Maryvillia arion 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 : 21-22

Maryvillia arion Walcott, 1916

Plates 22, 23

1916 Maryvillia arion Walcott , p. 400, pl. 64, figs 4–4c.

1942a Blountia arguta Walcott ; Resser, p. 8.

1956 Maryvillia arion Walcott ; Rasetti, p. 1267 (includes complete synonymy).

1961 Maryvillia arion Walcott ; Rasetti, p. 116, pl. 21, figs 14–15.

1965 Maryvillia arion Walcott ; Rasetti, p. 59, pl. 9, figs 22–26.

non 1992 cf. Blountia arion (Walcott) ; Pratt, p. 66, pl. 23, figs 22–25 [= Blountia sp. indet].

non 2000 Blountia arion (Walcott) ; Stitt & Perfetta, p. fig. 11.4, 11.5 [= Maryvillia cf. M. triangularis Lochman, in Lochman and Duncan, 1944 ].

Diagnosis. Evenly rounded anterior margin of cranidium. Frontal area occupies one-third (32%; 31–33) cranidial length. Steeply sloping preglabellar field grades into long (occupies two-thirds [66%; 65–67] of frontal area length), concave anterior border across broad, very shallow border furrow. Subtrapezoidal glabella barely elevated above fixigenae. Pygidium semielliptical, length (sag.) equal to 70% (68–74) of maximum width, with smooth external surface, shallow furrows. Axis extends beyond border furrow onto down-sloping border. Pitting on internal mould (corresponding to small projections on underside of exoskeleton) is bimodal, with larger pits scattered among small- er ones (Pl. 22 fig. 5).

Material. Holotype cranidium ( USNM 62826; Pl. 22, figs 1–3), one paratype cranidium ( USNM 62826, Pl. 22, figs 4–7) and one paratype pygidium ( USNM 62827, Pl. 23, figs 1, 2), all from the Nolichucky Formation; 0.8 km (0.5 mile) east of Rogersville, Hawkins County, Tennessee ( USNM locality 123b). A pygidium ( USNM 94968, Pl. 23, figs 3, 4) that was designated by Resser (1942a) as a paratype of Blountia arguta is a previously unillustrated specimen from the type lot of M. arion . Non-type specimens, also from the Nolichucky Formation in the Rogersville region, are a cranidium ( USNM 144603, Pl. 22, figs 8–11) and two pygidia ( USNM 144603, Pl. 23, fig. 5; USNM 144603, Pl. 23, figs 8, 9) from Rasetti’s (1965) locality cnm/2, and a pygidium ( USNM 144604, Pl. 23, figs 6.7) from Rasetti’s (1965) locality cnn/1. All from the Nolichucky Formation, Rogersville, Hawkins County, Tennessee (Walcott 1916).

Occurrence. Crepicephalus Zone, Nolichucky Formation , Tennessee. Type locality is USNM locality 123b, 0.5 mile east of Rogersville, Hawkins County. Rasetti’s (1965) localities (cnm/2, cnn/1, cnn/2, cnn/3, cnn/4, cnn/14), Nolichucky Formation at the Big Creek and Rogersville sections in Hawkins County , and Lost Creek and Russell Gap sections in Jefferson County , Tennessee .

Description. Cranidium subtrapezoidal in outline; width at palpebral lobe equal to 97% of length. Anterior margin evenly rounded. Frontal area occupies one-third (32%; 31–33) cranidial length (sag.); two-thirds (66%; 65–67) of frontal area consists of long, concave anterior border. Preglabellar and preocular fields steeply sloping, then gradually merge with border. Broad (sag., exsag.) anterior border furrow is gently impressed on internal mould. Glabella subtrapezoidal in outline with gently rounded front; three pairs of furrows barely perceptible only on internal mould. Glabellar width at palpebral lobe is 85% (83–86) of length; convexity low, only subtly raised above fixed cheeks. LO short, accounting for less than one-tenth (7%; 6–8) of glabellar length; crescent-shaped, tapers (tr.) before reaching edge of glabella. SO and axial furrows well incised on internal mould. Combined width of palpebral areas makes up 45% (45–47) of cranidial width. Palpebral lobes situated in front of glabellar mid-length; moderately curved, short, equal to 13% (13–14) of cranidial length. On internal mould, raised palpebral ridge extends from lobe to anterior corner of glabella.Anterior branches of facial suture nearly parallel, then curve into anterior margin; posterior branches diverge backward as faintly sigmoid curves. Posterolateral projections extend slightly backward, increasing curvature abaxially to a tapered point. Posterior border furrow well defined on internal mould, extending abaxially across threequarters of width (tr.) of posterolateral projection, terminating short of sutural margin. Dense pitting with bimodal size distribution preserved on internal mould; large pits are more than twice diameter of smaller ones.

