Blountia morgancreekensis, 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 : 18-20

Blountia morgancreekensis n. sp.

Plates 17–19

1942b Maryvillia hybrida Resser , p. 71, pl. 13, figs 14, 15 (only; figs 16, 17, the holotype pygidium, may belong to Coosella Lochman, 1936 ; see Palmer 1954, p. 722, and Rasetti 1956, p. 1268).

1954 Blountia nixonensis Lochman, in Lochman & Duncan ; Palmer, p. 722, pl. 79, fig. 4.

Diagnosis. Anterior cranidial margin well rounded. Frontal area long, equal to 29% (24–33) of cranidial length; anterior border furrow indistinct, with preglabellar field and anterior border separated primarily by a gradual change in slope; lateral profile of frontal area evenly concave. On internal mould, border defined by absence of caecal network. Pygidium with gently inflated pleural field and down-sloping border that accounts for a little less than one fifth (17%; 12–24) of pygidal length (sag.). Eight to nine axial rings expressed on internal mould. External surfaces of cranidia and pygidia smooth; pits (corresponding to small projections on underside of exoskeleton) well developed on internal moulds of fixigenae and pleural fields but absent on glabella and anterior portions of anterior border. Caecal network of preglabellar field expressed only on internal mould.

Name. For the type locality at Morgan Creek , Burnet County, Texas .

Material. All specimens are from the Cap Mountain Limestone , Riley Formation , Burnett County, central Texas. The holotype cranidium ( OU 238142; Pl. 18, figs 1–3), five paratype cranidia ( OU 238143–238145 , OU 238151–238152 ,), and seven paratype pygidia ( OU 238146–238150 , OU 238154, OU 238155) are from collection MCNe1 7.1. One paratype cranidium ( OU 238153) is from collection LB 0.8, and one paratype ( USNM 108675 b) is from 3.2 km southeast of Fall Creek. One cranidium from LB -0.15, three cranidia and four pygidia from LB 0.8, one pygidium from MCNe1 5.8–5.9, and 36 cranidia and 25 pygidia from MCNe1 7.1 were also available for study .

Occurrence. Aphelaspis Zone, Cap Mountain Limestone, Burnet County, Riley Formation , Texas in collections LB -0.15, LB 0.8, MCNe1 5.8–5.9, and MCNe1 7.1.

Description. Cranidium roughly trapezoidal in outline, with well-rounded anterior margin and constriction at palpebral lobe; width at palpebral lobe about 90% of length (88; 80–93). Frontal area long, accounting for about 30% cranidial length (29; 24–33). Anterior border furrow very shallow; barely perceptible on some specimens (e.g., Pl. 18, fig. 4). Preglabellar field differentiated largely from anterior border by slope change; on internal mould, bor- der lacks caecal network present on preglabellar field. Preglabellar field steeply slopes towards the anterior border and makes up 37% (28–52) of the frontal area length. Anterior border nearly flat. Glabella convex and lateral profile curves down anteriorly. Axial furrow faint on external surfaces of exoskeleton with slightly deeper preglabellar furrow; all furrows better expressed on internal mould. Glabella narrows slightly anteriorly to broadly rounded front; width at the posterior end of the palpebral lobes is three-quarters its length (75%; 69–81). SO and LO indistinct on exoskeleton but better defined on internal mould. On external surface, SO expressed only medially as finely-etched groove; on external mould, SO shallow but extends across full width of glabella; LO very short, makes up about one-tenth (9%; 7–11) of the glabellar length (sag.). Lateral glabellar furrows absent on external surface, with up to four pairs barely perceptible on internal mould (e.g., Pl. 18, figs 8, 9). Palpebral lobe very short, gently arcuate band, length equal to approximately one-tenth (11%; 8–13) total cranidial length, positioned in front of glabellar mid-length; convex palpebral ridge best expressed on internal mould, extending obliquely forward to reach axial furrow near anterior corner of glabella. Anterior branches of facial sutures diverge forward from palpebral lobes before curving evenly inward along anterior cranidial margin. Posterior branches diverge backward along curved path; curvature increases posteriorly, with sutures nearly parallel near posterior corners of cranidium. Palpebral areas of the fixed cheeks make up a combined two-fifths (39%; 36–45) of total width at cranidial mid-length; slope downward away from axial furrow. Posterior margin of posterior area deflected backward at about 15 degrees from transverse plane; smaller individuals (e.g., Pl. 17, fig. 11) have more strongly deflected margins. Postocular area curved steeply downward; extended into large, triangular posterolateral projection with pointed corners; smaller individuals (e.g., Pl. 17, fig. 11) tend to have rounded corners. Posterior border furrow faint on external surface but better defined on internal mould; shallows abaxially, disappearing at middle of posterolateral projection. Entire exoskeleton is smooth. Apart from furrows, fixigenae of external mould densely pitted (corresponding to small projections on underside of exoskeleton), with scattered pits on glabella; network of caecal markings on preglabellar and preocular fields.

