Blackwelderia repanda Öpik, 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 : 22-26

Blackwelderia repanda Öpik, 1967

Figs 10 , 11

1967 Blackwelderia repanda ; Öpik, p. 315–316, pl. 32, fig. 2, pl. 47, fig. 10.

1989 Blackwelderia sp. cf. B. repanda Öpik ; Wang, Mills, Webby & Shergold, p. 114–115, figs. 4R–T, 5A–I.?2007 Blackwelderia sp. cf. B. repanda Öpik ; Shergold, Laurie & Shergold, p. 31–32, fig. 11A–F.

Material. Ten cranidia, three librigenae, two hypostomes and twelve pygidia figured, CPC42243–CPC42268. 26 cranidia, 62 librigenae, nine hypostomes and 102 pygidia not figured (mostly fragments).

Description. Cephalon transversely subovate, up to 11 mm long (sag.). Cranidium trapezoidal in outline, length:width ratio approximately 53%, maximum width across posterior limbs of fixigenae, narrowest point of cranidium at the anterior margin; strongly convex (sag., tr.). Anterior margin straight (tr.). Posterior margin bowed anteriorly, except for the occipital ring that bulges posteriorly. Anterior branches of facial suture (γ–β) are difficult to distinguish. Glabella long (sag.), anteriorly truncate, trapezoidal in outline, strongly convex, with maximum convexity across midwidth, lateral slopes gently convex; width:length ratio of 71% to 74% (mean 72%; n = 3), occupying 89% of the cranidial length. Axial furrow narrow (tr.) and deep. S1 well defined, deep and narrow (exsag.), intersecting axial furrow approximately level with ε, directed slightly posteromedially for a short distance, before bifurcating with the anterior branch traversing a small distance anteromedially before becoming indistinct and the posterior branch continuing posteromedially. S2 and S3 short; S2 is directed posteromedially, whilst S3 is directed anteromedially. Occipital ring of moderate length (sag.), becoming slightly narrower abaxially, posterior margin strongly bowed backwards. SO wide (exsag.), straight to slightly bowed backwards medially in some individuals, moderately deep (sag.) and deepening abaxially. Anterior cranidial border narrow (sag.), moderately convex, slightly upturned. Anterior border furrow deep and wide (sag., exsag.), with deeper (possibly apodemal) pits situated at the anterolateral corners of the glabella. Preocular field, strongly convex, strongly downsloping toward the anterior border furrow, 21% of sagittal cranidial length between the anterior border and γ. Palpebral lobes reniform in outline, defined by a narrow (tr.), shallow palpebral furrow, anterior tip situated opposite S2, posterior tip situated opposite L1. Eye ridge weakly defined on external surface, slightly more defined on exfoliated surfaces, extending posterolaterally from just anterior to S 2 in a slightly anteriorly curved line towards the anterior tip of palpebral lobe. Palpebral area of fixigena slightly convex, downsloping towards the axial furrow, maximum width (tr.) is adjacent to 92% of the glabellar width. Postocular field moderately long (exsag.) and strongly downsloping toward the posterior border furrow. Posterolateral projections of fixigena strongly downsloping. Posterior border narrow (exsag), widening abaxially, separated from fixigenal field by deep, wide (exsag.) border furrow. Posterior border furrow straight proximally, then directed posterolaterally to ε.

Hypostome of moderate size, up to 5 mm in length (sag.), strongly convex (sag., tr.), anterior margin weakly curved, lateral margins concave anteriorly and convex posteriorly, posterior margin strongly rounded. Middle body anterior lobe strongly convex and ovate. Posterior lobe moderately convex, narrow (sag., exsag.), strongly crescentic; anterior wings short (tr.), subtriangular in outline. Middle furrow slit-like and shallow.

