PRELIMINARY REPORT ON MORPHOLOGICAL ANALYSIS OF POSSIBLE MAMMAL BURROWS IN THE SALT WASH MEMBER OF THE UPPER JURASSIC MORRISON FORMATION, SOUTHEASTERN UTAH

Large-diameter structures (LDS) in the Salt Wash Member of the Upper Jurassic Morrison Formation near Ticaboo, Utah, may represent burrows constructed by mammals, therapsids, and/or reptiles. The depositional environment in which the LDS are found is interpreted as a floodplain. The matrix around the burrows contained many slickensides. The LDS are located in the third paleosol down from the top of the section in the study area. Sandstones with lenses of pebble conglomerates that may represent flooding or avulsion events separate the paleosols. A second locality with similar lithologies was found east of Capitol Reef National Park near Torrey, Utah. Here the paleosols were thinner and the LDS were smaller in length and diameter. The LDS had two main morphotypes. Morphotype 1 was more common and composed of a vertical to subvertical helical shaft that lead to one or more horizontal tunnels. In two cases the tunnels were also helical. Longitudinal and transverse striations were visible on the top of LDS elements but poorly preserved due to the carbonate coating. Side and bottom surfaces lacked striations and were coated with smooth lumps of carbonate. Morphotype 2 had three occurrences and was composed of a single long (~2 m) subhorizontal tunnel. Though present again on the top of the LDS elements, there were fewer longitudinal and transverse striations preserved than on Morphotype 1. These striations were also distorted. The major surface texture was again lumpy carbonate. All LDS appear to have undergone significant architectural deformation due to the carbonate coating. With a high level of complexity and the presence of a turnaround in some specimens, the architecture of Morphotype 1 resembles mammal and therapsid burrows. Although no body fossils have yet been found in or associated with these LDS, the previous discovery of the fossorial Fruitafossor in the Morrison Formation in Fruita, Colorado, supports the interpretation of mammals as the tracemaker. The simpler architecture of Morphotype 2 most closely resembles the burrows of reptiles. The large size is comparable to burrows excavated by crocodilians or monitor lizards. The longitudinal and transverse striations on all LDS are interpreted as scratch marks from the manus and/or pes, as well as bite marks.