DEINONYCHOSAURIAN TRACE FOSSILS FROM THE LATE CRETACEOUS (MAASTRICHTIAN) CANTWELL FORMATION, ALASKA: TRACK MAKER INFERENCES USING COEVAL, INTERFORMATIONAL COMPARISONS

Recent exploration of the Late Cretaceous lower Cantwell Formation (LCF) in Denali National Park and Preserve (DENA) of Alaska reveals a diverse nonavian dinosaur track assemblage. However, inferences regarding the taxonomic identity of the track makers are hampered by a paucity of skeletal fossils, especially for rarer elements of the ichnofauna, such as theropods. In 2016, a new theropod-dominated track site, the Theropod Creek Slide (TCS) was found bearing a previously unrecognized two-toed morphotype preserved as surface layer tracks, some of which include skin impressions. Here, we infer the track maker’s identity by comparing the trace fossil record of the LCF with the body fossil record preserved in coeval horizons of the Prince Creek Formation (PCF) of northern Alaska, located approximately seven degrees of latitude further north. The PCF has produced the richest record of polar dinosaurian skeletal fossils known, with representatives of at least 14 species of nonavian dinosaurs, including seven species of theropods. Notably, the PCF is early Maastrichtian in age, with the most fossiliferous interval dated at 69.2 ± 0.5 Ma, nearly coeveal with a new U-Pb date of 68.8 ± 0.3 Ma derived from the TCS.

The two-toed tracks show that digit II was elevated above the ground, diagnostic of the theropod clade Deinonychosauria. At least five taxa of deinonychosaurs are known from the PCF, including four small dromaeosaurids (estimated hip height <0.5 m) and a much larger troodontid. The majority of new DENA two-toed tracks are exceptionally large, with a maximum foot length exceeding 20 cm and an estimated minimal hip height of 0.8-0.9 m. Based on size, pedal morphology and skeletal evidence from the PCF, and assuming the two formations showed comparable community structure, the largest DENA two-toed track maker was likely the same taxon as the large-bodied PCF troodontid. The TCS site significantly represents one of the few definitive deinonychosaurian tracks sites recorded in North America.