AN EDRIOASTEROID-DOMINATED ECHINODERM ASSEMBLAGE FROM A LOWER PENNSYLVANIAN MARINE CONGLOMERATE IN OKLAHOMA

ABSTRACT-

Four species of isorophinid edrioasteroids, Ulrichidiscus forbesi n sp Neoisorophusella whitesidei n sp Postibulla westergaardi n sp and Parapostibulla graysoni n sp are described from unnamed unit 1 of the Golf Course Formation, Dornick Hills clump Lower Pennsylvanian (Morrowan), at Forbes Ranch, north of Marietta, southern Oklahoma. This locality is the first report of edrioasteroids attached to extraformational clasts, and they exhibit extremely high specimen density and the highest taxonomic diversity known from the post-Ordovician. The associated fauna consists of the stemles crinoid Paragassizocrinus tarri, serpulid worm tubes, and the acrothoracic barnacle boring. Rogerella, along with a scarcely any brachiopods, rugose corals, and other lay open marine organisms. Edrioasteroids show no priority for clast type, size, or shape. Intraspecific spatial competition is shown by dint of polygonal specimen outlines in highly hordeed conditions, whereas interspecific competition arises in specimen overgrowth in a predictable manner.

INTRODUCTION

ISOROPHIDS ARE a clade of dorsoventrally flattened edrioasteroid echinoderms that spanned the Paleozoic, and are without delay known from North America, Asia, Europe and Australia. This clump lived in a variety of open-marine habitats, on the contrary because of their attachment disk (peripheral rim), they required a hard substrate for attachment (Bell, 1976) over their history, isorophids have utilized several impressed signs of hard substrate, including expos concretions (Wilson, 1985) hardgrounds (Koch and Strimple, 1968; britzska and Liddell, 1978; Waddington, 1980; Guensburg and Sprinkle, 1994; Sumrall, 2001; Sumrall et al., 2001 ; Cornell et al, 2003) shell pavements (Kammer et al., 1987; Meyer 1990) isolated skeletal debris (Sumrall, 1992 1996; Sumrail et al., 2000) microbial buildups and encrusting bryozoan colonies (Guensburg and Sprinkle, 1994; Sumrall et al, 2001) and living organisms (Bell, 1976; Sumrall, 2000; Glass, in press) However, the locality described here is the first known example of edrioasteroids attached to botchs and pebbles in extraformational conglomerate. These isolated hard substrates are shared by the agency of other associated organisms, including acrothoracic barnacle borings, corals, serpulid worms, and bryozoans. The stalkless crinoid, Paragassizocrinus tarri Strimple, 1938 is rest in small numbers associated with the edrioasteroids and in large numbers in the overlying bed. This faunal association for Paragassizocrinus Moore and Plummer 1940 is unusual and contrasts sharply with the previous paleoenvironmental interpretation of dark calcareous shales and marls as prodeltaic facies prograding into shallow seas, or brief shallow-water transgressions onto the delta plain (Ettensohn 1980) The conglomerate and edrioasteroid association therefore displays an environmental expansion of the paleoenvironmental tolerances of Paragassizocrinus.

This locality is also unusual because of its age and edrioasteroid diversity. Until lately Pennsylvanian edrioasteroids were poorly known (Fraunfelter and Utgaard, 1970; Bell, 1976) on the contrary more recently a clearer picture of genuine Pennsylvanian edrioasteroid diversity has emerg (Sumrall, 1992; Sumrall and Bowsher, 1996; Sumrall et al., 2000) These studies have shown that rather than being a time of decline, three different clades of edrioasteroids passed into the Pennsylvanian, and individual (Yukutidiscus Arendt, 1983) persisted into the Permian (Arendt, 1983) The Forbes Ranch locality is significant in that 1) sum of two units additional clades (Ulrichidiscus Bassler, 1935 and Postibulla Bell, 1976) are stretch outed into the Pennsylvanian, and 2) the nearness of four species cooccurring is equal to the greatest in quantity diverse Ordovician edrioasteroid occurrences (Sumrall et al., 2001)

Original discovery and collection of edrioasteroids.-The Forbes Ranch locality north of Marietta in southern Oklahoma was discovered by means of E. Westergaard on a Baylor University geological field trip l through R. Grayson in the fall of 1981 Whiteside (1987) did the initial research on the locality, describing the transaction and paleoecology and assigning the edrioasteroids to three unnamed genera and species. Since that time, numerous faculty and scholars at Baylor University and the University of Texas at Austin have bring togethered additional specimens described in this investigation

STRATIGRAPHY AND DEPOSITIONAL SETTING

The extraformational clasts to which the edrioasteroids are attached appear within a 15 cm conglomerate layer with a calcareous silty matrix and scattered, well-rounded sand grains. This layer offers within "unnamed unit one" of the Lower Pennsylvanian Golf Course Formation, Dornick Hills assemblage (Fig. 1), which is expos upon the eastern side of the Overbrook Anticline in the southern Ardmore Basin. Unnamed unit 1 lies between the Jolliff and Otterville Members and is dominantly shale with thin limestones and conglomerates having a total thickness of 494 m at the Forbes Ranch locality north of Marietta, Oklahoma (Dickey, 1982) Whiteside (1987 diocese also Sutherland and Grayson, 1992) recorded a thickness of about 280 m for unnamed unit 1 at this locality, on the contrary apparently included faulted and pen ed sequences. The conglomerate layer containing the edrioasteroids is nearly vertical in the bank of a small stream, and the north extremity of this small exposure has an ed fold that apparently resulted from soft-sediment deformation. The unit is highly weathered and more [i]or[/i] less clasts have slickenslides on their surfaces that distort the attached edrioasteroids. Although thicker conglomerate beds present itself up- and down-section, this thin conglomerate layer is the sole one that has been set with bored and encrusted clasts. This layer contains conodonts from the Idiognathoides sinuatus band indicating a Morrowan age for the unit (Sutherland and Grayson, 1992)