Pygidium semicircular in outline; length 71% (66–76) of maximum width; articulating facet on anterior corners. Articulating half-ring very short, gently curved, distinguishable only on internal mould. Articulating furrow about twice the length of half-ring; deep and visible on internal mould. Axis moderately convex and raised above pleural field; length (sag.) 83% (76–88) of pygidial length. Maximum axis width one-third (33%; 27–38) maximum pygidium width; tapers posteriorly. Eight or nine axial rings decrease in length (sag.) posteriorly, usually with low inflated bosses developed abaxially; additional terminal piece small and rounded, so that axis comprises at least nine to ten segments. Axial ring furrows effaced on external surface but well defined, transverse on internal mould; anterior furrows connected across crest of axis, but posteriormost furrows effaced medially. Anteriormost pleural furrow shallow on external surface, but deeper on internal mould; remainder of furrows effaced on external surface and weakly expressed on internal mould. Lateral and posterior borders down-sloping, separated from gently inflated pleural field by shallow border furrow (more firmly impressed on internal mould) that weakens towards posterior tip of axis (e.g., Pl. 19, fig. 5); length (sag.) equal 14% (10–17) maximum length of the pygidium but shortens towards anterior corners of pygidium. External surface of entire pygidium smooth; well preserved internal moulds show closely spaced pits (corresponding to small projections on underside of exoskeleton) on pleural field, with reduced density on axis and border.

Discussion. Palmer (1954) recognized that the paratype cranidium of Maryvillia hybrida Resser, 1942b (Pl. 17, figs 1–3) from the Cap Mountain Member was a species of Blountia , and assigned it to B. nixonensis Lochman (= B. bristolensis Resser ; see above). Numerous new sclerites from the Cap Mountain Member at the Morgan Creek and Lake Buchanan sections now demonstrate that Resser’s specimen records a distinct, new species, B. morgancreekensis , that differs from B. bristolensis in several characters. The frontal area of the cranidium is longer in B. morgancreekensis , occupying nearly 30% of cranidial length, versus about 20% in B. bristolensis . Moreover, the anterior border of B. morgancreekensis is defined largely by a gradual change in slope and, on internal moulds, by the loss of the caecal markings that are present on the preglabellar field (e.g., Pl. 17, figs 7–9, Pl. 18, figs 8, 9). As a result, the lateral profile of the frontal area is evenly concave. In contrast, there is a more abrupt change in slope between the preglabellar field and anterior border in B. bristolensis , with a distinct, albeit shallow, border furrow (e.g., Pls 2, 4); the border is proportionally shorter than in B. morgancreekensis (e.g., compare Pl. 2 with Pl. 18). The pygidium of B. bristolensis has more strongly inflated pleural fields so that there is a more pronounced change in slope at the border and, consequently, a border furrow that is more clearly defined (compare Pl. 3, figs 1, 2 and Pl. 5, figs 1–11 with Pl. 19, figs 1–11); the border of B. bristolensis is somewhat shorter, accounting for 10% of pygidial length, versus 14% in B. morgancreekensis .

Blountia morgancreekensis resembles B. angelae in morphology of the pygidium, including the expression of the axis and pitting on the internal mould (compare Pl. 15, figs 6–9 with Pl. 17, fig. 13 and Pl. 19). Cranidia are clearly separable in frontal area morphology: B. angelae has a sharp break between the preglabellar field and anterior border, and as a result, the anterior border furrow is also better defined (compare Pl. 15, figs 3, 5 with Pl. 17, figs 3, 8 and Pl. 18, figs 7, 9).

Blountia mimula (e.g., Pl. 1, figs 5–7) and B. newfoundlandensis (e.g., Pl. 20) differ from B. morgancreekensis in their shorter frontal areas that also have an abrupt break in slope between the preglabellar field and anterior bor- der, and their less strongly curved anterior cranidial margins. The pygidium of the former species is separable from that of B. morgancreekensis in the nearly effaced border furrow (e.g., Pl. 1, fig. 9), whereas the pygidium of the latter has a border that expands conspicuously adaxially, and a shorter axis with seven, rather than eight or nine axial rings in front of the terminal piece (e.g., Pl. 21, figs, 1, 6, 9). The pitting of the internal mould of B. morgancreekensis is not expressed in B. newfoundlandensis

Blountia morgancreekensis is similar in frontal area proportions to B. janei (Pl. 8), although the latter has a sharp break in slope between the preglabellar field and anterior border. Pygidial differences are decisive, with B. janei having an elongate pygidium with a longer border and shorter axis with six, rather than a minimum of eight, segments in front of the terminal piece (compare Pl. 7, figs 1–8 with Pl. 17, fig. 13 and Pl. 19).