VARIATIONS ON A THEME: SHALLOW SCRATCHES INDICATE A DIVERSITY OF BEHAVIORS BY INQUILINE TRACE-MAKERS IN THE UPPER CRETACEOUS PIERRE SHALE

The recurrence of shallow grooves that form the elements of meandering trails on the interior of mollusk shells has enhanced our knowledge of behaviors and putative trace-makers that occurred in cryptic habitats of the Pierre Shale. Prior evidence of inquilinism, mobile organisms utilizing cryptic habitats for food resources or protection from predators in Cretaceous ammonite shells, has come from fecal pellets and the interred body-fossils of crabs and fishes. The Pierre Shale scratches are found in a variety of arrangements along the flanks of ammonite and inoceramid internal molds, in positive relief. The first evidence of a locomotion or foraging trace preserved on a hard substrate is represented by trails of repeated sets of scratches in an overlapping plaited pattern. This most closely resembles Parundichna, a trace attributed to coelacanth fishes raking soft sediment with their fins, but differs from it in having rectilinear scratches along the midline and a lateral undulating series of scratches. It is also much smaller in diameter (~4 mm) and, of course, on a hard substrate. Prolonged inhabitation of particular baculitid shells by animals moving around inside produced heavily bioeroded surfaces of randomly oriented scratches. Fishes and isopod and decapod crustaceans, which have mobile, semi-rigid appendages, are all potential culprits of loosely arranged shallow scratches. Traces of grazing bioerosion are represented by Radulichnus inopinatus Voigt and an unnamed Radulichnus ichnospecies; these traces are attributed to the activity of patellogastropods (limpets) and polyplacophorans (chitons), respectively. They are comprised of relatively simple, repeated modular units, corresponding to a molluscan radula, which scraped across a continuous surface. Development of the Gnathichnus Ichnofacies was contemporaneous with colonization of the cryptic seafloor habitats by bryozoans, brachiopods, and serpulid worms. Cryptic communities of the Western Interior Seaway comprised a discontinuous mosaic of biogenic cavities supplied by the shells of ammonites and other mollusks during episodic mass die offs.