Peabody's legacy: the Moenkopi Formation (Middle Triassic, Anisian) tetrapod ichnofauna—updates from an extensive new tracksite in NE Arizona, USA

The Lower-Middle Triassic Moenkopi Group/Formation in the southwestern USA has yielded a famous tetrapod skeletal and ichnofossil fauna. A new locality in the Holbrook Member of the Moenkopi Formation (Anisian) of northeastern Arizona appears to be the most extensive Middle Triassic tetrapod tracksite in North America. The ichnofossil-bearing level is close to the base of the Holbrook Member and several meters below the overlying Shinarump Formation (Upper Triassic, Carnian) of the Chinle Group. The track-bearing bed is a mudstone layer overlain by a massive-to-laminar, tabular sandstone body that represents sheet-like non-channelized flow deposited by flooding and preserves the collected natural casts of the tracks. A low diversity invertebrate ichnoassemblage dominated by Scoyenia on the track-bearing surface represents the Scoyenia ichnofacies. The paleoenvironment can be characterized as an intermittently subaerial/subaqueous setting on a nonmarine riverine floodplain. The tetrapod ichnoassemblage is dominated by archosaur tracks with the chirotheriids Chirotherium barthii, C. rex, Isochirotherium marshalli, Synaptichnium diabloense, S. pseudosuchoides, and small Rotodactylus cursorius. Synaptichnium and Rotodactylus are the most abundant tetrapod ichnogenera at this location. Chirotheres occur with different size classes, possibly suggesting a mixed archosaur community with individuals of different growth stages and ages. Biostratigraphically, the composition of the assemblage and the presence of the ichnospecies Chirotherium barthii in the Holbrook Member indicate the Chirotherium barthii biochron for this unit. The lack of Chirotherium sickleri supports former conclusions about paleobiogeographic peculiarities of the North American assemblage, if compared to early Anisian ichnoassociations of Europe, where Chirotherium barthii is commonly associated with C. sickleri. Interestingly, the new tracksite lacks archosauromorph/lepidosauromorph tracks such as Rhynchosauroides or therapsid tracks such as Dicynodontipus that co-occur at other localities.