Morphology and taphonomy of an exceptional trackway from the Flathead Sandstone (Middle Cambrian) of Wyoming

Abstract

Two sandstone slabs from the Cambrian Flathead Sandstone of Wyoming exhibit different preservational modes of an exceptionally large and unusual trackway. On the smaller slab, the trackway is a depression on the top of the bed (concave epirelief). A greater length of the trackway is preserved on the larger slab, where it is a raised feature on the bottom of the bed (convex hyporelief). The latter trace is a cast of the original trackway made on a now-missing mud layer, whereas the epirelief trace likely represents an undertrack. The original trackway consisted of two parallel pockmarked furrows separated by a broad ridge. Individual depressions in furrows cannot be matched across the ridge and do not contain discrete scratch marks. At one place the linear succession of imprints comprising one furrow changes to a scattering of discrete pits. Although taxonomic identity of the trackway’s maker is uncertain, the animal was bilaterally symmetrical and had paired appendages like an arthropod or an onychophoran. The sole of the large slab exhibits several unilobate traces that intersect the major trackway as well as one another. Their smooth surfaces, lack of marginal ridges, and discontinuous nature suggest that they were made by burrowers or furrowers following the sand/mud interface subsequent to casting of the major trackway. Their origin remains problematic.