Blountia bristolensis Resser, 1938

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 : 11-12

Blountia bristolensis Resser, 1938

Plates 2–5

1938 Blountia bristolensis Resser , p. 65, pl. 12, fig. 24.

1938 Maryvillia bristolensis Resser , p. 87, pl. 12, fig. 38.

1944 Blountia nixonensis Lochman, in Lochman & Duncan , p. 43, pl. 4, figs 7–12.

1965 Blountia bristolensis Resser ; Rasetti, p. 58, pl. 10, figs 1–2, pl. 11 figs 9–12.

non 1965 Blountia bristolensis Resser ; Palmer, p. 29, pl. 1, figs 1, 2, 4 [= Blountia nevadensis n. sp.].

Diagnosis. Frontal area 25% (23–26) of cranidial length, with sharp change in slope between preglabellar field and anterior border, and well-defined anterior border furrow. Seven thoracic segments. Pygidium semielliptical in outline with inflated pleural field, well-defined border furrow, and down-sloping border of even width occupying 10% (8–12) of pygidial length (sag).

Material. The holotype is a pygidium ( USNM 94942; Pl. 5, figs 1, 2) from the Nolichucky Formation, reservoir on Mumpower Creek, 5.6 km north of Bristol, Virginia ( USNM locality 36v). Other material includes the holotype cranidium of Maryvillia bristolensis Resser ( USNM 94963, Pl. 2, figs 1–3) and two previously unillustrated cranidia from the type lot (both catalogued as USNM 94963, Pl. 2, figs 4–9). Previously unillustrated paratypes of B. nixonensis Lochman, in Lochman & Duncan , include three cranidia ( USNM 511426, Pl. 4, figs 1–3; USNM 511429, Pl. 4, figs 4–6; USNM 511430, Pl. 4, figs 7–9) and four pygidia ( USNM 511427, Pl. 5, figs 9–11; USNM 511432, Pl. 5, figs 4–6; USNM 511433, Pl. 5, figs 7, 8; USNM 511434, Pl. 5, figs 1–3).

Occurrence. Aphelaspis Zone, Nolichucky Formation , Rasetti’s (1965) collections cnp/17, cnp/14, and cnq/ 14 in Tennessee; USNM locality 36v near Bristol, Virginia ( Resser 1938). Aphelaspis Zone, Pilgrim Formation, Nixon Gulch, Horseshoe Hills , Montana ( Lochman and Duncan 1944).

Discussion. Rasetti (1965, p. 58) noted that the pygidium and the cranidium that Resser (1938) named Blountia bristolensis and Maryvillia bristolensis , respectively, are from the same collection, and he considered them to represent a single species under the former name. The cranidium of Blountia bristolensis Resser (1938) has a relatively short frontal area that occupies 25% (23–26) of cranidial length, and with a preglabellar field that varies from slightly longer to slightly shorter than the anterior border. There is an abrupt change in slope at border furrow. An exoskeleton assigned to the species by Rasetti (1965) (Pl. 3, figs 3–7) has similar frontal area proportions to Resser’s cranidia and appears to be conspecific.

Both Palmer (1965) and Rasetti (1965) considered Blountia nixonensis Lochman, in Lochman & Duncan, 1944 to be conspecific with Blountia bristolensis . Specimens figured by Lochman & Duncan (1944, pl. 4, figs 7–12) could not be located at the USNM, but their previously unillustrated paratypes (Pls 4, 5) are closely comparable to B. bristolensis , and we agree that they belong to the same species. However, sclerites from the Mendha Formation in Nevada that Palmer (1965, pl. 1, figs 2–3) identified as Blountia bristolensis are assigned to a new species, Blountia nevadensis (Pl. 16). Cranidia of the latter (Pl. 16, figs 1–3, 8) differ clearly in having a longer frontal area that features a nasute border that narrows abaxially. The pygidium of B. nevadensis is relatively narrower (length equal to 69% of maximum width, versus 55% in B. bristolensis ).

Cranidia of B. newfoundlandensis n. sp. (Pl. 13, figs 5–10, Pl. 20, Pl. 21) from the Cow Head Group, western Newfoundland, resembles B. bristolensis in possessing a relatively short frontal area. The former species differs by having a narrower glabella (glabellar width is 69% of length versus 79% in B. bristolensis ) and palpebral lobes situated slightly farther forward. Pygidial differences are decisive in separating these species. The pygidium of B. newfoundlandensis (Pl. 21) displays a border that expands noticeably towards the axis to occupy18% of pygidial length (sag.) In contrast, the border of B. bristolensis maintains a nearly even width along the pygidial margin, and is relatively shorter, accounting for 10% (8–12) of pygidial length (sag.).