Blountia montanensis Duncan, in Lochman & Duncan, 1944

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 : 13-14

Blountia montanensis Duncan, in Lochman & Duncan, 1944

Plate 9

1944 Blountia montanensis Duncan, in Lochman & Duncan, 1944, p. 53 , pl. 8, figs 29–34.

non 1965 Blountia montanensis Duncan, in Lochman & Duncan ; Rasetti, p. 57, pl. 9, figs 13–20 [= B. tennesseensis n. sp.]. non 1992 Blountia montanensis Duncan, in Lochman & Duncan ; Pratt, p. 66, pl. 24, figs 13–15 [= B. sp. indet.]

Diagnosis. Frontal area short, occupying 23% (22–25) of cranidial length; entire frontal area down-sloping. Convex anterior border slightly longer than preglabellar field; anterior border furrow well defined. Glabella weakly tapered. Anterior branches of facial sutures subparallel. Posterior edge of posterolateral projection deflected gently backward, with weak expansion (exsag.) of posterior border.

Material. Holotype cranidium ( USNM 126836 , Pl. 9, figs 4–6) and five paratype cranidia ( USNM 126837 , Pl. 9, figs 1–3; USNM 126838 , Pl. 9, figs 10–13; USNM 126839 , Pl. 9, figs 7–9) from the Pilgrim Formation, Dry Wolf Creek section ( Lochman & Duncan, 1944), Little Belt Mountains , Montana.

Occurrence. Crepicephalus Zone, Pilgrim Formation, Dry Wolf Creek section (horizons 38/4, 4.5, and 4.5a), Little Belt Mountains, and Half Moon Pass section (horizon 9.4), Big Snowy Mountains, Montana ( Lochman & Duncan 1944).

Discussion. Rasetti (1965) figured several sclerites from the Nolichucky Formation of Tennessee as Blountia montanensis (Pl. 13–14) but several differences separate the cranidia from Lochman & Duncan’s types (Pl. 9), and they are assigned to B. tennesseensis n. sp. Both sets of cranidia share a short frontal area in which the anterior border is slightly longer than the preglabellar field, and subparallel anterior branches of the facial sutures. However, B. tennesseensis has narrower palpebral areas of the fixigenae (combined width is 30% of cranidial width across the palpebral lobes, versus 40% in Lochman & Duncan’s types), and the posterior branches are less strongly divergent, so that the posterolateral projection shortens (exsag.) more abruptly abaxially.

A cranidium from the Rabbittkettle Formation , northern Canada, identified by Pratt (1992, pl. 24, figs 13–14) as B. montanensis , is misidentifed. The types have a noticeably longer anterior border, and and the preglabellar field is also longer in most specimens (e.g., Pl. 9, figs 1–10) .

Blountia bristolensis (e.g., Pl. 2) is also close to B. montanensis in the length of the frontal area of the cranidium. However, the former has a flat, rather than convex anterior border, anterior branches of the facial sutures that are distinctly divergent, a wider palpebral area of the fixigenae, more strongly tapered glabella, and less divergent posterior branches of the sutures, so that the posterolateral projection narrows abruptly and is distinctly triangular in outline. Blountia newfoundlandensis (Pl. 20) has palpebral lobes that are farther forward on the cranidium than in B. montanensis , so that the posterolateral projection is longer (exsag.) adaxially but it also tapers more abruptly as a consequence of the weakly divergent posterior branches of the facial sutures.