TWO NOVEL ICHNOSPECIES: LARGE-DIAMETER VERTEBRATE BURROWS IN THE UPPER JURASSIC MORRISON FORMATION, SOUTHEASTERN UTAH

Abstract

Large-diameter structures in the Salt Wash Member, Upper Jurassic Morrison Formation in southeastern Utah, are interpreted as mammal burrows. All burrow types are found in pedogenically modified clayey mudstone overlain by sandstone channel deposits. The environment of deposition is interpreted as river channel and floodplain. Two types of burrows, found at two localities, reveal the hidden biodiversity of small vertebrates in an area with a paucity of such body fossils. Morphotype 1 exhibits a vertical to subvertical helical shaft leading to a subhorizontal tunnel. The helical shaft has a mean depth of 71.4 cm from the interpreted paleosurface and the mean path length is 99.4 cm. The mean dip of the whorls in the helices is 39.2°. The mean tunnel length is 42.3 cm. Both shafts and tunnels are ovoid in cross section with the horizontal diameter slightly larger than the vertical. The shaft averages 9.2 cm wide and 7.3 cm tall. The tunnel averages 10.7 cm wide and 10.7 cm tall. The tracemaker that produced the burrows was likely a fossorial mammal that used them as a shelter when not foraging above ground. These burrows are assigned to domichnia and represent a new ichnospecies, Daemonelix martini. Morphotype 2 burrows are networks of interconnected shafts and tunnels at angles of 0–89º. Segments of the network, shafts and tunnels, are straight, curved, or helical. The mean length of a section is 30.7 cm. The tracemaker that produced the burrows was likely a social fossorial mammal that used them for a variety of purposes and is categorized as polychresichnia. The burrows represent a new ichnogenus and ichnospecies, Fractussemita henrii. Both morphotypes have ridges and knobs preserved on the burrow walls. Some surficial morphology is interpreted as scratch marks from the claws and/or teeth of the tracemakers. Primitive mammals are the most likely tracemakers for both morphotypes based on comparison to the architectural and surficial morphologies of fossil and extant vertebrate burrows. The burrows reveal the actions of small vertebrates not recorded by body fossils showing potential partitioning of the environment and availability of resources for small vertebrates. The burrows in the Salt Wash Member reflect the depth of vadose zone at the time of excavation. Their preservation shows the way minerals were moved through the soil.