HORSESHOE CRAB TRACE FOSSILS AND ASSOCIATED ICHNOFAUNA OF THE PONY CREEK SHALE LAGERSTÄTTE, UPPER PENNSYLVANIAN, KANSAS

The uppermost Pennsylvanian Pony Creek Shale Member of the Wood Siding Formation in Wabaunsee County, Kansas, contains exceptionally preserved horseshoe crab (Xiphosura) fossils. Associated with these fossils is a diverse ichnofauna that includes horseshoe crab traces. The deposit has been interpreted as a marginal marine deposit. More specifically the Pennsylvanian Pony Creek Shale Member has been interpreted as a transition between multiple depositional environments (from non-marine to marginal marine & higher intertidal to non-marine).

More than 50 xiphosuran trace fossils were recovered from the Pony Creek Shale Member, representing xiphosuran crawl trails of varying levels of detail, resting traces (four adult traces of positive relief and one negative relief trace of a juvenile), and a locomotion trace from a xiphosuran telson. These traces are associated with body fossils belonging to Paleolimulus signatus, which is inferred to be the trace maker, and this makes the deposit valuable for studying xiphosuran behavior. In addition to these traces, two specimens represent the ichnogenus Rhizocorallium, and there are also several burrows of different kinds. The trace fossils probably come from a tidal flat (marginal marine or non-marine) depositional environment, as traces from xiphosurans have been associated with marginal marine and non-marine environments, and the presence of mud cracks indicates reoccurring sub-aerial exposure. Overall the trace fossils improve the understanding of the environment in which these xiphosurans lived, as well as some of their life habits.

Multiple bromalites were also recovered from the member, showing traces of predation, as well as fecal matter and regurgitation. Some of these bromalites show visual indication of initial pyritization. The bromalites were likely produced by a variety of animals, but some appear to have been produced by the xiphosurans.