Late Pennsylvanian invertebrate paleontology of Bruton Canyon, northern Sierra Oscura, Socorro County, New Mexico
— Barry S. Kues

The excellently exposed Upper Pennsylvanian sequence in Bruton Canyon, first studied by Thompson (1942), consists of several members of the Missourian-Virgilian Atrasado Formation (named as formations by Thompson) and the overlying Bursum Formation (Madera Group). The Burrego, Story, Del Cuerto, and Moya Members of the Atrasado Formation yielded marine invertebrate collections dominated by brachiopods; crinoid fragments, rugose corals, bryozoans, and fusulinids are subsidiary elements of these assemblages, with molluscs being rare. The Burrego assemblage is unusual in its abundance of the large productide Echinaria, generally an uncommon genus in New Mexico Pennsylvanian strata. By far the largest collection (>1400 specimens), from the middle part of the Del Cuerto Member, is dominated by brachiopods (19 species), with solitary rugose corals and crinoid debris next in abundance. This Del Cuerto assemblage is dominated numerically by the brachiopod genera Composita and Enteletes (73% of all brachiopod specimens), with moderate numbers of Phricodothyris and the small productide Hystriculina, but only small numbers of the other brachiopod species. This is the only assemblage known in the Pennsylvanian of New Mexico in which Enteletes, which is absent from most Late Pennsylvanian strata, is a dominant element. Its association with locally dense concentrations of solitary rugose corals in the Del Cuerto Member is similar to a few assemblages in the Late Pennsylvanian of the Midcontinent region, and suggests that the genus flourishes only in a restricted and specific set of ecological conditions. All of the Atrasado assemblages are strongly dominated by stenohaline groups, and lived in well-oxygenated, offshore, shallow-marine environments that experienced no salinity fluctuations and little turbidity.