Middle Pennsylvanian, late Atokan-early Desmoinesian echinoderms from an intermontane basin, the Central Colorado Trough

Journal of Paleontology, Nov 1998 by Webster, G D, Houck, Karen J

Late Desmoinesian:

Upper part of Hermosa Formation (Girty, 1903):

Eupachycrinus? sp.

Latest Atokan:

Pinkerton Trail Limestone (Strimple and Miller, 1971): Synarmocrinus molasensis (Strimple and Miller, 1971) Aglaocrinus sp. cf. A. marquisi (Moore and Plummer 1940)

Aexitrophocrinus minuramulosus Strimple and Miller 1971

Desmoinesian:

Madera Formation (Tischler, 1963; Strimple, 1976) Ulocrinus sp.

Erisocrinus coloradoensis (Strimple, 1976)

Exaetocrinus maderaensis Strimple, 1976

Sciadiocrinus ovalis (Strimple, 1951)

Dicromyocrinus beldenensis is the oldest Pennsylvanian (late Atokan) crinoid known from Colorado (Table 1). The specimen is from an arenaceous and argillaceous grainstone float slab from the lower two-thirds of the Belden Formation. The cup lacks the stem and arms, but does not show evidence of abrasion. The Belden Formation consists of interbedded black shale, lime mudstones, and skeletal wackestone with thin anhydrite beds (DeVoto et al., 1986). To the south it increases in sandstone content and contains some turbidites. It was deposited in the Central Colorado Trough to the north of the Sawatch Uplift (DeVoto et al., 1986).

Preservation of the Minturn crinoids is basically good, except some specimens show diagenetic pressure solution pitting. Clays in the matrix are deformed by compaction, commonly have slickensided surfaces, and often partly enclose disarticulated ossicles and parts of the cup. Solution weathering in the high mountain environment of the Minturn area has shallowly to deeply etched the surfaces of exposed cups and disarticulated ossicles, many beyond identification.

Cromyocrinids dominate the Minturn faunas (Table 1) and each of the above faunal lists contain one or more cromyocrinid taxa, except for the occurrence of Eupachycrinus? sp. in the upper part of the Hermosa Formation. No detailed stratigraphic information, description or illustration of Eupachycrinus? sp. was given by Girty (1903). At that time the generic characters were loosely interpreted and a number of inadunate taxa were assigned to Eupachycrinus and subsequently reassigned to other genera as listed by Bassler and Moodey (1943). The identification of Girty's specimen is very questionable and will not be considered further.

The common occurrence to dominance of cromyocrinids in the late Atokan and Desmoinesian faunas of Colorado and the Desmoinesian part of the Naco Formation of Arizona (Webster and Lane, 1970; Webster, 1981) reflects their adaptation to the environmental conditions of the intermontane basins of the region, which includes the tolerance of some clays and fine to medium grained sand particles present in some of the limestones wherein the cromyocrinids are found. Cromyocrinids are common elements in many Carboniferous and Permian crinoid faunas worldwide (i.e., Moore and Plummer, 1940; Yakovlev and Ivanov, 1956; Strimple and Watkins, 1969; Lane and Webster 1966; among others). They are often associated with erisocrinid and pirasocrinid inadunates, and less commonly with flexibles (faunal lists in Bassler and Moodey, 1943; numerous papers by various authors describing faunas from the Midcontinent of the U.S. and Russia; Webster, unpublished data).