New Mexico Geological Society Annual Spring Meeting — Abstracts

Tetrapod footprints are known from the Upper Triassic Chinle Group in Wyoming, Utah, Colorado, Arizona and New Mexico. Most of these tracks are from the upper part of the Chinle Group in strata of the Rock Point Formation and its correlatives-strata of Apachean (late Norian-Rhaetian) age. Tracks older than Apachean are relatively rare in the Chinle Group, apparently due to the presence of more facies favorable to track preservation in the younger portion of the Chinle than in pre-Apachean strata. One of the oldest and most interesting pre-Apachean records is an apparent dinosaur track from the lower part of the Chinle at Fort Wingate, New Mexico. This specimen is from Museum of Northern Arizona (MNA) locality 530, catalogued as MNA V1581, and was originally described by Hasiotis et al. (1994; GSA Abstracts w/Programs, 26(6), 17). MNA V1581 is a single track preserved in convex epirelief on a matrix of olive-gray siltstone. The middle digit is 43 mm long, and is flanked by shorter digits that are 29 and 25 mm long (Fig. 1). The digits are long and slender, and the tips are pointed. The digits converge posteriorly to a "heel" that appears to be rounded, although it may not be complete. Each digit appears to preserve phalangeal pads, in the formula 2-3-4. On face value, MNA V1581 appears to be the tridactyl pes impression of a theropod dinosaur, usually referred to the ichnogenus Grallator. This seems a reasonable identification of MNA V1581, especially given its relatively long middle toe (mesaxonic), which is characteristic of Grallator. However, given that MNA V1581 is a single track, we consider the identification tentative. Therefore, contra previous assertions, this is not the track of a new ichnospecies, the oldest dinosaur from the Chinle, or an unexpected record. The Bluewater Creek Formation, and thus the track, is of Adamanian (latest Carnian) age, and probable trackmakers (small theropods) were diverse and locally abundant by Adamanian time. The oldest dinosaurs, including theropods, from the Chinle Group are from demonstrably older (Otischalkian) strata. Consequently, tracks of Grallator are not unexpected in the Chinle Group generally and the Bluewater Creek Formation in particular. Other tracks associated with MNA V1581 include a poorly preserved track of Brachychirotherium(?) and another, unidentifiable trace, both catalogued as MNA V3303.