Blountia angelae, 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 : 17

Blountia angelae n. sp.

Plate 15

1965 Blountia mimula Walcott ; Rasetti, p. 59, pl. 10, figs 3–7.

Diagnosis. Cranidia with well-rounded anterior margin. Frontal area occupies 30% (30–31) of cranidial length (sag.), with down-sloping preglabellar field and flat border. Pygidium semielliptical in outline, length 64% (62–66) of maximum width. Down-sloping border long, occupying 14% (12–16) of pygidial length; border furrow well defined. Seven distinct axial rings in front of terminal piece of least three short segments. Exoskeleton smooth, internal mould displays dense pitting.

Name. For the senior author’s mother, Angela Armstrong.

Material. Holotype cranidium ( USNM 144602 , Pl. 15, figs 1–3), paratype cranidium ( USNM 144602 , Pl. 15, figs 1–3) and two paratype pygidia ( USNM 144602 , Pl. 15, fig. 6; USNM 144602 Pl. 15, figs 7–9) from the Aphelaspis Zone, Nolichucky Formation, Hurricane Hollow (cnr/17) section ( Rasetti 1965), Union County, Tennessee.

Occurrence. Aphelaspis Zone, Hurricane Hollow section (cnr/17) ( Rasetti 1965), Nolichucky Formation, Union County, Tennessee.

Description. Cranidium subtrapezoidal in outline with a rounded anterior margin and constriction at palpebral lobes. Frontal area equals 30% (30–31) of cranidial length (sag.); preglabellar field accounts for 36% (35–38) of frontal area length, down-sloping into flat border separated by border furrow, which varies from well defined to being faint medially. Anterior border long, shortening slightly abaxially. Convex glabella subtrapezoidal in outline with well-rounded front, raised above fixed cheeks; axial and preglabellar furrows well-defined. Glabellar width at palpebral lobes equal to 75% (72–78) of glabellar length (sag.). Lateral glabellar furrows absent on testate surface. Posterior margin of occipital ring arcuate; SO not expressed on exoskeleton. Palpebral areas down sloping; combined width comprises 40% (41; 38–45) cranidial width at palpebral lobes. Palpebral lobes short, slightly curved bands, equal to 15% glabellar length; positioned in front of glabellar mid-length.Anterior branches of facial sutures diverge slightly before curving towards anterior margin. Posterior branches extend obliquely back, deflecting posteriorly to genal angle. Posterior border furrow well impressed, nearly transverse across posterolateral projections. Posterior margin directed obliquely back, gently curved towards posterior corner of cranidium. Exoskeleton smooth.

Pygidium semielliptical in outline, sagittal length 64% (62–66) of maximum width. Long lateral and posterior borders, occupying 14% (12–16) pygidial length (sag.) but shorten somewhat anteriorly. Border down sloping, separated from pleural field by well-incised border furrow that weakens slightly posteriorly. Articulating facet present on anterior corners. Axis convex, raised above pleural field; comprises one-third (31%) of maximum pygidial width, tapering at border furrow. Articulating half-ring very short (3% axis length), slightly curved band; articulating furrow double the length of half-ring, both expressed on internal mould. Seven transverse axial rings plus two or three incorporated into terminal piece; defined on internal mould, faintly expressed on exoskeleton. Rings decrease in width posteriorly with gently inflated bosses developed abaxially and subtle crest raised along sagittal line. Pleural field gently inflated. Anteriormost pleural furrow long (exsag.), deep groove; remaining pleural furrows weakly defined on internal mould and barely perceptible on exoskeleton. Testate surface smooth; dense pitting (corresponding to small projections on underside of exoskeleton) on internal mould, with increased concentration on the pleural fields.

Discussion. These specimens were originally identified as Blountia mimula by Rasetti (1965) but, as discussed above, they belong to a different, new species. Comparisons with B. mimula , B. cora , B. janei , and B. gaspensis are presented earlier in the text. Blountia angelae differs from B. bristolensis in having longer frontal area (accounting for 30% cranidial width compared to 23%), a gently sloping preglabellar field, and a shallow anterior border furrow. The pygidia share well-defined border furrows, but B. angelae is relatively longer, with length equal to 64% of maximum width, versus 55% in B. bristolensis .