Blackwelderia reineri, Opik, 1967

Smith, Patrick M., Paterson, John R. & Brock, Glenn A., 2018, Trilobites and agnostids from the Goyder Formation (Cambrian Series 3, Guzhangian; Mindyallan), Amadeus Basin, central Australia, Zootaxa 4396 (1), pp. 1-67 : 50-52

Biaverta reineri Öpik, 1967

Fig. 25

1967 Biaverta reineri ; Öpik, p. 371–372, pl. 39, figs 1–5.

?1989 Biaverta sp. indet.; Wang, Mills, Webby, Shergold, p. 113, fig. 4Q.

Material. Twelve cranidia figured, CPC42387–CPC42398. 24 cranidia not figured (mostly fragments).

Description. Largest cranidium 5 mm long (sag.). Cranidium subtrapeziform, length:width ratio 65%, maximum width across posterolateral projections of fixigena; narrowest point of cranidium at the γ–γ section, where it is about 60% of maximum cranidial width (tr.). Anterior margin strongly rounded; elevated medially being retroverted. Posterior margin strongly bowed forward (exsag.), moderately expanding abaxially. Anterior branches of facial suture (γ–α) converge anteriorly in a broad arc. Posterior branches of facial suture strongly curve towards posterolateral corners of cranidium. Axial furrow broad and indistinct. Glabella narrow (tr.) and moderately convex, with maximum convexity at about midwidth, width:length ratio of 81% to 82% (mean 81%; n = 3), occupying about 68% cranidial length; slightly tapering forward, width (tr.) across anterior 52% maximum width of glabella. Lateral glabellar furrows narrow (tr.) and deep. S1 short (tr.), directed transversely, becoming slightly wider (exsag.) adaxially before terminating. S2 parallel to and shorter (tr.) than S1; S3 effaced. Occipital ring moderately wide (sag.), narrowing abaxially; SO moderately narrow (sag.), bowed forward medially, deep and deepening abaxially. Faint circular bacculae, positioned opposite and behind L1. Anterior border strongly upturned medially; poorly defined, lacking anterior border furrow. Preglabellar field of moderate length, making up 26% of total sagittal length; strongly concave, preocular field strongly convex and strongly downsloping adaxially towards preglabellar field. Palpebral lobes absent. Eye ridges faintly defined on exterior surface, more visible exfoliated cranidia; extending from the anterolateral corners of the axial furrow in a slight anterior curve towards γ; eye ridges appear faintly bifurcated. Palpebral area situated forward on the cranidium, slightly forward of the glabella anterior; short (exsag.); convex, downsloping towards axial furrow and preglabellar field. Postocular field long (exsag.), 68% of sagittal cranidial length; wide (tr.), 72% of the maximum cranidial width; slightly downsloping medially towards the axial furrow and posteriorly near the posterior border furrow. Posterolateral projections of fixigena are flat with a steep downturn at the lateral extremities. Posterior border narrow (exsag.) and slightly wider abaxially. Posterior border furrow deep, narrow (exsag.), disappearing abaxially at the steep downturn near lateral extremities.

Prosopon consists of minute granules interconnected to form a dense reticulated pattern covering the palpebral area. Small, dense granules cover the anterior border, glabella, preocular field, eye ridges and posterolateral projections. Preglabellar field with faint caecal veins all directed anteriorly. Axial furrow smooth.

Hypostome, rostral plate, librigena, thorax and pygidium unknown.

Discussion. These distinctive cranidia are commonly found in the upper sandy limestone beds of the GOY section. The Goyder Formation specimens are essentially identical to Biaverta reineri Öpik, 1967 from the Mindyallan Mungerebar Limestone in the Georgina Basin (Öpik 1967) . Specimens from both localities have the characteristic slightly upturned anterior border, preglabellar field of moderate length that is laterally strongly concave and medially slightly convex, as well as a prosopon of dense minute granules (compare Fig. 25A–M with Öpik 1967, pl. 39, fig. 1–5). The specimens from the Goyder Formation only vary marginally from the holotype found in the Mungerebar Limestone in possessing well developed lateral glabellar furrows and eye ridges, although this may be attributed to preservational quality; the Mungerebar Limestone material is relatively poorly preserved, especially the coarsely silicified specimens (e.g., Öpik 1967, pl. 39, fig. 3–5). Biaverta reineri is distinguished from the only other named species, B. biaverta Öpik, 1967 , by its less upturned anterior border and longer preglabellar field.

Specimens of B. reineri from the Goyder Formation also resemble a single, poorly preserved cranidium described by Wang et al. (1989) as Biaverta sp. from the Mindyallan Boshy Formation, N.S.W. Both the Goyder Formation material and N.S.W. specimen share the slightly upturned anterior border and longer (sag.) preglabellar field that is typical of B. reineri . Given that the N.S.W cranidium has been distorted and is coarsely preserved in sandstone, it is only questionably synonymised with the Goyder Formation material.

Occurrence. GOY section horizon 132.7 m ( Fig. 3 ).

Distribution. Goyder Formation, Amadeus Basin, Northern Territory. Mungerebar Limestone, Georgina Basin , Northern Territory and Queensland. Possibly the Boshy Formation, Koonenberry Belt, New South Wales. All occurrences are Cambrian Series 3, Guzhangian (Mindyallan) in age.

Figures

FIGURE 3. Generalised lithostratigraphy and taxon ranges through the Goyder Formation at the GOY section. Sampled horizons are indicated on the right side of the section, with sample numbers representing tape meterage along the section line (see Fig.2). Measurements on the left side of the section are in meters (true thickness) from the base of the Goyder Formation. SF = Shannon Formation, PSS = Pacoota Sandstone. Solid black circles represent the occurrences of taxa present in the GOY section. Hollow circles represent the approximate stratigraphic occurrences of taxa found at nearby spot localities. Ranges of taxa found only atspot localities approximate, and has been inferred using lithological, faunal and geographical information.

FIGURE 25. Biaverta reineri Öpik, 1967. All specimens come from sample GOY/173.6. A, N, O, P, CPC42387, partial cranidium; A, dorsal view; N, oblique anterolateral view; O, lateral view; P, anterior view. B, CPC42388, partial cranidium.C, I,CPC42389,partial cranidium;C, dorsal view;I, close up ofornament.D, CPC42390,partial cranidium.E, CPC42391, partial cranidium. F, CPC42392, partial cranidium. G, CPC42393, partial cranidium. H, CPC42394, partial cranidium. J, CPC42395, partial cranidium. K, CPC42396, partial cranidium. L, CPC42397, partial cranidium. M, CPC42398, partial cranidium. All scale bars are 1 mm.