452
Journal of Paleontology 92(3):442–458
Diagnosis.—Length of genal spine at least one-fifth that of sagittal cephalic length. Posterior margin of glabella drawn out posteriorly into long occipital spine, estimated to be approxi- mately half glabella length (sag.).
Occurrence.—Collections ICS-10567 and ICS-10568, upper part of the Murray Shale, locality CM3 (type locality, Appendix), Chilhowee Mountain, Blount County, Tennessee, U.S.A. Also from the Murray Shale in the closely adjacent Little River Gap area (Walcott, 1890, 1891; Appendix). These occurrences are provisionally assigned to the mid- to upper Montezuman Stage, Waucoban Series, traditional “lower” Cambrian of Laurentia (see above), which is likely to fall within provisional Stage 3, Series 2 of the developing global chronos- tratigraphic zonation of the Cambrian System (Peng et al., 2012).
Description.—Cephalon semicircular in outline; proximal portion of posterior cephalic margin oriented very slightly pos- teriorly when traced distally, distal portion flexed anteriorly by ~20° relative to proximal portion at rounded adgenal angle located less than half of distance from axial furrow to base of genal spine. Greatest observed cephalic length estimated to be ~18.8mm (sag.). Genal spine broad-based, inner margin of spine smoothly arcs into distal portion of posterior cephalic margin, base of spine transversely opposite posterior portion of lateral or posterior margin of LO; length unknown, but at least one-fifth cephalic length (sag.; Fig. 4.2–4.6, 4.8). Intergenal spine absent or reduced to small dorsal swelling on posterior cephalic border immediately distal to adgenal angle. Cephalic border of low dorsal convexity, poorly defined around entire cephalon by very shallow border furrow; width of anterior border opposite junction of ocular lobes with LA estimated to be slightly less than length (exsag.) of LO. Glabella bullet-shaped in outline, generally tapering anteriorly; ~74 −83% of cephalic length (sag.), preglabellar field short (sagittal length approxi- mately equal to or slightly more than that of anterior cephalic border). Maximum width of LA ~87% basal glabellar width (tr.). Posterior margin of glabella strongly convex posteriorly, drawn out posteriorly into long, broad-based occipital spine; length of occipital spine unknown, but estimated to be approximately half glabella length (sag.; Fig. 4.7). All glabellar furrows extremely shallow. SObarely incised over axis, deepest midway between sagittal line and axial furrow, extremely shal- low or not incised adjacent to axial furrow, abaxial end slightly anterior to adaxial end. LO subtrapezoidal, slightly widens anteriorly, lateral margins bow outward slightly; more-or-less confluent with L1 anterodistally, ~15–20% glabellar length (sag., excluding occipital spine). S1, S2, and S3 barely visible, shallower than SO, clearest abaxially, absent over axis. S1 approximately parallel to SO; S2 approximately transversely oriented; S3 oriented slightly posterolaterally when followed distally. L1 subtrapezoidal, slightly narrows anteriorly; length (exsag.) ~20% glabellar length (sag., excluding occipital spine). L2 subtrapezoidal, slightly narrows anteriorly; length (exsag.) ~15% glabellar length (sag.). L3 subquadrate to subtrapezoidal, slightly widens anteriorly; length (exsag.) ~10% glabellar length (sag.). Axial furrow shallow at lateral margins of LO and L1, shallows anteriorly, absent around anterior margin of LA. LA
slightly wider (tr.) than long (sag.), separated from extraocular area by a subtle break in slope, weakly convex dorsally; widest point at intersection with inner margin of ocular lobes. Weak parafrontal band extends around lateral and anterior margins of LA; anteriorly confluent with extremely weakly defined, broad plectrum; posteriorly merges with outer margin of ocular lobe. Each ocular lobe diverges from exsagittal line at ~42 − 51° (measured from most abaxial point of outer margin to anterior contact of outer margin with LA) or ~30° (measured from pos-
terior tip to contact of inner margin with glabella), crescentic, flat-topped, posterior tip approximately transversely opposite distal tip of SO or posterior portion of lateral margin of L1; anterior portion more subdued in relief than posterior portion, summit lower than LA and separated from it by break in slope; inner margin poorly defined from interocular area; ocular furrow not apparent. Interocular area slopes outwards and down away from glabella (subhorizontal on USNM 633932, Fig. 4.1); almost twice as wide (tr.) as ocular lobe and ~75 −110% width (tr.) of extraocular area opposite S1 (compare Fig. 4.5, 4.6 to Fig. 4.2). Intergenal ridge and posterior ocular line run poster- olaterally behind ocular lobe, converge at intergenal swelling. Fine granulations over entire surface on well-preserved speci- mens. Terrace lines on cephalic doublure at base of genal spines (Fig. 4.5, 4.6). Hypostome, rostral plate, thorax, and pygidium unknown.
Etymology.—Named for the location of its discovery, Chilhowee Mountain, Tennessee.
Materials.—The species is known from the holotype plus seven additional specimens: USNM 18446 (external mold; Fig. 4.8), USNM 633932 (internal mold; Fig. 4.1), USNM 645832 (part and counterpart; Fig. 4.2),USNM645833 (part and counterpart; Fig. 4.5, 4.6), USNM 645834 (part and counterpart; Fig. 4.7), USNM 645835 (external mold), USNM 645836 (counterpart).
Remarks.—Specimens of Buenellus chilhoweensis n. sp. are preserved as internal and external molds in shale. On some specimens, key morphological features such as the occipital spine are better exhibited on the external mold. Latex peels of the external molds were not made due to the friable nature of the shale: damage to the already very limited number of specimens available was deemed too likely to occur. Instead, internal and external molds of those specimens are figured herein. Buenellus chilhoweensis n. sp. is very similar to Buenellus
higginsi. The most striking differences are in the length of the genal spine (longer in Buenellus chilhoweensis n. sp. than in Buenellus higginsi) and in the size of the axial structure on the occipital ring (a long, prominent spine in Buenellus chilho- weensis n. sp. versus a much smaller spine or node in Buenellus higginsi). No obvious, consistent interspecific differences in other aspects of cephalic shape were observed. Quantitative exploration for any subtle interspecific difference in shape is rendered futile for several reasons. First, the general effacement of the cephalon of both species makes many morphometric variables hard to identify and consistently measure (e.g., width of the cephalic border, or dimensions of particular glabellar lobes). Second, the available sample size is cripplingly low for Buenellus chilhoweensis n. sp., so that the ability to discern
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