Leiostegiidae, Smith & Paterson & Brock, 2018

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 : 21-22

Leiostegiidae gen. et sp. indet.

Fig. 9

Material. Seven pygidia figured, CPC42236–CPC42242. Eleven pygidia not figured (mostly fragments).

Description. Pygidium up to 5.2 mm long (sag.), semicircular, moderately convex, length:width ratio of 50% to 51% (mean 50%; n= 3). Anterior margin slightly curved, with the articulating half-ring being more strongly convex; anterolateral corners moderately well rounded. Posterior margin curvature slightly stronger than posterolateral portions. Axis prominent, of moderate width (tr.), strongly tapered posteriorly, width:length ratio of 86%, occupying about 72% of sagittal length of pygidium. Narrow (sag.), articulating half-ring, well defined by a shallow, narrow (sag.) articulating furrow. Two well-defined axial rings, a third faintly developed, separated by shallow, narrow (sag.) inter-ring furrows; furrows only faintly visible medially. Terminal piece of moderate length (sag.), defined posteriorly by gentle change in slope; small, narrow postaxial ridge present, almost extending to posterior margin. Axial furrow deep and narrow (tr.). Pleural regions moderately convex, with four narrow, variably developed pleural furrows that become shorter and fainter posteriorly and terminate well before the pygidial margin; furrows become more posterolaterally directed towards the posterior. Border lacking furrow, differentiated by a slight change in slope near posterior and lateral margins.

Posterior and lateral borders of pygidium covered by small, closely spaced pits. Smaller pits on border gradually transition into larger, slightly more widely spaced pits and small granules on the pleural ribs. Pleural furrows smooth.

Discussion. These pygidia resemble various leiostegioid taxa. The Goyder Formation specimens share a few characteristics with species of Chuangia , which exhibit an almost identical pygidial outline, 3–4 axial rings, and sometimes a distinct postaxial ridge (e.g., Zhang & Jell 1987, pl. 89, fig. 10, pl. 90, figs 7–10, pl. 91, figs 1, 9; Choi et al. 2008, figs 15.8, 15.11). Some pygidia of Chuangia also have a similar pitted ornament, at least on exfoliated specimens (e.g., Zhang & Jell 1987, pl. 90, fig. 10, pl. 91, figs 1, 11, pl. 94, figs 1, 2), but generally differ in having weakly developed furrows on the pleural field. The pygidia described herein also resemble Pagodia (Idamea) baccata Öpik, 1967 from the Pomegranate Limestone in the Georgina Basin , which have a similar pygidial outline and 3–4 axial rings (Öpik 1967, pl. 17, figs 5–7; Shergold 1982, pl. 13, figs 6, 7). However, the Goyder Formation pygidia differ in having a postaxial ridge and more tapered axis. Due to the lack of distinctive pygidial features and an absence of cranidia from the GOY section or spot localities, this taxon is left under open nomenclature.

Occurrence. GOY section horizons 73.2 and 83.9 m ( Fig. 3 ).

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 9. Leiostegiidae gen. et sp. indet. A–C, E–I come from sample GOY/97; and D comes from sample GOY/109.2. A, H, I, CPC42236, partial pygidium; A, dorsal view; H, lateral view; I, posterior view. B, CPC42237, partial pygidium. C, CPC42238, partial pygidium. D, CPC42239, partial pygidium. E, CPC42240, partial pygidium. F, CPC42241, partial pygidium.G, CPC42242, partial pygidium.All scale bars are 2 mm, except B, H = 1 mm.