CARYOCARIS (ARTHROPODA, PHYLLOCARIDA) FROM THE MAZARN SHALE AND ITS BIOSTRATIGRAPHIC POTENTIAL

A moderately well-preserved fauna of phyllocarid arthropods is present in a 0.5m interval of the Mazarn Shale (Early Ordovician, Ibexian) near Paron, Arkansas. The phyllocarid fossils consist of carapaces with posterior spinules and a stout, tapering carapace horn. One specimen consists of 3 abdominal segments articulated with a telson and furcal rami. The carapaces are preserved in a variety of perspectives (laterally and dorsal-ventrally compressed) as graphitic carbon compressions that are lightly phosphatized on bedding planes of dark gray shale. Some of the specimens display a reticulate texture that is likely to be in part original, and partly from post mortem deformation. The Mazarn was deposited in a deep marine setting near the continental slope. The phyllocarids occur in a thin interval which also contains abundant graptolites, including Phyllograptus typus and Glossograptus sp., and minor occurrences of inarticulate brachiopods. Together with its interpreted paleogeographic setting along the southern margin of Laurentia, the isolated nature of this fossiliferous layer suggests that this interval may represent a brief time of nutrient upwelling. Internal deformation resulting from Ouachita orogenesis strongly impacted the preservation of the Mazarn fauna, distorting the phyllocarid carapaces. Regular thecal spacings on P. typus were used to determine the absolute elongation of the graptolites in the plane of bedding. Because the phyllocarids are preserved in the same beds as the graptolites, we were able to determine the original dimensions of the carapaces by using the calculated elongation from P. typus. The overall morphology of the phyllocarids combined with corrected dimensions support the identification of these phyllocarids as Caryocaris curvilata (some authors place C. curvilata in synonymy with C. maccoyi). Examination of previously published material described as C. curvilata from the Caryocaris Shale of Nevada and new collections from apparently coeval strata in Nevada reveals that the Caryocaris from the Caryocaris Shale is C. stewarti, not C. curvilata. Caryocaris curvilata appears to be restricted to the late Ibexian and early Whiterockian, and C. stewarti is an early Ibexian species. These observations suggest that caryocarids may be biostratigraphically useful.