ELDONIIDS AND ASSOCIATED TRACE FOSSILS FROM THE MIDDLE CAMBRIAN SPENCE SHALE, UTAH

The early middle Cambrian (Series 3, Stage 5) Spence Shale, Wellsville Mountains, Utah, first studied by Charles Walcott in 1898, has provided a nonmineralized fauna of exceptional preservation similar to the Burgess Shale, Chengjiang, Kaili, as well as the Wheeler and Marjum formations of Utah. Two forms of nonmineralizing discoidal eldoniids have been recovered: a small version similar to the Burgess Shale Eldonia ludwigi, and a new form, with certain morphological differences from E. ludwigi, that is associated with planar traces and accumulations of small shell debris of restricted size ranges. The debris consists of empty shells (1-8mm) of hyolithid conchs and opercula, trilobite fragments and spines, Scenella, algae, minor brachiopods, and indeterminate shells. The new species was probably benthic, not pelagic, responded to bottom currents, and trapped food in its tentacles. Trace fossils of Gordia and Planolites have been identified within the central areas of the eldoniids and in the debris aggregates. This is the first report of traces associated with eldoniids. It is generally assumed that trace fossils indicate dysoxic or oxic bottom water conditions. Previously the Spence Shale was considered deposited under anoxic conditions, however, the eldoniids appear to represent an in situ community of benthic organisms inhabiting a shallow shelf environment. Combined with the trace fossil evidence, deposition most likely occurred in an environment that was oxic to dysoxic. This evidence supports recent studies that suggest several cycles of sedimentation occurred within the Spence Shale. More evidence is needed to determine whether these eldoniid communities occur elsewhere in the Spence Shale, and what taphonomic conditions allowed this preservation to occur.