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

Abstract

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.

Chapter 9 Coral biogeography in the Late Ordovician (Cincinnatian) of Laurentia

“…In total through its history, the Red River -Stony Mountain Province occupied a vast area of Laurentia (Elias 1981(Elias , 1983(Elias , 1985(Elias , 1991(Elias , 1995. This includes the central and western USA and most of Canada ( Fig.…”

“…It is considered to be the ancestor of the characteristic Red River -Stony Mountain species G. robusta ( Fig. 9.3a), which was widespread in cratonic areas of the province and gave rise to numerous other species (Elias 1985). The major early Cincinnatian transgression provided an opportunity for corals to disperse from the continental margin into the area of the Red River -Stony Mountain Province, and new, endemic species appeared during that event.…”

“…Some characteristic Red River -Stony Mountain genera and species dispersed widely within the province, but others had limited distributions. Evolutionary events that yielded species and subspecies endemic to particular depositional areas contributed to the high provincial diversity (Elias 1985(Elias , 1991. There was greater communication between some places than others.…”

“…Corals are incompletely known in the Red River -Stony Mountain Province (e.g. Flower 1961;Nelson 1963Nelson , 1981Elias 1985) and especially in the 'Continental Margin' Area (e.g. Dixon 1974;Bolton 1980;Elias et al 1994).…”

Extinction, invasion, and sequence stratigraphy: Patterns of faunal change in the Middle and Upper Ordovician of the eastern United States

“…The timing of the extirpations and immigration event correspond with those seen among the brachiopods, except that none of the corals made it into the C1 sequence as did two of the brachiopod genera. Corals are known to occur throughout Mohawkian and Cincinnatian strata in the central and western United States and Canada (Browne, 1964(Browne, , 1965Elias, 1982Elias, , 1983Elias, , 1985 and indicate that the regional extirpations resulted in a geographic range restriction of these taxa. Stratigraphic ranges for corals in the central Appalachians are poorly known and will require further study.…”