Blountia mimula Walcott, 1916

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 : 10-11

Blountia mimula Walcott, 1916

Plate 1

1916 Blountia mimula Walcott , p. 399, pl. 61, figs 4, 4a–c.

1938 Blountia mimula Walcott ; Resser, p. 63, pl. 12, figs 18–19.

non 1965 Blountia mimula Walcott ; Rasetti, p. 59, pl. 10, figs 3–7 [= Blountia angelae n. sp.].

Diagnosis. Frontal area slightly more that one-quarter (27%; 25–29) cranidial length, with gently sloping preglabellar field and flat anterior border nearly equal in length (sag.); anterior border furrow well-defined. Moderately convex glabella with width at palpebral lobe equal to 75% of glabellar length. Seven thoracic segments. Where expressed, pygidial border 15% of total length of pygidium; border furrow barely perceptible to entirely effaced.

Material. The holotype is a nearly complete exoskeleton ( USNM 62781 ; Pl. 1, figs 1–4) from the Nolichucky Formation at Shields Ridge, Jefferson County, Tennessee ( USNM loc. 120). Paratypes are a cranidium ( USNM 62873 ; Pl. 1, figs 5–7) and pygidium ( USNM 62872 ; Pl. 1, figs 8–10) from the Nolichucky Formation, 17.7 km (11 miles) northwest of Knoxville , Knox County , Tennessee ( USNM loc. 107c) .

Description. Cranidium exclusive of posterolateral projection subrectangular in outline, width at palpebral lobe equal to 75% of length, with evenly rounded anterior margin. Frontal area long, occupying about one-quarter cranidial length (27%; 25–29), preglabellar field comprises about 40% (39; 36–42); gently slopes down with subtle increase in slope anteriorly to flat anterior border; anterior border furrow shallow. Moderately convex glabella subtrapezoidal in outline, narrowing slightly forward, with well-rounded anterior margin. Glabellar width at posterior end of palpebral lobe about 75% (74; 70–78) its length. Glabella surface smooth, lacking lateral glabellar furrows and with only a hint of SO near axial furrow on some specimens (Pl. 1, fig. 1). Posterior margin of LO gently curved. Palpebral lobe short, equal to about one-tenth (12%; 10–15) cranidial length, situated in front of glabellar mid-length. Anterior branches of facial suture diverge before curving along anterior cranidial margin; posterior branches extend at about 45 degrees, increasing curvature posteriorly. Posterolateral projections extend laterally with backwardly curved posterior margins, tapering to a sharp point. Posterior border furrow weakly expressed across two-thirds of posterior margin, becoming entirely effaced beyond that point.

Thorax comprised of seven flat, transverse segments that taper to short, down-sloping pleural spines. Pleural furrow very faint but expressed near fulcrum. Axis is convex and raised above pleural lobes, occupies about one-third (31%) thoracic width at anterior, narrowing (tr.) posteriorly. Axial furrows shallow but clearly defined grooves.

Pygidium subcircular in outline, maximum length more than half of width (57%; 50–64). Axis convex, raised above pleural field, width one-third (32%; 21–33) maximum pygidial width (tr.). Axial furrows well-incised and shallow at axial tip. Faint axial ring furrows expressed at margin of axis in some individuals but effaced completely in others; at least seven axial rings present. Pleural field gently raised above down-sloping border, unfurrowed apart from barely perceptible pleural furrow at anterior. Lateral and posterior border furrows faint. Border length 15% maximum pygidial length. Entire exoskeleton smooth.

Discussion. Walcott’s (1916) types of Blountia mimula suggest that there may be size-related variation in glabellar width, which ranges from 64% of cranidial width at palpebral lobe in the holotype (Pl. 1, fig. 1) to 79% in the larger paratype (Pl. 1. fig. 5). The anterior cranidial margin is less strongly curved in the paratype. The pygidia also show variation in proportions; the length of the pygidium of the holotype is 50% of width (Pl. 1, figs 1, 3), whereas in the paratype (Pl. 1, fig. 9), the proportion is 64%.

Sclerites in Tennessee identified by Rasetti (1965, pl. 10, figs 3–7) as Blountia mimula do not appear to be conspecific with the holotype and represent a new species, B. angelae (Pl. 15). These species are differentiated most clearly by pygidial morphology: the pygidium of B. angelae has a clearly defined border furrow on both testate surfaces (Pl. 15, fig. 6) and internal moulds (Pl. 15, figs 7–9), whereas B. mimula is effaced (Pl. 1, figs 1–3. 9, 10). Cranidia of B. angelae differ from B. mimula in frontal area proportions: the anterior border is noticeably longer in the former (compare Pl. 1, figs 1, 5 with Pl. 15, figs 1, 4). In addition, the palpebral areas are wider in B. angelae (compare Pl. 1, fig. 5 and Pl. 15, figs 1, 4).

Cranidia of B. bristolensis Resser, 1938 , including its synonyms Maryvillia bristolensis Resser, 1938 and Blountia nixonensis Lochman, in Lochman & Duncan, 1944 (see below), have anterior branches of the facial sutures that are subparallel, or nearly so (e.g., Pl. 2, Pl. 3, fig. 5), whereas as those of B. mimula are more strongly diveregent (e.g., Pl. 1, fig. 5). The frontal area of B. mimula is longer (29% total cranidial length compared to 23% in B. bristolensis ), and the palpebral area of the fixigena is distinctly narrower (compare Pls 2, 4 with Pl. 1, figs 1, 5). Pygidial contrasts are even stronger, with the firmly impressed border furrows of B. bristolensis (Pl. 3, figs 1, 2, Pl. 5) clearly differing from the effaced condition of B. mimula . The articulated specimens of both B. mimula and B. bristolensis have seven thoracic segments, but the latter species has a wider axis (38% of thorax width versus 30% in B. mimula ).

Pygidia of B. cora Lochman, in Lochman & Duncan, 1944 , from the Pilgrim Formation of Montana, show effacement of the border furrow that resembles the condition in B. mimula (compare Pl. 1, figs 3, 9 with Pl. 6, figs 8, 9 and Pl. 7, figs 9, 10, 14). If the paratypes are assigned correctly, the pygidium of B. cora lengthens considerably during holaspid ontogeny (e.g. compare Pl. 6, fig. 9 and Pl. 7, fig. 9). At a similar size, pygidia of B. mimula are relatively longer (e.g., compare Pl. 1, fig. 3 and Pl. 6, fig. 9). Cranidia of B. cora also show substantial ontogenetic changes in proportions, with strong positive allometry of the frontal area (e.g., compare Pl. 6, figs 1–7 and Pl. 7, figs 11–13). However similarly-sized cranidia of B. cora (Pl. 6, figs 4, 6) and B. mimula (Pl. 1, fig. 5) are dramatically different, with the latter having a frontal area that is less than half of the relative length of the former, and a much shorter preglabellar field.

Pygidial anatomy also separates B. janei Lochman, in Lochman & Duncan, 1944 , also from the Pilgrim Formation, from B. mimula . The pygidium of the former is proportionately longer and has a shorter axis throughout holaspid ontogeny (Pl. 7, figs 1–8).