Enigmatic structures in an Upper Pennsylvanian (Kasimovian) marine limestone

Enigmatic structures in the shape of inverted rounded cones are very common in a discrete horizon of the upper Stoner Limestone Member (Stanton Limestone Formation, Lansing Group, Upper Pennsylvanian) in southeastern Nebraska. These structures are 100-300 mm in diameter, but rare, very large examples attain 1300 mm in diameter. On bedding planes, individual structures are conspicuously expressed as groups of concentric rings, some of which appear as low ridges (rugae). At least some of these rings correspond to concentric cone sheets (hollow cones) extending downward into the rock mass; these cone sheets are sometimes separated clearly from each other by discontinuous mud linings or drapes. The largest structures usually have smaller ones nested within them. The Stoner Limestone structures differ from described types of nonbiological and biological conelike structures, yet they must have formed in soft sediment prior to burial. Although they have a superficial similarity to such plug-shaped trace fossils as Conichnus and Conostichus, the purported dwelling traces left by cnidarians, several salient points with respect to shape, structure, and occurrence distinguish the Stoner Limestone structures from such traces. A trace-fossil hypothesis for the origin of the structures remains plausible--possible feeding structures--albeit problematic. Study of the Stoner Limestone concentric-conelike structures indicates that careful reexamination of trace fossils and sedimentary structures in Midconinent Pennsylvanian limestones is in order, as well as a reconsideration of how unique physical and diagenetic properties of carbonate sediments in general might affect the preservation of such features.