DIVERSITY AND DISTRIBUTION OF VERTEBRATE BURROWS IN THE LOWER JURASSIC NAVAJO SANDSTONE, SOUTHEASTERN UTAH: IMPLICATIONS FOR HYDROLOGY, ECOLOGY, AND CLIMATE

Six morphotypes of vertebrate burrows range from simple to complex and appear to be mostly mutually exclusive of each other. Morphotypes (M) I–III are inclined (< 15º angle) and gently curved with an elliptical cross section (Length-to-Width ratio [L:W] ~2:1) and a medial ridge along the burrow base. The surficial morphology contains thin, elongated subparallel scratch marks along the floor and walls. These burrows are tentatively assigned to Reniformichnus, whose diameter and length that do not overlap in size: 1) MI is < 7 cm in diameter; 2) MII is < 15 cm in diameter; and 3) MIII is < 40 cm in diameter. Morphotype IV is less curved, inclined at < 30º, with a L:W ratio ~1.4, and a more subtle medial ridge along the burrow base. The surficial morphology exhibits thin and elongated subparallel scratch marks but mostly along the boundary between the floor and walls of the burrow. Morphotype V and VI have complex architectural morphology with high-density networks of subhorizontal to horizontal tunnels and inclined and helical ramps ~4 to 21 cm in diameter and ~1 to 10 cm tall that are irregularly interconnected at Y- and T-branched tunnels with an average L:W ratio 2.2. Each complex is preserved as differentially weathered circular to elliptical, low-relief mounds ~1 m tall and as large as 40 m by 40 m in plan view. The surficial morphology exhibits paired scalloped features along the walls but is mostly smooth. Morphotype VI differs from MV by having fewer burrows with larger (15–30 cm) diameters, fewer Y- and T-branched connections, and no distinct mounds.

The hydrologic, ecologic, and climatic implications of these burrows are significant to the Navajo erg system. They played a role in pedogenesis and nutrient dynamics by improving rainfall infiltration, air exchange between the atmosphere and soil, and turning-over sediment. Ecologically, the constructors (reptiles, therapsids, mammals) represented predators or omnivores at higher or the highest trophic level, feeding on arthropods and other vertebrates, which, in turn, would have been consumers of other arthropods and vegetative matter. They provided shelter for smaller vertebrates and much smaller invertebrates and conduits for seed germination in the subsurface. The burrows indicate a landscape stabilized by increased precipitation also able to support primary producers.