Librigena up to 10 mm in length excluding spine. Lateral margin, including that of genal spine, moderate to strongly curved anteriorly. Posterior margin curved slightly forward distally before abruptly curving rearwards to become the inner margin of the genal spine. Genal field subtrapeziform, 75% of librigenal width (tr.) along posterior border, moderately convex. Lateral border moderately well defined, widest near the posterior border. Lateral and posterior border furrows wide (exsag.) and moderately deep. Low eye socle present, separated from librigenal field by wide, shallow eye socle furrow. Genal spine blade-like, approximately the same length as the librigenal field.

Pygidium up to 10 mm long (sag.), subtrapezoidal in outline excluding spines, moderately convex, sagittal length:width ratio of 47% to 54% (mean 50%; n=4) excluding spines. Anterior margin slightly curved, with the articulating half-ring being slightly convex. Axis prominent, moderately narrow (tr.), moderately tapered posteriorly, width:length ratio of 70%, occupying about 85% of sagittal length of pygidium. Articulating half-ring incompletely preserved, appears to have been short (sag.). Four well defined axial rings present, separated by deep, wide (sag., exsag.) inter-ring furrows; a faint fifth axial ring present in some specimens, separated from the terminal piece by a poorly developed inter-ring furrow. Terminal piece short (sag.). Axial furrow deep and wide (tr.). Pleural regions slightly convex, with four wide (exsag.), moderately deep pleural furrows. Furrows are contiguous with the lateral extremities of the axial rings; anterior furrows directed slightly backwards from the axial furrow and distally curved backwards, and becoming shorter and more backwardly directed posteriorly, terminating before reaching margin. Pleural ribs strongly convex. Faint, shallow, narrow (exsag.) interpleural furrows on first and second pleural ribs, extending the entire length of the rib. Border of moderate width (sag., exsag.), defined by change in convexity from the pleural region; with seven pairs of spines, first and fifth pair are considerably longer and proximally wider (tr.). Anterior first to fourth pair of spines extending posterolaterally and become progressively more backwards pointing towards the tips. Posterior sixth and seventh spine pairs directed straight back.

Prosopon consists of granules of various sizes. Glabella, palpebral areas and postocular fields of fixigena as well as the posterolateral projections are densely covered in small granules with medium-sized granules randomly and equidistantly dispersed in-between. Occipital ring densely covered in small granules. Librigenal field densely covered in small and medium-sized pustules with a faint anastomosing network of genal caeca also discernable, librigenal border and spine densely covered with small granules. Pygidial axis, ribs and spines densely covered with small granules. Medium-size pustules interspersed on the axial rings and pleural ribs.

Rostral plate and thorax unknown.

Discussion. This taxon was originally reported as “Damesellinae, fragments” from Ross River Gorge at locality NT187 by Öpik (1967, appendix 2, p. 16). The specimens from the GOY section and locality 87-008 are remarkably similar, although librigenae from the former site exhibit a slightly shorter genal spine. This minor difference is either ontogenetic (the 87-008 specimens are slightly larger) or taphonomic (GOY specimens are preserved in limestone rather than a coarse sandstone). The GOY specimens also exhibit minor variation in the degree of glabellar taper during ontogeny. The lateral glabellar margins of smaller holaspid specimens converge anteriorly at approximately 20° ( Fig. 10D, H and I ), whilst larger specimens ( Fig. 10A, B and E ) have slightly less convergence at approximately 12°.

The Goyder Formation specimens are considered conspecific with Blackwelderia repanda Öpik, 1967 from the Georgina Basin. The specimens share a short (sag.) upturned anterior border, a relatively short (sag.) anterior border furrow, a slightly tapered glabella, narrow (tr.) S1 and S2 furrows, faint eye ridges, a pygidium with the first and fifth pair of spines longer (exsag.) and wider (tr.) than other spine pairs, as well as a granulose and pustulose prosopon. The Goyder Formation specimens are also similar to material identified as Blackwelderia sp. cf. B. repanda by both Wang et al. (1989) and Shergold et al. (2007) from the Mindyallan of N.S.W. and Western Australia, respectively. Although the specimens from N.S.W. are distorted, they have a short (sag.) anterior border, a slightly tapered glabella, narrow (tr.) and deep S1 and S2 furrows, faint or absent eye ridges, and a pygidium with long and wide first and fifth spine pairs ( Wang et al. 1989, fig. 4R–T, 5A–I). Likewise, the specimens from Western Australia, although poorly preserved in coarse sandstone, exhibit these same features ( Shergold et al. 2007, fig. 11A–F). The specimens from Western Australia differ in having a slightly more tapered glabella with a rounder anterior, and fainter pleural furrows on the pygidium. However, this falls within the range of intraspecific variation seen in other members of the genus. The synonymy of the Western Australian material is queried only because of the incomplete and limited material illustrated by Shergold et al. (2007).

