GREGARIOUS BEHAVIOR RECORDED IN THE TRACKS OF AN EARLY MIDDLE JURASSIC SYNAPSID

Approximately 100 individual tracks of the ichnogenus Brasilichnium occur on a single foreset surface of the early Middle Jurassic Aztec Sandstone in Valley of Fire State Park of Southern Nevada. These tracks were kindly brought to our attention by Steven and Evan Theodore. The tracks occur in twelve subparallel trackways that traverse up the slip face of a dune. Individual trackways are up to 1.5 m long, and their bearings deviate from one another by a maximum of 26 degrees.

Some of the tracks show impressions of four distinct toes, while others show only three. Individual tracks range in length from 2.0 cm to 3.8 cm and in width from 2.2 cm to 4.1 cm. Length:width ratios range from 0.68 to 1.13. This range of ratios slightly exceeds, but is in good agreement with, the range reported by Reynolds (2006) for Brasilichnium in the Aztec Sandstone of the Mescal Range of eastern California.

Brasilichnium tracks are inferred to have been made by a synapsid quadruped, perhaps a tritylodont therapsid (Loope, 2006). Brasilichnium is locally fairly common in the Navajo Sandstone, and it has also been documented in the correlative Aztec Sandstone of the Mescal Range of California. The Valley of Fire occurrence reported here is the first known occurrence of this ichnogenus in Nevada.

In all other occurrences of which we are aware, Brasilichnium occurs in solitary trackways. The presence of multiple, subparallel tracks suggests that the Brasilichnium trackmaker was, at times, highly gregarious. This may be the earliest evidence of gregarious behavior in a synapsid.