Maryvillia triangularis (Lochman, 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 : 23

Maryvillia triangularis (Lochman, in Lochman & Duncan, 1944)

Plates 24, 25

1944 Blountina triangularis Lochman, in Lochman & Duncan , p. 57, pl. 8, figs 12–18.

1956 Blountina triangularis Lochman ; Rasetti, p. 1268 [included in list of species that Rasetti considered to be synonyms of Maryvillia arion Walcott ].

Diagnosis. Anterior border narrows abaxially with well-incised border furrow; steeply sloping preglabellar field. Moderately convex glabella that narrows (tr.) anteriorly to a broadly rounded front, and very shallow axial furrows. Posterolateral projections long and triangular, with posteriorly deflected margins. Pygidium subtriangular in outline with faint border, axial, and pleural furrows that all weaken posteriorly. Pitting (corresponding to small projections on underside of exoskeleton) preserved on internal mould.

Material. Figured specimens include the holotype cranidium ( USNM 127159 , Pl. 16, figs 6–8), four paratype cranidia ( USNM 127161 , Pl. 24, figs 1–3; USNM 127161 , Pl. 24, fig. 4; USNM 127161 , Pl. 24, fig. 5; USNM 127161 , Pl. 24, figs 6–8), and four paratype pygidia ( USNM 127160 c, Pl. 25, fig.. 9; USNM 127160 d, Pl. 25, figs 1–3; USNM 127161 , Pl. 25, figs 4, 5; USNM 127161 , Pl. 24, figs 9, 10) from the Pilgrim Formation, Half Moon Pass section ( Lochman & Duncan 1944), Big Snowy Mountains , Montana .

Occurrence. Crepicephalus Zone, Pilgrim Formation, Half Moon Pass section, Lochman and Duncan’s (1944) horizon 9.2, Big Snowy Mountains , Montana.

Discussion. Blountia nasuta (Pl. 12, figs 1–6) resembles Maryvillia triangularis by having a roughly triangular anterior cranidial margin, and a pygidium is also triangular in outline (Pl. 12, figs 7–12). However, B. nasuta differs in having a longer frontal area (accounting for 37% cranidial length, versus 28% in M. triangularis ) with a longer preglabellar field (occupying 46% of frontal area, versus 30%), and a flat, rather than concave border. The facial sutures of B. nasuta diverge forward from the palpebral lobes, whereas the anterior branches of M. triangularis are nearly parallel. The pygidium of B. nasuta is noticeably flatter (e.g., compare Pl. 12, figs 7. 9, 12 and Pl. 25, figs 2, 3, 5), with a shorter axis and longer (sag.) border.