Webster and Hageman—Buenellus chilhoweensis n. sp. (lower Cambrian Chilhowee Group)
equivalent) of the original 1889 site. However, lack of sufficient exposure currently hinders such a test.
Refined age assignment for the Murray Shale.—Buenellus chilhoweensis n. sp. is known only from the vicinity of the type locality and therefore offers no current utility for species-level correlation or biostratigraphy. However, the phylogenetic affinity of Buenellus chilhoweensis n. sp. can be used—with caveats—to indirectly constrain the age of the Murray Shale. The new species is hypothesized to be most closely related to Buenellus higginsi Blaker, 1988, the type and only other known species of Buenellus Blaker, 1988 (see Systematic Paleontology section). Buenellus higginsi is known only from the Sirius Passet Lagerstätte in the lower portion of the Buen Formation of North Greenland (Blaker and Peel, 1997; Babcock and Peel, 2007; Ineson and Peel, 2011). That Lagerstätte has been con- sidered to belong to the middle to upper part of the Montezuman Stage, based on the fact that it bears a trilobite (and is therefore younger than the pre-trilobite Begadean Series) and is below strata that contain early Dyeran trilobites (Palmer and Repina, 1993; Blaker and Peel, 1997; Palmer and Repina inWhittington et al., 1997; Babcock and Peel, 2007); acritarch biostratigraphic data are also consistent with that age assignment (summarized by Babcock and Peel, 2007). To the extent that such closely related species as Buenellus higginsi and Buenellus chilho- weensis n. sp. are likely to be of generally similar geologic age, the upper part of the Murray Shale is provisionally hypothesized to be of mid- to upper Montezuman age (i.e., between ~518Ma and 515.5Ma sensu Peng et al., 2012; Fig. 2.2). A Montezuman age for Buenellus chilhoweensis n. sp. is also congruent with the biostratigraphic constraints on the age of the Murray Shale imposed by other sources of data (see above): we are unaware of any data that unambiguously contradict this age inference. The hypothesis that the Murray Shale (or at least its upper
part) is coarsely age-equivalent to the lower part of the Buen Formation comes, of course, with the non-trivial caveat that the correlation is based solely on the two lithostratigraphic units that were deposited in widely separated basins on the Iapetan and Innuitian margins of Laurentia, respectively, sharing a genus in common. A hypothesis of age-equivalence of strata is most robust when those strata are from geographically closely spaced sections and share species in common, because under such conditions the assumptions regarding isochrony of local stratigraphic ranges are less prone to dramatic violation (e.g., Landing et al., 2013). We therefore do not claim precise age- equivalence of the upper Murray Shale and lower Buen Formation within the Montezuman Stage (although such equivalence is possible), and we stress that our provisional age assignment for the Murray Shale (Fig. 2.2) is a working hypothesis that should be tested with additional data.
Materials and methods
Repositories and institutional abbreviations.—All Murray Shale trilobite specimens in this study are housed in the paleo- biology collection of the U.S. National Museum of Natural History (USNM). The first specimen was found in situ on the exposure; all other specimens were recovered from bulk samples extracted from the outcrop. The bulk samples were
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taken from a stratigraphic interval ~50 cmthick that included the horizon on which the first specimen was found. Comparative data for species belonging to other nevadioid genera were obtained through study of specimens housed within the collec- tions of the Institute for Cambrian Studies (ICS), University of Chicago. Fossil-bearing localities within the Murray Shale are described in the Appendix.
Morphometric data.—Traditional morphometric data (linear dimensions and angles) were taken from digital images of spe- cimens (for the Tennessee material) or from scans of published images (for Buenellus higginsi). Data were collected using the ImageJ software (
http://rsb.info.nih.gov/ij/index.html). Values for some variables were estimated on incompletely preserved or moderately effaced specimens, but only when those estimates were replicable within a small margin of error (typically <0.05mm on large cephala). Values for variables relating to transverse measurements that span the sagittal line were obtained on some specimens by doubling a transverse mea- surement from the sagittal line to one end-point of the variable. Such estimates are designated as “approximate” values in the description. Measurement error introduced through these approximations is likely to be negligible.
Terminology.—The morphological terminology applied herein largely follows that of Palmer and Repina (1993) and Whittington and Kelly in Whittington et al. (1997), with modifications to olenelline terminology proposed by Webster (2007a, b, 2009) and Webster and Bohach (2014).
Systematic paleontology
Order Redlichiida Richter, 1932 Suborder Olenellina Walcott, 1890 Superfamily “Nevadioidea” Hupé, 1953
Remarks.—Palmer and Repina (1993; also Palmer and Repina in Whittington et al., 1997) included Buenellus alongside Nevadia Walcott, 1910, Nevadella Raw, 1936, Cirquella Fritz, 1993, and Pseudojudomia Egorova in Gorjansky et al., 1964 within the Family Nevadiidae Hupé, 1953. That familial desig- nation has been accepted by most other workers (e.g., Blaker and Peel, 1997; Jell and Adrain, 2003; Babcock and Peel, 2007). Lieberman (2001) found that taxa traditionally assigned to the Nevadiidae formed part of a paraphyletic grade between the Fallotaspidoidea Hupé, 1953 and [Olenelloidea Walcott, 1890+ Judomioidea Repina, 1979]; he termed that grade the “Nevadioidea” Hupé, 1953 and did not define families within it. Relationships among “nevadioids” are far from settled, however: Buenellus was not included in that cladistic analysis, for example; nor was Limniphacos Blaker and Peel, 1997, another possible nevadiid from North Greenland. A forth- coming, more comprehensive cladistic analysis of olenelline trilobites will resolve relationships among these taxa (Webster, in preparation). Pending publication of that new analysis, and given the uncertainty over monophyly of the traditional Neva- diidae, we herein conservatively avoid family-level assignment within the “Nevadioidea.”
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