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

Although the crinoids of the Minturn Formation are a little older (latest Atokan to early Desmoinesian vs. Missourian and Virgilian) than the faunas reported by Pabian et al. (1989), at the generic level, they contain representatives of two of the assemblages that Holterhoff (1988) recognized. These genera have the same arm structure as their younger counterparts and should represent relatively similar feeding strategies and paleoenvironments. As noted above, the carbonates of the Minturn Formation are interpreted to represent the transgressive phase or highstand of a cycle with reduced elastic influx (Houck, 1993). However, reduced elastic influx in an intermontane fan-delta setting would probably be equivalent to moderate or perhaps high elastic influx in the Midcontinent. This suggests that the ability to feed within an environment of some elastic influx, whether at highstand or lowstand, controlled the composition of the crinoid fauna and community.

More recent interpretations of the feeding efficiency of the filtration fans (Baumiller. 1993: Holterhoff. 1997a. 1997b) susgest modification of the interpretations of Pabian et al (1989) are needed. The combination of these studies is applied to the Minturn faunas. Two types of filtration fans or guilds (Holterhoff, 1997a, 1997b) are present in the Minturn crinoids. Only the nonpinnulate flexible Aexitrophocrinus minuramulosus belongs to the open filtration fan guild. All other taxa have 10 or more pinnulate arms, with the density of the filtration fan depending upon the taxon, and belong to the high-density filtration-fan guild. Baumiller (1993) noted that the dense filtration fans operate at a maximum in higher energy environments whereas open filtration fans are more effective in lower velocity environments. The adult or gerontic specimens of all Minturn crinoids are relatively large forms. A search for microcrinoids in washed residues was unsuccessful. Pabian and Strimple (1970) found that Late Pennsylvanian crinoids of the Midcontinent showed a preference of large forms in the nearshore facies and small forms in the offshore facies. Holterhoff (1996, 1997a, 1997b) noted that diversity increased from nearshore settings to mid-shelf before decreasing into outer shelf regimes. It should be noted that the noncolunal generic diversity (2-7) of the Minturn faunas is less than half of the minimal average (15) for the Midcontinent nearshore faunas as given by Holterhoff (1996, 1997b). The combination of dominantly dense filtration fan adaptations, large form, and low diversity for the Minturn crinoid fauna suggests a nearshore, high-energy environment. For these reasons, we interpret the Minturn crinoids as nearshore faunas; they are not interpreted as mid-shelf and offshore faunas. Lacking a more comprehensive study of intermontane faunas and recognition of a greater number of faunas from nearshore to offshore settings in the intermontane environment, it is inappropriate at this time to make a direct comparison of the Minturn faunas with those of two midcontinent nearshore settings.