New Mexico Geological Society Annual Spring Meeting — Abstracts

Near Ghost Ranch in Rio Arriba County, New Mexico, NMMNH locality 3845 (the Snyder quarry) is stratigraphically high in the Petrified Forest Fonnation of the Chinle Group and of Revueltian (Norian) age. Besides an extensive fossil bone assemblage, which apparently formed in response to a catastrophic wildfire, the quarry yields a small array of nonmarine invertebrates from mudstones and conglomerates a few meters above the pIincipal bone bed. A single, incomplete conchostracan from the mudstone has fine growth lines, a carapace length of ~5 mm and can be tentatively assigned to the polymorphic genus Lioestheria. It indicates the presence of a shallow, ephemeral pond of probable high alkalinity soon after Snyder bone-bed accumulation. Mudstone above the main bone bed also yielded a 48 mm long by 19 mm wide decapod specimen that is wide-bodied and short-tailed, unlike other known Triassic crustaceans. This animal represents the first decapod body fossil from the Triassic of New Mexico and is the oldest eubrachyuran crab.

The most abundant invertebrates from the Snyder quarry are unionid bivalves that form an allochthonous assemblage in conglomerate above the main bone bed. These unionids are elongate to ovate in outline, thin shelled, and have abmpt anterior ends, about 19 radiating umbonal ridges and radiating riblets on the beak. Metrics (in mm, N = 16) are length =25-58, height =10-29 (length/height ~2) and beak length =7-16. They are best assigned to Antediplodon terraerubrae (Meek, 1875) sensu Good, 1998, though we recognize that application of the name Antediplodon Marshall, 1929 to Triassic unionids is problematic. A. terraerubrae is a common Revueltian unionid from Chinle
Group strata in New Mexico, Arizona and Utah.