Skolithos linearis Haldeman, 1840 at its early Cambrian type locality, Chickies Rock, Pennsylvania: Analysis and designation of a neotype

The trace fossil Skolithos, with its simple, vertical to very slightly inclined, tubular form, is widely recognized and globally distributed in early Cambrian to Holocene strata, representing deep-sea to continental paleoenvironments. The type species, Skolithos linearis, was first described in 1840 by Samuel S. Haldeman, based on his observations of early Cambrian quartzite at Chickies Rock, a well-known landmark on the east bank of the Susquehanna River, in Pennsylvania. The original description was meager; no illustrations were published, no holotype was established, and no specimens directly associated with Haldeman are known to exist today. A “neoholotype” proposed by Howell (1943) is invalid, as it does not conform to requirements of the International Code of Zoological Nomenclature. As a result, the iconic ichnogenus Skolithos is inadequately characterized and some of the numerous ichnospecies that have subsequently been named are not unequivocally assignable to it. In order to stabilize this important ichnogenus and to provide a basis for the taxonomic revision it urgently requires, we have studied S. linearis at its type locality. Our new material is described and illustrated here, and we have designated a neotype, accessioned by the U.S. National Museum of Natural History (Smithsonian Institution). The stratigraphy and regional geologic setting of Chickies Rock, the sedimentology and environment of deposition of cross-bedded quartzite with abundant S. linearis constituting classic “pipe rock,” and the structural deformation of this fabric are reviewed. Given the simple morphology of Skolithos, disparate organisms in several phyla have been proposed as its maker. We concur in the general view that Cambrian trace fossils assigned to S. linearis were most probably made by burrowing phoronids or polychaete annelids. The global abundance of largely monospecific pipe rock in analogous Cambro-Ordovician settings is recognized as a characteristic feature of the ecology of shallow-marine environments of that time. The stage has now been set for a thorough re-evaluation of all existing ichnospecies assigned and related to the ichnogenus Skolithos.