THERAPSID BURROWS IN THE LOWER JURASSIC NAVAJO SANDSTONE, SOUTHEASTERN UTAH

Northwest of Moab, Utah, the Lower Jurassic Navajo Sandstone contains tetrapod burrows at four distinct stratigraphic levels. The most extensive burrowed horizon is an ~2 m thick interval 24-26 m above the base of the Navajo and can be identified over tens of square kilometers. These burrows have subcircular cross sections and begin as shallow, low-angle diagonal shafts that lead to multiple terminal chambers. Branching is irregular and variable, no burrow linings or scratch marks are evident and burrow walls are mostly smooth. The burrows cross-cut bedding and have a fill that is identical to the host rock, but the filled burrows preferentially weather from the host rock. Most burrows have a 10-20 cm diameter, and total lengths are up to 60 cm, though most are much shorter. In many places, the burrows form complex mazes that are concentrated in mounds that are 1-2 m in diameter and 0.3-0.9 m thick. Locally, the upper part of the burrowed interval also contains rhizoliths, which are readily distinguished by their knobby surface texture, relatively small diameters (< 10 cm), distinct (calcareous or siliceous) infilling, vertical orientation and, in some cases, downward branching. The tetrapod burrows from the Navajo Sandstone resemble therapsid burrows from the Lower Triassic of the South African Karoo. Therefore, a therapsid (probably tritylodontid) apparently produced most or all the burrows in the Navajo Sandstone. The extensive burrowed horizons in the Navajo Sandstone indicate multiple episodes of stable, relatively wet landscapes during which a profusion of small tetrapods thrived. This refutes previous ideas of a relatively barren Navajo landscape and also indicates an abundance of Early Jurassic therapsids in the American West that is not documented by body fossils.