Large-diameter burrows in pedogenically modified floodplain deposits in the Salt Wash Member, Upper Jurassic Morrison Formation, southeast Utah, U.S.A., are interpreted to have been constructed by mammals. They are distinguished as Daimonelix martini isp. nov., which exhibits a helical shaft down to a horizontal tunnel with a mean depth of 71.4 cm from the inferred palaeosurface. The mean path length of the shaft is 99.4 cm; mean dip of the whorls is 39°. The mean tunnel length is 42.3 cm. Shafts and tunnels are oval or elliptical in cross section with the horizontal diameter slightly larger than the vertical (ratio of ~1.26:1); 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 was likely a fossorial mammal that used the burrow as a den to shelter when not foraging above ground; the burrows are domichnia. The other from the same member is Fractisemita henrii igen. nov. et isp. nov., a network of interconnected shafts and tunnels; shaft and tunnel segments are straight, curved, or helical. The segments are at angles of 0–89°; mean length of a section is 30.7 cm. The cross sections of all elements are oval or elliptical; the mean width is 6.3 cm and the mean height is 4.9 cm (ratio of ~1.29:1). The burrows are interpreted as the work of a social mammal and represent multiple tracemaker behaviours: protection, denning, foraging, and possibly food storage. The burrows are polychresichnia. Surficial morphologic features preserved on the burrow walls of both types are interpreted as scratches made by the tracemaker claws and/or teeth. 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.
