to Trichichnus; mayflies (Insecta: Ephemeroptera) = short, U-shape, large diameter burrows similar to Arenicolites; biting midge larvae (Diptera: Ceratopogonidae) = irregular surface trails similar to Haplotichnus; and grasshoppers or crickets (Orthoptera: Gryllidae, Acrididae) = hopping traces as cm-scale cf. Saltator. Most potential tracemakers are known from Cretaceous amber and compression fossils. Subvertical, 3–10 cm diameter burrows 20–50 cm deep are attributed to freshwater crayfish. Swimming trails produced by ray-finned and lobe-finned fishes assignable to Undichna occur in black shale and interbedded sandstone-mudstone. Subhorizontal burrows 12–15 cm in diameter with longitudinal scratch marks were likely produced by small mammals. Hundreds of footprints in a megatracksite record the co-occurrence of four sizes of hadrosaur, suggesting postnatal supervision and social behavior. Six morphotypes of bird footprints record various behaviors. Ichnofossil evidence suggests that organism activity was recorded in fluvial sediments in summer months when aquatic and terrestrial communities were more active. The climate was more akin to the present-day area of the U.S.-Canada border.