REAPPRAISAL OF ECHINODERMS FROM THE OIL CREEK FORMATION, MIDDLE ORDOVICIAN (WHITEROCKIAN/DARRIWILIAN), ARBUCKLE MOUNTAINS AND CRINER HILLS, SOUTHERN OKLAHOMA

The limestone beds of the Oil Creek Formation in southern Oklahoma were described in the 1930s as consisting largely of echinoderm skeletal remains, but no new echinoderm taxa were described until the 1980s because of the scarcity of complete specimens. By 1974 only a few articulated specimens had been found in ranch and roadside exposures. Field work done in 1974-1979 for a Ph.D. dissertation (Lewis, 1982) relied heavily on a single Interstate-35 roadcut in the Arbuckle Mountains; this was relatively fresh at that time but now is largely overgrown. Most of the complete specimens came from a storm-dominated, lower-offshore facies characterized by megaripples and overturned clasts often covered with holdfasts. This unique roadcut is no longer productive, but related echinoderms have now been found from earlier units in western states. A manuscript in preparation describes the unpublished Oil Creek echinoderms and compares them with related taxa from other units.

Published Oil Creek taxa from the dissertation are the crinoid Archaetaxocrinus burfordi (Lewis, 1981) and the rhipidocystid eocrinoid Mandalacystis dockeri (Lewis et al., 1987). Unpublished blastozoans include two specimens of a new asteroblastid diploporan genus; many deltoid plates and two bibrachials of a parablastoid later named Parabolablastus elongatus (Sprinkle & Sumrall, 2008); two partial thecae of a thin-plated, pustular, new paracrinoid?; and distinctive plates of the glyptocystitid rhombiferan Hadrocystis n. sp. Crinoids include a large partial crown of the camerate Reteocrinus n. sp.; three calyces of two new species of the hybocrinid Hybocrinus; a possible new species of the cladid Palaeocrinus; and a possible new species of the cladid Carabocrinus with no? RR-OO slits.

The Oil Creek echinoderm fauna most closely correlates with that in the Kanosh Shale (western Utah) and the Antelope Valley Formation (central Nevada), which share many of the same groups and some of the same genera. Echinoderm diversity in Laurentia slowly increased in the Early and Middle Ordovician before radiating to much higher levels in the better-known Late Ordovician. This diversity pattern is somewhat different than that seen in brachiopods or bryozoans. It is unclear what role preservation and facies may have played in this diversity pattern.