New Mexico Geological Society Annual Spring Meeting — Abstracts

Sphenothallus Hall 1847 is a widely distributed invertebrate taxon that ranges in age from Early Cambrian to Permian. Its phylogenetic affinities are problematic, but it is generally considered to be either an annelid or cnidarian. We document the first New Mexico record of Sphenothallus, which was collected from New Mexico National Museum of Natural History locality 4667, in the Upper Pennsylvanian (Missourian) strata of the Atrasado Formation, near Tinajas Arroyo in Socorro County. A single specimen (NMMNH P-37916) assigned to Sphenothallus sp. was found in a 4-m thick unit of thinly laminated, dark gray shale stratigraphically low in the Atrasado Formation. Deposition of this unit took place within a coastal plain lacustrine environment as indicated by the sediments and by conchostracans, insects and terrestrial plants. As Sphenothallus typically occurs in low-energy shallow marine environments, it is thus considered an allochthonous element of the fossil assemblage. P-37916 is preserved along the bedding plane as part and counterpart. It consists of an incomplete crushed sphenothallid tube that has a length of 35 mm and a maximum width of 10 mm. One end of the tube gently tapers and is presumably the more proximal end, although the specimen lacks any basal attachment structure or apertural opening. Near this end a small segment of the tube is displaced that retains a thickened lateral margin. The tube lacks annulations and is covered in places by carbon. Numerous (~200) irregularly circular holdfasts are attached to the tube surface. Holdfast diameters range from 0.25 mm to 1 mm, indicating that fastening occurred over a period of time. None of the holdfasts retain their own tubes. Although both tube and holdfast morphology is characteristic of the genus Sphenothallus, there is insufficient material to make a specific assignment. Sphenothallus holdfasts have been reported on various surfaces including sphenothallid tubes, brachiopods, crinoids and hardgrounds. Historically, Sphenothallus has been often misidentified or neglected; making it probable that it is more common in New Mexico than has been previously recognized.