A PENNSYLVANIAN (MORROWAN) OSTRACODE FAUNA FROM TEXAS

Journal of Paleontology, Jan 2004 by Hoare, Richard D, Merrill, Glen K

ABSTRACT-A Lower Pennsylvanian (Morrowan) silicified ostracode fauna occurs just above the conformable Mississippian-Pennsylvanian boundary within the Barnett Formation in Texas. The fauna contains several taxa that were previously restricted to the Upper Mississippian and numerous taxa only known from the Lower Pennsylvanian. New taxa proposed include Amphissites (Amphissites) morrowanensis n. sp., Polytylites subrectus n. sp., Kirkbyella (Berdanelld) delicata n. sp., Kindlella proscillata n. sp., Libumella walkerorum n. sp., Moorites tumidus n. sp., Leptoderos arytaina n. gen. and sp., and Cribroconcha prolixa n. sp.

INTRODUCTION

AN ABUNDANT and diverse ostracode fauna was obtained from the upper Barnett Formation exposed on the J. R. Walker Ranch, which is located approximately 7.0 km southwest of Lampasas, Lampasas County, Texas (Fig. 1). The precise location is 31�02�17''N, 98�15'06''W or 14RNK71403387 on the Nix 7 �' quadrangle. The locality is on the east side of a stock tank in the northwest-facing fault scarp that is one of the many along the northern edge of the Llano Uplift resulting from late Paleozoic, post-Ouachita extensional tectonics. This particular fault block is surrounded by Cretaceous sedimentary rocks and is one of the most northeasterly outcrops properly assigned as part of the Llano Uplift. It exposes lithostratigraphic units from the Ellenburger Group to the Marble Falls Limestone. The upper portion of the Barnett Formation straddles the Mississippian-Pennsylvanian boundary (Fig. 2). The uppermost portion differs lithologically and geochemically from the bulk of the Barnett Formation here and elsewhere. It represents one of a half dozen stratigraphic intervals in the Carboniferous of central Texas that contain elevated levels of iridium, other noble metals, and rare earths (Grayson and Merrill, 1988). The 1.5 m at the top of the Barnett Formation at this locality is so enriched (Orth et al., 1987).

The sample was taken from a thin (1-4 cm) limestone lens with phosphatic oolites within the phosphatic greensand. Acetic acid residues of this limestone yield exquisitely preserved conodonts and the silicified ostracodes upon which this study is based. Surrounding rocks contain no ostracodes and much more poorly preserved conodonts, indicating that early cementation and silicification of this lens led to enhanced preservation, as opposed to surrounding sediments where taphonomic and diagenetic vagaries during stratigraphic condensation in a somewhat starved basin took their toll. The sampled limestone is 67 cm above the base of the Pennsylvanian based on the first occurrence of Declinognathodus noduliferus (Ellison and Graves, 1941) as defined by Baesemann and Lane (1985, p. 98) and approved by the IUGS subcommisson on Carboniferous stratigraphy (Wagner et al., 1985, p. 60).

PREVIOUS WORK

The early history of Mississippian-Pennsylvanian ostracode studies has been well documented by Robinson (1978) and Sohn and Jones (1984) indicating taxonomic, biostratigraphic, and paleoecological usefulness. In addition to the numerous studies referred to in those papers are several others of that time frame pertinent to the present study [e.g., Batalina (1926), Roth (1928), Kegel (1932), Harris and Lalicker (1932), Swartz (1936), Scott and Borger (1941), Egorov (1950), Hennigsmoen (1953), Elias (1958), Rozhdestvenskaja (1959), Erlich (1964), Bless and Jordan (1970, 1972), Schallreuter (1973), and Knox (1977)] which are mainly taxonomic studies.

More recently, taxonomic studies by Sohn (1977), Dewey and Fahreus (1987), Christopher et al. (1990), Dewey (1992), Becker (1997), Hoare et al. (1999), Olempska (1999), and Hoare and Mapes (2000); biostratigraphic studies by Bless (1983), Sohn (1983, 1986), Melnyk and Maddocks (1988b) and Jones (1989); and paleoecologic studies by Melnyk and Maddocks (1988a), Becker and Bless (1990), Dewey et al. (1990), and Dewey and Puckett (1991) have been useful.

Ostracode classification is still in a state of flux since that published in 1961 in the Treatise of Invertebrate Paleontology edited by Moore. The classification by Abushik et al. (1990), on Paleozoic ostracodes, appears to be overly split up at the family level. Whatley et al. (1993) published a suprageneric classification which still contains several uncertain allocations. During the past several years Becker published numerous papers related to the classification of various families which culminated in his 2002 paper on a classification scheme for Paleozoic ostracodes, which also includes uncertain assignments at the ordinal, subordinal, and superfamily levels. Progress on a revision of the Treatise volume has been discontinued, which is unfortunate in terms of ostracode studies, including classification (R. L. Kaesler, personal commun., 2002).

THE FAUNA

The 31 taxa present (Table 1) indicate a diverse and abundant fauna with some Mississippian relationships. Gortanella longispina (Jones and Kirkby, 1886b), Polytylites superus (Croneis and Gale, 1939), P. trilobus (Croneis and Gale, 1939), the subgenus Berdanella Sohn, 1961b, Proparaparchites fubulus Cooper, 1941, and Bairdia girtyi Sohn, 1960, all were previously restricted to Mississippian occurrences. Sixteen of the included Pennsylvanian species, as known at present, are restricted to Morrowan units.