Invertebrate paleontology of the Upper Triassic Snyder quarry, Chinle Group, Chama basin.

The Snyder quarry in the Chama basin of north-central New Mexico is stratigraphically high in the Petrified Forest Formation of the Chinle Group and of Revueltian (Norian) age. Besides an extensive fossil vertebrate assemblage, the quarry yields a small assemblage of nonmarine invertebrates from mudstones and conglomerates a few meters above the principal bonebed. A single, incomplete conchostracan carapace from the mudstone has fine growth lines, granular sculpture, no longitudinal striae and a carapace length of ~ 6 mm. It can be tentatively assigned to the polymorphic genus Lioestheria, and it indicates the presence of a shallow, ephemeral pond of probable high alkalinity soon after Snyder bonebed accumulation. Mudstone above the main bonebed also yielded a 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 from a conglomerate above the main bonebed. These unionids are elongate to ovate in outline, thin shelled and have abrupt anterior ends and prominent umbonal ridges. They are tentatively assigned to Antediplodon terraerubrae (Meek, 1875) sensu Good, 1998, though we conclude that application of the name Antediplodon Marshall, 1929 to Chinle Group unionids is problematic. Shell shape of the Snyder quarry unionids suggests they lived in a high velocity stream, and they are preserved articulated as an allochthonous assemblage in fluvial conglomerate. A. terraerubrae is a common Revueltian unionid from Chinle Group strata in New Mexico, Arizona and Utah