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


statistical significance for any subtle differences between the species is greatly compromised. Finally, both taxa are only known from moldic specimens preserved in shale, and all specimens exhibit some degree of breakage or deformation induced by taphonomic compaction. This not only further reduces sample size for quantitative analysis (because not all variables can be reliably measured on all specimens), but also complicates the interpretation of any such analyses. Compaction-related deformation is known to inflate the degree of variation seen in fossils (see Webster and Hughes, 1999 and Webster, 2015 for quantitative analyses of the effects of compaction on cephalic shape in olenelloid trilobites, and see Babcock and Peel, 2007 for a discussion of compaction-related variation in Buenellus higginsi). The high degree of variation in proportional width of the extraocular area in Buenellus chilhoweensis n. sp. (compare Fig. 4.5, 4.6 to Fig. 4.2) probably relates to different collapse patterns in response to compaction of an originally strongly convex extraocular area (similar to that seen in non-compacted specimens of Pseudojudomia egregia). The differences in genal spine size and form of the axial


structure on the occipital ring are sufficiently marked that they are robust against such taphonomic issues and thus provide a defensible means for diagnosing the Appalachian form as a distinct species. These differences are interpreted as interspe- cific rather than ontogenetic in nature because they are expressed in comparably sized specimens (sagittal cephalic lengths range from ~10.7mm to ~18.8mm in the sample of Buenellus chilhoweensis n. sp., and from ~5.6mm to ~24.1mm in the studied sample of Buenellus higginsi).


Discussion


Buenellus chilhoweensis n. sp. is the oldest known trilobite from the Iapetan margin of Laurentia. Occurring within the Murray Shale, Buenellus chilhoweensis n. sp. is older than the trilobites found in the uppermost Chilhowee Group (Helenmode/Anti- etam Formation) of the southern and central Appalachians (see above; Fig. 2.2). The oldest reported trilobites in the northern Appalachians of the U.S.A. occur in the Cheshire Formation of Vermont, Massachusetts, and New York State (Dwight, 1887; Walcott, 1888; Gordon, 1911; Shaw, 1954; Knopf, 1962; Landing, 2007, 2012). The Massachusetts occur- rence documented by Walcott (1888), however, might be sourced from the top of the stratigraphically older Pinnacle Formation, as noted by Landing (2007, 2012, and references therein). Landing (2007, 2012) also reported an unidentifiable trilobite fragment from the top of the Bomoseen Member of the Nassau Formation in the Taconics of New York State, which is believed to be age-equivalent to the upper Pinnacle Formation. All those specimens were historically identified (sometimes tentatively) as “Olenellus” (Dwight, 1887; Walcott, 1888; Gordon, 1911) and have been taken to infer a Dyeran age (Landing, 2007, 2012). The northern Appalachian occurrences and identifications are currently being re-evaluated (Webster and Landing, in preparation), but to date no specimens have been observed that would indicate a Montezuman age. The oldest trilobites known from western Newfoundland and Lab- rador occur in the basal Forteau Formation (Schuchert and Dunbar, 1934; North, 1971; Stouge and Boyce, 1983), for


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which a mid-Dyeran age is well established (Palmer and James, 1979; Debrenne and James, 1981; James et al., 1989). The oldest trilobites reported from eastern Greenland occur in the Bastion Formation and are also of mid-Dyeran age (Poulsen, 1932; Cowie, 1971; Skovsted, 2006; Stein, 2008). The apparent absence of Montezuman trilobites from the


Iapetan margin of Laurentia was curious, given that trilobites of that age have been reported from the adjacent Innuitian margin (Buenellus higginsi, see above) and that Montezuman-age trilobites—including fallotaspidoids, which occur in lower- most Montezuman strata—are diverse and abundant along the Cordilleran margin (e.g., Fritz, 1972, 1973; Nelson, 1976, 1978; Hollingsworth, 2005, 2007, 2011). Was there some paleobio- geographic reason why trilobites did not invade the Iapetan margin until much later? Discovery of Buenellus chilhoweensis n. sp. resolves that dilemma: trilobites did inhabit the Iapetan margin, at least locally, during the Montezuman. Trilobites in rocks of this age are clearly difficult to find in the Appalachians, but the Chilhowee Mountain discovery demonstrates their pre- sence and encourages further effort in the field to more fully document their early history there. Detailed correlation of lower Cambrian strata between the


Iapetan and Cordilleran margins of Laurentia has proven diffi- cult. The difficulty arises in part from the absence of a detailed and widely applicable biostratigraphic zonation of the Iapetan strata (although a local zonation scheme has been developed for the upper Dyeran of eastern New York State; Bird and Rasetti, 1968). Recent and ongoing fieldwork in the Appalachians is yielding new fossils that can promote the development of an Iapetan margin biostratigraphy and assist in circum-continental correlation (e.g., Hageman and Miller, 2016; Webster and Landing, 2016; this study). Sequence stratigraphic data might also prove useful in this endeavor. For example, attempts have been (and continue to be) made to correlate the late Dyeran “Hawke Bay Regression” around Laurentia and further afield (e.g., Palmer and James, 1979; Palmer, 1981; Landing et al., 2002, 2006; Bordonaro, 2003; Landing, 2012; Geyer and Vincent, 2015; Webster and Landing, 2016). Sequence strati- graphic interpretations and sometimes sea level curves are being developed for Montezuman and lower Dyeran strata of both the Iapetan (Whisonant, 1974; Mack, 1980; James et al., 1989; Barnaby and Read, 1990; Lavoie et al., 2003; Landing, 2007, 2012; Tull et al., 2010; see above) and Cordilleran margins (Hollingsworth 2005, 2007, 2011; Dilliard et al., 2007, 2010; English and Babcock, 2010; Webster and Bohach, 2014). Dis- covery of Buenellus chilhoweensis n. sp. within the upper part of the Murray Shale provides a valuable biostratigraphic calibra- tion for the depositional sequences recognized along the Iapetan margin. With this and future fossil discoveries in the Chilhowee Group, it might ultimately become possible to recognize time- equivalent sedimentary packages around Laurentia and thus add sequence stratigraphy to the arsenal of tools for high-resolution circum-continental correlation of lower Cambrian rocks. Finally, the occurrence of Buenellus chilhoweensis n. sp.


and Isoxys chilhoweanus in the Murray Shale, and of Buenellus higginsi and Isoxys volucris Williams, Siveter, and Peel, 1996 in the Sirius Passet Lagerstätte, is noteworthy. Buenellus is known only from those two units and therefore is either poorly sampled or appears to have been rather limited in its environmental


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