SPHENOTHALLUS-LIKE FOSSILS FROM THE UPPER MARTINSBURG FORMATION (UPPER ORDOVICIAN) IN NORTHEASTERN TENNESSEE

The occurrence of rare Sphenothallus-like fossils is documented for the first time in the strata of the upper Martinsburg Formation (Shermanian Stage, Caradocian Epoch) at the Thorn Hill locality in northeastern Tennessee. These strata consist of skeletal limestones (with common brachiopods, bryozoans, and trilobites) interbedded with gray siltstone and shale and interpreted as tempestites or storm deposits of a shallow marine shelf environment. The Sphenothallus-like remains are black to dark gray carbonized tubes found in siltstone beds of the upper parts of the tempestite successions, oriented parallel to bedding and commonly subparallel to each other. The tubes are up to 60 mm long and 0.5-2 mm wide. They are unbranched, straight or slightly bent, occasionally twisted and wrinkled, and not significantly tapered. Many tubes exhibit straight fractures that are parallel to each other and perpendicular or at an angle to tube elongation. All observed tube terminations were formed by breaking along these and other irregular fractures during transport and deposition by storm waves and currents. The tube fracturing in part also reflects taphonomic modification during burial.

In cross section, the well-preserved tubes are elliptical (compressed) or circular (uncompressed) and filled with silty sediment, scattered organic material and calcite cement. SEM examination revealed bumpy and irregular to smooth tube surfaces and simple walls, 3-80 µm thick, with fine laminations parallel to the tube walls. These laminations and lack of other distinctive structure are similar to the characteristics of Sphenothallus (Hall, 1847), a relatively rare tube-dwelling marine worm of Cambrian to Permian age. The size and shape of the Martinsburg fossils resemble the remains of Sphenothallus bicarinatus (Girty, 1911). The Martinsburg fossils, however, show only limited evidence for longitudinal thickenings (producing two lateral ridges on opposite sides of the tube) and for lateral tube tapering or widening, which are common features of most Sphenothallus species. Future studies of these Sphenothallus-like fossils, including geochemical analyses of their carbonized remains, will attempt to further improve identification of these rather rare and mysterious fossils.