Marine invertebrate assemblages from the Late Pennsylvanian (Virgilian) Holder Formation, Dry Canyon, Sacramento Mountains, south-central New Mexico

The Holder Formation along Dry Canyon consists of a cyclic succession of Virgilian marine siliciclastics, marine carbonates, and nonmarine siliciclastics, including a large basal algal bioherm complex, deposited on a narrow shelf just west of the shoreline of the Pedernal land mass. Holder marine invertebrates have been little studied; here two quite different middle Virgilian assemblages from an 80-m-thick (262-ft-thick) section along US‐82 are discussed. The first assemblage, from a 1-m-thick (3-ft-thick) dark-gray calcareous mudstone within a regressive interval approximately 50 m (164 ft) above the base of the section, is dominated by gastropods and bivalves (96% of specimens) with the gastropod Retispira espinasa and the bivalve Polidevcia arata being especially abundant. Other common gastropods are species of Taosia, Colpites, Hypselentoma, and Goniasma, and other common bivalves are cf. Sedgwickia and Myalinella. Some mollusc species are also present in the overlying Laborcita Formation, but a few have not previously been reported from New Mexico. The absence of most stenohaline groups and low taxonomic diversity (35 species) of this