Crinoids of the Oil Creek Formation, lower Middle Ordovician (Whiterockian/Darriwilian), Arbuckle Mountains and Criner Hills, southern Oklahoma

Abstract The Middle Ordovician Oil Creek Formation in the Arbuckle Mountains of southern Oklahoma has been well known for its echinoderm-rich limestones for decades. Despite the abundance of separate plates that accumulated in shallow-water, storm-swept environments, complete specimens are rare. Consequently, no new echinoderm taxa were described from the formation prior to the work of the authors. Construction of Interstate Highway 35 in the late 1960s provided rare access to relatively fresh exposures in the 1970s when many of the specimens described herein were collected. The site now is largely overgrown. Crinoids described in this paper include (1) the hybocrinid Guensburgocrinus lineatus n. gen. n. sp., which is apparently intermediate between Syndiasmocrinus Guensburg and Sprinkle, 2022 (Early Ordovician) and Hybocrinus Billings, 1857 (Late Ordovician); (2) ? Hybocrinus nanobrachiatus n. sp.; (3) three species of the cladid ? Palaeocrinus Billings, 1859; (4) two species of the cladid ?? Carabocrinus Billings, 1857, both lacking radial-oral plate slits; and (5) the camerate Reteocrinus oklahomensis n. sp., the oldest known species in the genus. Additional crinoid diversity is indicated by unassigned separate calyx plates. Crinoids appear associated with several species of blastozoans including parablastoids, rhombiferans, diploporitans, and paracrinoids. UUID: http://zoobank.org/eb6434b1-05dd-4c7a-8007-a62680C66860