Blountia nevadensis, 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 : 17-18

Blountia nevadensis n. sp.

Plate 16

1965 Blountia bristolensis Resser ; Palmer, p. 29, pl. 1, figs 1, 2, 4.

Diagnosis. Frontal area long, occupying 31% cranidial length, with gentle change in slope between preglabellar field and anterior border; faint anterior border furrow. Nasute anterior border expands medially. Preglabellar field equal to about half length (sag.) of border. Fixed cheeks equal one-third cranidial width with long (exsag.) posterolateral projections curved backward abaxially. Pygidium semielliptical in outline with length 70% (68–73) of maximum width; posterior border accounts for 18% (17–19) of total length. Border furrow shallow but clearly defined. Axial furrows become indistinct towards tip of axis. Axial rings inflated abaxially into low knobs on both external surface and internal mould.

Name. For the occurrence of this species in the Mendha Formation in Nevada.

Material. Holotype cranidium ( USNM 141505 ; Pl. 16, figs 1–3), one paratype cranidium ( USNM 141505 b, Pl. 16, fig. 8), one paratype free cheek ( USNM 141504 , Pl. 16, figs 4, 5), and two paratype pygidia ( USNM 141506 , 141506 b, Pl. 16, figs 6, 7, 9) from Highland Range , Nevada ( Palmer 1965).

Occurrence. Aphelaspis Zone, House Range , Utah; Highland Range and Yucca Flat, Nevada.

Description. Cranidium roughly triangular in outline with nasute anterior margin; width at palpebral lobes equals 80% sagittal length. Frontal area long, about one-third (31%) cranidial length; preglabellar and preocular fields even in length. Anterior border nasute, with maximum width medially and decreasing (exsag.) abaxially, occupying 59% of frontal area length (possibly even shorter in the incomplete paratype, Pl. 16, fig. 8). Preglabellar field gently down sloping with gradual change into flat anterior border; weakly expressed anterior border furrow. Strongly convex glabella subtrapezoidal in outline with broadly rounded front; at mid-point, glabellar width equals 70% of length. Occipital furrow faint and curved gently backward, LO gently curved, even in length across glabella; accounts for 8% (7–8) of glabellar length (sag.). Fixed cheeks wide, combined width 40% (40–41) of cranidial width at palpebral lobes. Palpebral lobes slightly curved and oblique, equal to 11% maximum cranidial length. Facial sutures in front of palpebral lobes diverge slightly outward then curve towards anterior margin. Posterolateral projections curve sharply backwards and taper to a point. Posterior border furrow shallow and disappears distally; posterior border expands abaxially.

Free cheek narrow, extending into short, pointed genal spine. Shallow lateral border furrow separates narrow (tr.; exsag.), roughly triangular librigenal field from broad (tr.), convex lateral border. Eye small, globular; ocular suture absent. Conspicuous terrace ridges on margins of lateral border.

Pygidium semielliptical in outline, length (sag.) 70% (68–73) of maximum width. Axis convex, raised above pleural field, occupies about one-third (32%; 31–33) of pygidial width at anterior.Axial furrows moderately incised, become faint at terminus of axis. Abaxially, axial rings gently inflated into low knobs. Posterior border 18% (17–19) of pygidial length (sag.), narrows slightly abaxially. Border furrow shallow becoming indistinct towards axis. Pleural field smooth, no trace of pleural furrows on exoskeleton; raised above down-sloping border.

Discussion. As discussed above, sclerites from the Mendha Formation, Nevada, that Palmer (1965) identified as Blountia bristolensis represent a new species. The medially expanded, nasute anterior border on the cranidium clearly separates B. nevadensis from B. bristolensis (e.g., Pl. 2) and almost all other species of Blountia . Comparisons with B. nasuta Rasetti (Pl. 12), the only other species with a similar, nasute anterior border, are presented in earlier in the text.