CONODONTS FROM THE MIDDLE ORDOVICIAN OIL CREEK FORMATION, SOUTH-CENTRAL OKLAHOMA

The Oil Creek Formation consists of more than 190 m of carbonate and siliciclastic rocks in outcrops in the western Arbuckle Mountains, south-central Oklahoma. The formation represents a transgressive-regressive cycle produced in a shallow, tropical sea that covered the eastern Midcontinent during the Middle Ordovician (Whiterockian Series). Sixty-four samples taken at approximately 3-m intervals from the Oil Creek yielded a collection of over 45,000 conodont elements. These elements are assigned to at least 25 species, 3 of which represent new species.

The Oil Creek conodont fauna is dominated (over 80%) by Neomultioistodus compressus Harris and Harris, Trigonodus sinuosus (Mound), and Drepanoistodus angulensis (Harris). N. compressus is joined by a new species of Neomultioistodus in the upper Oil Creek. Other notable species represented in the collection include Ansella jemtlandica (Lofgren), Chosonodina rigbyi Ethington and Clark, Dischidognathus primus Ethington and Clark, Fahraeusodus marathonensis (Bradshaw), Oistodus multicorrugatus Harris, Oistodus cristatus Ethington and Clark, Oistodus n.sp., Parapanderodus striatus (Graves and Ellison), Paraprioniodus costatus (Mound), Paraprioniodus n.sp., Protopanderodus gradatus Serpagli, Pteracontiodus cryptodens (Mound), Pteracontiodus gracilis Ethington and Clark, and four species of Histiodella.

The large collections of Histiodella from the Oil Creek offer a unique perspective on the evolution of this biostratigraphically important genus. Histiodella is characterized by a skeletal apparatus composed of geniculate coniform, ramiform, and carminate pectiniform elements. Histiodella serrata Harris, H. sinuosa (Graves and Ellison), H. n.sp., and H. holodentata Ethington and Clark are represented in the Oil Creek. Species assignment is based on characteristics of the carminate pectiniform elements. Other elements in the apparatus show modification but are rare and tend to be more conservative. In addition to modification of individual elements, the Histiodella lineage shows a trend toward decreasing relative abundance of ramiform and geniculate coniform elements. Collections of Histiodella n.sp. and H. holodentata from the middle to uppermost Oil Creek contain no ramiform elements and few geniculate coniform elements.