Solitary Rugose Corals of the Upper Ordovician Montoya Group, Southern New Mexico and Westernmost Texas

The Upper Ordovician (middle Edenian to upper Richmondian) Montoya Group of southern New Mexico and westernmost Texas comprises, in ascending order, the Second Value Dolomite, Aleman Formation, and Cutter Dolomite. Solitary rugose corals in the Second Value are Grewingkia robusta (Whiteaves, 1896), Bighornia sp. cf. B. patella (Wilson, 1926), Streptelasma divaricans (Nicholson, 1875), a new species of Neotryplasma, and Salvadorea? spp. A and B. Salvadorea kingae kingae Nelson, 1981, G. franklinensis n. sp., and G. crassa alemanensis n. subsp. occur in the Aleman. Taxa found in the Cutter are S. kingae cutterensis n. subsp., G. sp. cf. G. franklinensis, and B. sp. cf. B. patella.

Grewingkia robusta is the most abundant species in the solitary rugosan assemblage that is present within the Second Value. This assemblage apparently inhabited comparatively deep-water environments. Most of the corals lived in relatively high-energy conditions, but epizoic forms favored low-energy niches. Salvadorea kingae is the most common taxon in the assemblage that characterizes the Aleman-Cutter. This is probably a comparatively shallow-water assemblage. The dominant taxon inhabited relatively low-energy environments, while less common species lived in higher energy conditions. If the distribution of solitary rugose corals in the area of Montoya deposition was related primarily to water depth, a paleobathymetric gradient from relatively deep in the west to predominantly shallow in the southeast existed through Second Value–Aleman time. During Cutter time, water was relatively deep in the southwest and northeast, and predominantly shallow in the southeast. If the degree of environmental restriction was the principal factor limiting the distribution of Montoya solitary Rugosa, open normal marine environments were predominant in the southeast and uncommon in the north and west.

Montoya representatives of Grewingkia, Bighornia, and Salvadorea indicate that the area of deposition was situated within the Red River–Stony Mountain Solitary Coral Province, which occupied most of North America during Late Ordovician time. All species of these genera are typical “epicontinental” forms. Neotryplasma, the only “continental margin” taxon, reflects a cratonic margin paleoposition. The discovery of Streptelasma divaricans within Edenian-Maysvillian strata in the Montoya is consistent with an hypothesis that solitary Rugosa were introduced to the Richmond Solitary Coral Province of eastern North America during an early Richmondian transgression.

Within the Red River–Stony Mountain Province, geographic speciation and dispersion seem to have been important factors in the evolution and diversification of Grewingkia robusta and related taxa, including G. haysii selkirkensis n. subsp. from the Selkirk Member of the Red River Formation in southern Manitoba. In Salvadorea, speciation events within the New Mexico–Texas area and Williston Basin were apparently rapid, and coincided with onsets of clastic deposition. Evolutionary change within this genus has not been recognized in the Hudson Bay Basin, where there were no clastic influxes.

In the Aleman-Cutter sequence of the Montoya Group, recognition of specific intervals bearing solitary rugose corals may permit detailed biostratigraphic, and possibly chronostratigraphic, correlation. The change from a Grewingkia-dominated assemblage to a Salvadorea-dominated assemblage was not synchronous throughout the Red River–Stony Mountain Province, and the ranges of widely distributed species such as G. robusta and S. kingae cannot be considered isochronous from basin to basin. Within particular basins, endemic taxa having restricted stratigraphic ranges, such as G. crassa alemanensis, can be useful biostratigraphic markers. Streptelasma divaricans remains useful as a Richmondian index fossil in strata within the area occupied by the Richmond Province.