Blackwelderia repanda can be distinguished from other species attributed to this genus by its shorter (sag.) anterior cranidial border and anterior border furrow, the pygidial axis consisting of four (or possibly five) axial rings, faint interpleural furrows on the first and second pleural ribs, seven pygidial spines, with the first and fifth being longer (exsag.) and wider (tr.) than the others, as well as a granulose and pustulose prosopon. Blackwelderia repanda is most comparable to B. gibberina Öpik, 1967 , from the Georgina Limestone , which has similar cranidial and pygidial features (cf. Öpik 1967, pl. 48, figs 1–3), but differs from B. repanda only in having the first pygidial spines slightly wider and longer and the fifth spine pair being narrower and shorter. Öpik (1967) also suggested that B. gibberina differed in having an almost transverse anterior cranidial border and pygidium without interpleural furrows.

Occurrence. GOY section horizons 73.2, 83.9, 132.7 and 140.7 m ( Fig. 3 ). Specimens also recovered from 87- 0 0 8 and GOYWEST.

Distribution. Goyder Formation, Amadeus Basin, Northern Territory; O’Hara Shale, Georgina Basin , Northern Territory and Queensland; Boshy Formation, Koonenberry Belt, New South Wales. Possibly the Skewthorpe Formation, Bonaparte Basin, Western Australia. 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 11. Blackwelderia repanda Öpik, 1967. A comes from spot locality 85-008; and B–N come from GOY/173.6. A, CPC42258, partial pygidium. B, J, CPC42259, partial pygidium; B, dorsal view; J, close up of pygidial ornament. C, D, I, CPC42260, partial pygidium; C, lateral view; D, dorsal view; I, close up of pygidial spine ornament. E, CPC42261, partial pygidium. F, CPC42262, partial pygidium. G, CPC42263, partial pygidium. H, CPC42264, partial pygidium. K, CPC42265, partial pygidium.L, CPC42266, partial pygidium. M, CPC42267, partial pygidium.N, CPC42268, partial pygidium. All scale bars are 2 mm, except A = 4 mm, E, F, I, N= 1 mm, and J = 0.5 mm.

FIGURE10. Blackwelderia repanda Öpik, 1967. A–N, Q, R, U comefrom sampleGOY/173.6; and O, P, S, T come from spot locality 85-008. A, B, J, K, L, CPC42243, partial cranidium; A, dorsal view, latex cast; B, dorsal view; J, anterior view; K, lateral view; L, oblique anterolateral view. C, CPC42244, partial cranidium. D, CPC42245, partial cranidium, latex cast. E, CPC42246, partial cranidium. F, CPC42247, partial cranidium. G, CPC42248, partial cranidium. H, CPC42249, partial cranidium.I,CPC42250,partial cranidium.M, CPC42251, close up of cranidial ornament.N, CPC42252,close up of librigenal ornament.O, P, CPC42253,partial librigena; O, dorsal view; P, lateral view. Q, R, CPC42254, partial librigena; Q, dorsal view; R, lateral view. S, CPC42255, partial cranidium. T, CPC42256, partial hypostome.U, CPC42257, partial hypostome. All scale bars are 2 mm, except A–C, E, L, Q, R = 4 mm, F, H, I, U = 1 mm, and M, N = 0.5 mm.