ABUNDANT TRACE FOSSILS OF SAND-SWIMMING REPTILES PRESERVED IN CROSS-STRATA DEPOSITED HIGH ON THE FLANKS OF GIANT JURASSIC DUNES

Although subtropical dune fields support the lowest animal biomass of any terrestrial ecosystem, diverse modern vertebrates have evolved parallel behavior patterns to cope with the hot, dry, unstable substrate and the low prey availability of these environments. In modern sand seas, some lizards, small snakes, and the Namibian golden mole “swim” through cohesionless dune sand in pursuit of their insect prey. Because the temperature of the surface sand is commonly above 70 C during mid-day, some of these animals come to (or near) the dune surface only during the night.

Sinuous trace fossils made by animals tunneling through loose, dry dune sand are locally abundant in southern Utah within the Middle Jurassic Entrada Sandstone and the Upper (?) Jurassic Bluff Sandstone. The sharply delineated, cylindrical traces range from 20-50 mm in diameter and are found within thinly laminated wind-ripple deposits that are also replete with 5-mm insect burrows. Traces terminate upward against flat-topped cones of coarse sand that filled angle-of-repose pits. The pits formed in dry surficial sand during animal entry; coarse sand was preferentially trapped in the pits during subsequent wind storms. The abundance of coarse- and medium-grained sand in these deposits gives strong support to the sand-swimming interpretation of the traces: such materials have very low water retention and lose grain-to-grain cohesion quickly after wetting events. Tunnels rarely dip more steeply than 22 degrees, and their sinuosity suggests that elongate animals used lateral undulation to propel themselves through the sand. The animals also generated branched feeding burrows by intermittent forward probing and backward movement through use of rectilinear locomotion-a pattern of movement used by modern amphisbaenians.

The ancient traces are most abundant in the uppermost parts of very thick (up to 35 m) compound sets of cross-strata that were generated by relatively small dunes migrating along and down the lee slopes of giant dune ridges. These traces provide new evidence of reptiles that were: 1) either limbless or with reduced limbs; (2) elongated; and (3) quite large compared to their modern ecological analogs. They were insectivorous, and were adapted to hunting their prey within a soft, mobile substrate in a hot, arid, sparsely vegetated environment.