Microfossils from the upper Sylvan Shale (Late Ordovician), Oklahoma, USA
Mats Eriksson1 and Stephen A. Leslie2
1 Department of Geology, University of Lund, Sölvegatan 13, S–223 62 Lund, Sweden. E–mail: Mats.Eriksson@geol.lu.se
2 Department of Earth Sciences, University of Arkansas at Little Rock, 2801 South University Little Rock, Arkansas 72204–1099. E–mail: email@example.com
Key words: Scolecodonts. Conodonts. Sylvan Shale. Upper Ordovician. Oklahoma.
We present new information on scolecodonts and conodonts from the upper part of the Sylvan Shale of Oklahoma, U.S.A. Previous studies of the Upper Ordovician Sylvan Shale (Richmondian = Ashgill) (Amplexograptus manitoulinensis to the Dicellograptus complanatus graptolite zones; Amorphognathus ordovicicus conodont zone) have focused on its potential as a source rock for hydrocarbons and part of its faunal content (e.g., inarticulate brachiopods, graptolites, chitinozoans, acritarchs, and possible foraminifera). The Sylvan Shale is extremely meagre macroscopic sessile or vagrant shelly fossils content is most likely a result of a thixotropic sediment water interface. The greenish–grey Sylvan Shale represents a transgressive succession that was laid down in an epicratonic setting within, and along, the southern Oklahoma aulacogen.
Microfossils from the upper 30 meters of the Sylvan Shale at a locality south of the intersection of I–35 and US Highway 77 in Oklahoma were studied. Scolecodonts dominate the fauna, but chitinozoans, some organic spherules, and the possible foraminiferan, Thuramminoides, were also recovered. Conodonts are very rare. We recovered only ten identifiable elements belonging to two species, Plectodina tenuis and Amorphognathus sp., all showing CAI = 1, indicating heating temperatures of less than 50–80° C.
Scolecodonts from the Sylvan Shale were previously virtually unknown. The recovered specimens indicate a low abundance (highest c. 30 specimens/kg) and a low diversity jawed polychaete fauna. The recovered 250 specimens belong to eight genera, Kettnerites, Hadoprion, Oenonites, Atraktoprion, Leptoprion, Kalloprion, Protarabellites, Ramphoprion, and six families. The assemblage is strongly dominated by the new paulinitid species Kettnerites sp., generally forming more than 50% of the scolecodonts, and differs rather conspicuously from other assemblages of both Laurentia and Baltica, particularly by its high relative frequency of paulinitids and by the lack of taxa characteristic in other regions. Approximately coeval assemblages (Vormsi and Pirgu stages) from the eastern Baltic region are dominated by the placognath genera Mochtyella, Vistulella, and Xanioprion, and the labidognath genus Pteropelta. By comparison, the late Richmondian faunas of the type Cincinnatian region, U.S.A., are dominated by polychaetaspids and particularly Oenonites species, which are very rare in the Sylvan Shale. These differences most likely result from differences in environmental setting. It appears that bottom conditions, which likely limited the macroscopic sessile and vagrant shelly fauna either directly or indirectly, affected the species composition of the polychaete fauna. This also indicates that different species had different modes of life (e.g., benthic, nekto–benthic, nektonic). Additionally, the rarity of conodonts is intriguing because it suggests that either the bottom conditions limited the food source of these organisms, or perhaps the bottom condition itself was limiting to the mode of life of conodonts.
Received: February 15, 2003
Accepted: June 15, 2003