New Mexico Geological Society Annual Spring Meeting — Abstracts

Calcareous microfossils identified as polychaete worm tubes and assigned to the genus Spirorbis have been reported from the Middle Triassic (Anisian) Anton Chico Member of the Moenkopi Formation in Cibola County, New Mexico, from the Late Triassic (Norian) Bull Canyon and Redonda Formations in Quay County, New Mexico and from the Late Triassic (Carnian) Tecovas Member of the Dockum Formation in Crosby County, West Texas. However, restudy of these microfossils indicates they are attached, vermiform gastropods, not worm tubes. Thus, examination of shell microstructure reveals two layers: an internal, undulatory, laminar layer and an external, microcrystalline layer. This differs from the accretionary microstructure characteristic of polychaete tubes. Furthermore, both layers of the Triassic microfossil shells are pierced by punctae, a characteristic feature of vermiform gastropod shells, but not of polychaete worm tubes.

Similar attached microgastropods have been recognized in Late Paleozoic, marginal marine sediments. However, this is their first recognition in Triassic strata and in strata of nonmarine origin. These fossil gastropods are not closely related to modem vermiform gastropods (Vermetidae), but were analogous, mucus-based filter feeders. These filter feeders mostly attached themselves to aquatic vegetation in clear, warm Triassic ponds and rivers.