AN UNUSUALLY LARGE LATE DEVONIAN DISCINID BRACHIOPOD FROM FORT LEWIS MOUNTAIN, VIRGINIA

Discinids are inarticulate brachiopods possessing a chitino-phosphatic shell which range from the Ordovician to present. Most are relatively small with a round to slightly oval outline and ornamented with concentric ridges and furrows. Discininds are rare in the Foreknobs Formation and only one species, Orbiculoidea newberryi, has been recovered from the Blizzard and Red Lick Members in sections between Pulaski and Alleghany Counties, Virginia. The current material comes from the Red Lick Member on Fort Lewis Mountain in Roanoke County, Virginia, which is in the same region, but displays a totally different morphology. Orbiculoidea newberryi has an almost circular shape with a diameter of about 20 mm with a posteriorly located pedicle foramen. The specimens from Fort Lewis Mountain are ovoid averaging 47.9 mm long and 34.5 mm wide with an almost centrally located pedicle foramen. The posterior margin tapers forming valves with an egg-shaped margin. Fossilization has preserved some of the original shell material in a fauna typically represented by sediment replacement casts.

The Fort Lewis Mountain discinids most resemble O. grandis from the Late Devonian of New York but are more ovoid in shape. Other North American forms such as O. ampla and O. alleghania have very convex dorsal valves, a feature not seen in the present material, and O. magnifica has its pedicle openings in a different location. Gigadiscina lessardi from the Lower Devonian of Algeria are the same size as the Fort Lewis Mountain specimens and also have a tapering posterior margin, but are much broader with a very convex dorsal valve, but the geographic and temporal separation makes it unlikely they are closely related. All the Fort Lewis Mountain discinids were found as individual specimens rather than in clusters. Clustering, which has been observed in both fossil and extant discinids, has been interpreted as a defensive response to grazing predators such as chitons and gastropods. The lack of clustering in the Foreknobs discinids suggests that grazing predators may have been absent from the environment.