Webster and Hageman—Buenellus chilhoweensis n. sp. (lower Cambrian Chilhowee Group)


443


trilobite. This exposure is located in one of the general areas described by Walcott (1890) as a source for his initial fossil discoveries, and might even represent a re-discovery of the original fossil-bearing locality (Hageman and Miller, 2016; see below). Hageman and Miller (2016, p. 146, fig. 7d) briefly documented the discovery of the locality and illustrated the new specimen, but no formal description of the taxon was provided. Subsequent visits to the locality yielded several additional spe- cimens. Herein, we provide a formal description of that trilobite —named Buenellus chilhoweensis n. sp.—and review other body fossil occurrences within the Murray Shale. We demon- strate that Buenellus chilhoweensis n. sp. is the oldest known trilobite from the Iapetan margin of Laurentia, and we discuss the significance of the trilobite in terms of the much-needed biostratigraphic constraint it provides on the timing of events during the early evolution of that margin.


Geologic setting, lithostratigraphy, and age of the Chilhowee Group


Figure 1. Map of eastern U.S.A. showing trend of Ediacaran to lower Cambrian Chilhowee Group (gray shading) in southern and central Appalachians. Star symbol indicates location of Chilhowee Mountain, Blount County, Tennessee, where the fossils discussed herein were collected.


(e.g., Walcott, 1910; stratigraphic divisions for the Cambrian of Laurentia follow Palmer, 1998). However, subsequent sys- tematic revisions have greatly restricted the inclusivity of the genus (e.g., Palmer and Repina, 1993; Palmer and Repina in Whittington et al., 1997; Lieberman, 1998, 1999). With the recent reassignment of many species of “Olenellus” sensu lato to other genera, occurrences of Olenellus sensu stricto are apparently restricted to the mid- and upper Dyeran (provisional Cambrian Stage 4; Peng et al., 2012) (Webster, 2011 and references therein; Webster and Bohach, 2014). The historical records of “Olenellus” within the Chilhowee Group must, therefore, be re-evaluated in light of modern systematics in order to exploit their full biostratigraphic potential. Unfortu- nately, re-evaluation of the Murray Shale record is hampered by: (1) the absence of any description or illustration of specimens; and (2) the failure of subsequent workers to collect any addi- tional trilobite material, despite concerted efforts. The lack of success is due in part to poor and confusing descriptions of field localities (see below) and the apparent rarity of specimens. Indeed, several workers have expressed doubt regarding the supposed stratigraphic provenance of the material reported by Walcott and Keith (see below). In 2016, SJH discovered an exposure of the Murray Shale on Chilhowee Mountain that yielded a cephalon of an olenelline


Following the late Neoproterozoic breakup of Rodinia, the newly formed Iapetan margin of Laurentia evolved from a tectonically active rift margin to a passive, thermally subsidingmargin (Rankin, 1976; Thomas, 1977). The Ediacaran through lower Cambrian stratigraphic succession of the southern and central Appalachians records this rift-to-drift transition (Figs. 1, 2; Thomas, 1977, 2014; Mack, 1980; Bond et al., 1984; Simpson and Sundberg, 1987; Simpson and Eriksson, 1989, 1990). The extensional rift phase is represented in Tennessee by the Neoproterozoic Ocoee Super- group, which is a sequence of turbidites and mass flow deposits that accumulated in a large intracratonic rift basin (Tull et al., 2010; Thomas, 2014 and references therein). The overlying Chilhowee Group represents the basal siliciclastic portion of the initial trans- gressive depositional cycle (Sauk Sequence; Sloss, 1963) that blanketed the Iapetan margin during the thermal subsidence phase. Although sedimentary facies are laterally variable in thickness and composition (Walker et al., 1994), and stratigraphic nomenclature varies from region to region (Mack 1980), the Chilhowee Group can be considered in three successive packages. The lower Chilhowee Group, 400–1200m thick, consists


of the laterally equivalent Unicoi and Cochran formations in Tennessee and southwestern Virginia (Fig. 2.1). The Weverton Formation of northern Virginia, Maryland, and Pennsylvania has usually been considered to be a northern lateral equivalent of the lower Chilhowee Group (e.g., King, 1949; King and Ferguson, 1960; Cudzil and Driese, 1987; Walker and Driese, 1991), but has recently been proposed to correlate to the younger Nebo Quartzite (Smoot and Southworth, 2014). The lower Chilhowee Group formed as coalescing alluvial fans, braided stream, and overbank floodplain deposits with local mudflows in fluvial, deltaic, to shallow marginal marine environments (Mack, 1980; Simpson and Eriksson, 1989, 1990; Tull et al., 2010; Smoot and Southworth, 2014). Undated amygdaloidal basalt flows are locally present in the braidplain sediments (lower and middle) part of the Unicoi Formation in northeastern Tennessee and southwestern Virginia, but the upper Unicoi Formation probably represents an early phase of transgressive sedimentation on a passive margin (Simpson


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