Pygidium semielliptical in outline, length (sag.) equal to 70% (68–74) of maximum width. Very short articulating half-ring; articulating furrow slightly longer and well-incised only on internal mould. Maximum axis width about one-third (36%; 34–38) of pygidial width, tapers posteriorly and extends beyond border furrow. Twelve axial rings plus terminal piece expressed on internal mould. Axial furrows shallow, weakening towards terminal piece. Axis raised only slightly above pleural field. Pleural furrows weakly expressed on internal mould, indistinct on exoskeleton. Border down-sloping; approximately one-tenth maximum pygidial length (sag.); differentiated from pleural field by shallow border furrow which weakens posteriorly, becoming indistinct at axis tip. External surface smooth, except for terrace ridges along posterior margin. Internal mould has dense pitting with bimodal size distribution similar to cranidial pitting.

Discussion. The types of Maryvillia arion are a cranidium and two pygidia (Walcott 1916b, pl. 64, fig. 4) from Nolichucky Formation near Rogersville, Tennessee; the cranidium and one pygidium are reillustrated in this paper, along with a cranidium from the type lot that has not been figured previously (Pl. 22, figs 1–8, Pl. 23, figs 1–2). Resser (1942a) considered the pygidia to have been associated incorrectly with the cranidia, and assigned them to a new species, Blountia arguta . Rasetti (1956) later suggested that Resser was incorrect, and returned them to M. arion , which is followed here. The pygidium of M. arion is similar to B. mimula , the type species of Blountia , but differs by having a more strongly rounded outline, a more clearly defined border furrow, a longer axis that is barely raised above the pleural fields, and 12 rings in front of the terminal piece, versus 7–8 in B. mimula . The cranidium of B. mimula differs by having a far more elevated, convex glabella, anterior branches of facial sutures diverging outwards from palpebral lobes, rather than proceeding straight forward as in M. arion , and with the posterior margin of the posterolateral projection curved strongly backward.

Rasetti (1956) also considered Blountina triangularis Lochman, in Lochman & Duncan, 1944 to be a junior synonym of M. arion . Restudy of Lochman & Duncan’s types (Pls 24, 25) shows that these species share several characters supporting assignment of the former to Maryvillia , including a concave anterior border on the cranidium, a glabella that is just slightly raised above the fixed cheeks, and facial sutures that extend nearly straight forward from palpebral lobes. However, pygidial anatomy indicates unequivocally that they are distinct species. The pygidium of M. triangularis (Pl. 24, figs 9, 10; Pl. 25, figs 1–5, 9) is narrower and distinctly subtriangular in outline, which contrasts with the broad, posteriorly rounded outline of M. arion (Pl. 23), which also has a relatively longer axis. The cranidium of M. triangularis differs most obviously by a crudely triangular-shaped outline, with an anterior border that is longest medially (sag.) rather than maintaining a more even length along the anterior cranidial margin. In addition, the margins of posterolateral projections are directed posteriorly (Pl. 24, fig. 5, Pl. 25, fig. 6) rather than nearly transversely (Pl. 22, figs 1, 5).

Sclerites from the Rabbitkettle Formation of northern Canada that Pratt (1992, pl. 23, figs 23–25) assigned to cf. Blountia arion are incomplete and difficult to assess. However, the strong convexity of both the glabella and the pygidial axis separate them immediately from Maryvillia , and we consider them to represent an indeterminate species of Blountia .

Stitt & Perfetta (2000, fig. 11.4, 11.5) illustrated two pygidia from the Deadwood Formation under the name, Blountia arion (Walcott) . The subtriangular outlines of these sclerites demonstrate that they are misidentified. They can be compared to M. triangularis , although if the larger of the two specimens is complete, it is shorter and wider than any of Lochman and Duncan’s types (Pl. 24, figs 9, 10; Pl. 25, figs 1–5, 9).