Blountia tennesseensis, Westrop & Eoff, 2020

Westrop, Madison Armstrong Stephen R. & Eoff, Jennifer D., 2020, Systematics of a survivor: the Cambrian kingstoniid trilobite Blountia Walcott, 1916 across the Marjuman-Steptoean (Guzhangian-Paibian) extinction interval in Laurentian North America, Zootaxa 4804 (1), pp. 1-79 : 15-16

Blountia tennesseensis n. sp.

Plates 13, 14

1965 Blountia montanensis Duncan, in Lochman & Duncan ; Rasetti, p. 57, pl. 9, figs 13–20.

Diagnosis. Cranidium strongly arched in lateral view. Frontal area short, equal to 20% (19–21) of cranidial length, and flexed steeply downward. Glabella occupies two-thirds (67%; 66–70) of cranidial width across palpebral lobes. Palpebral area of fixigena very narrow, equal to 24% (20–26) of glabellar width. Anterior branches of facial sutures nearly parallel between palpebral lobe and anterior border furrow; posterior branches diverge backward along curved path so that cranidial width at palpebral lobe equals 70% (69-73) of width between posterior corners of cra- nidium. Pygidium with axis of seven axial rings and terminal piece of at least two segments occupying 82% (80–84) of pygidial length. Border furrow faint; border maintains nearly even width along pygidial margin.

Name. For the type area of this species in Tennessee .

Material. All specimens are from the Nolichucky Formation , Rogersville , Hawkins County, Tennessee ( Rasetti 1965). The holotype cranidium ( USNM 144599 ; Pl. 14, figs 1–3) is from collection cnm/2. Paratype cranidium and two pygidia ( USNM 144599 , Pl. 13, figs 4–6, Pl. 14, figs 4–6) are from collection cnm/2, and a cranidium, pygidium and librigena ( USNM 144598 , Pl. 13, figs 1–3, 7–9, Pl. 14, figs 7, 8) from collection cnk/1 .

Occurrence. Crepicephalus Zone, Nolichucky Formation, Hawkins County , Tennessee at Rasetti’s (1965) localities cnk/1, cnm/2, cnn/1, and cnn/3.

Description. Cranidium strongly arched, with width at palpebral lobe equal to 87% (86–88) of cranidial length; expands posteriorly, so that width at palpebral lobe equal to 70% (69-73) of maximum width at posterolateral projection.Axial and preglabellar furrows finely etched grooves. Glabella roughly barrel-shaped, with well rounded anterior margin; width at palpebral lobe equal to 74% (72–77) of glabellar length; occupies 80% (79–81) of cranidial length and 67% (66–70) of cranidial width at palpebral lobe. Lateral profile convex, flexed steeply downward anteriorly. Glabella unfurrowed aside from merest hint of SO on some specimens (e.g., Pl. 13, fig. 1). Frontal area with convex anterior border occupying 65% (61–67) of length; anterior border furrow clearly defined, forwardly convex curve. Preglabellar and preocular fields steeply sloping; in lateral view, nearly continuous with slope of anterior part of glabella (e.g., Pl. 14, fig. 2). Palpebral area slopes down from axial furrow, width equal to 24% (20–26; lowest value in largest cranidium, Pl. 14, fig. 1) of glabellar width at palpebral lobe. Palpebral lobe centred in front of glabellar mid-length, equal to 21% (19–23) of glabellar length; palpebral furrow shallow, expressed in part by change in slope of palpebral area at palpebral lobe. Convex, oblique palpebral ridge expressed on some specimens (Pl. 13, fig. 1). Anterior branches of facial sutures nearly parallel between palpebral lobe and border furrow before curving inward along anterior cranidial margin. Posterior branches diverge along curved path, becoming nearly parallel at posterior border. Posterior area flexed steeply downward; posterior margin deflected backward. Shallow posterior border furrow nearly transverse at axial furrow but curves forward distally. Posterior border without independent convexity, expanding distally, with minimum width at axial furrow equal to 25% (18–30; lowest value on smallest cranidium, Pl. 13, fig. 4) of maximum width at sutural margin. External surface of exoskeleton smooth.

Free cheek with visual surface attached. Weakly inflated librigenal field slopes down to convex lateral border; border furrow well defined groove, shallowing slightly posteriorly. Border with terrace ridges at outer margin; remainder of exoskeleton smooth.

Pygidium semielliptical in outline, length equal to 61% (60–62) of maximum length; articulating facet at anteri- or corner. Axial furrows distinct grooves, shallowing somewhat posteriorly. Axis convex, tapering gently backward, width at first ring equal to 29% (28–29) of maximum pygidial width; occupies 82% (80–84) of pygidial length. Axial rings and ring furrows faint on external surface of exoskeleton (Pl. 13, figs 7–9), but expressed clearly on internal mould (Pl. 14, figs 5, 6). Seven rings plus terminal piece of at least two segments; transverse rings furrows deepest at axial furrows, shallowing slightly medially. Pleural fields gently convex. External surface with shallow pleural furrow at anterior; other furrows barely perceptible. Internal mould with faint, oblique pleural furrows. Border furrow shallow on external surface of exoskeleton; border slopes downward, maintains nearly even width. External surface smooth aside from a few terrace ridges at pygidal margin.

Discussion. The combination of a short frontal area, parallel anterior branches of the facial sutures, and a very narrow (tr.) palpebral area of the fixigenae separates cranidia of B. tennesseensis (Pl. 13, figs 1–6, Pl. 14, figs 1–3) from those of other species of Blountia . The pygidia (Pl. 13, figs 7–9, Pl. 14, figs 1–3) are most like those of B. newfoundlandensis (Pl. 21), which differ in showing adaxial expansion of the border and has a distinct border furrow. The border furrow of B. tennesseensis is ill defined even in small specimens.

The cranidium assigned incorrectly to B. montanensis by Pratt (1992, pl. 24, figs 13, 14; see earlier discussion of B. montanensis ) is close to B. tennesseensis in possessing a narrow palpebral area of the fixigena. However, it differs in having a shorter anterior border than both small (Pl. 13, figs 4–6) and large (Pl. 14, figs 1–3) specimens of the latter. The pygidium illustrated by Pratt (1992, pl. 24, fig. 15) is smaller than any of the types of B. tennesseensis . It has a border that narrows abaxially, whereas the border of B. tennesseensis maintains a far more even width (e.g., Pl. 13, figs 7–9; Pl. 14, fig. 4).