Paraacidaspis, Poletaeva, 1960

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 : 39-40

Paraacidaspis ? priscilla (Öpik, 1967)

Fig. 19

1967 Saukia ? priscilla; Öpik, p. 250–251, pl. 30, figs 5–7.

Material. Six pygidia figured, CPC42330–CPC42335. Four pygidia not figured (mostly fragments).

Description. Pygidium up to 5 mm long (sag.), semicircular, only slightly convex, length:width ratio of 65%. Anterior margin slightly arched forward. Posterior margin broadly rounded, slight anterior inflection in margin medially near the postaxial ridge. Axis prominent, moderately narrow (tr.), tapering posteriorly, width:length ratio of 71%; small, occupying about 60% of sagittal length of pygidium. Articulating half-ring narrow (sag.), defined by narrow (sag.), shallow articulating furrow. Five moderately well developed to faint axial rings present, separated by shallow, narrow (sag.) inter-ring furrows. Terminal piece small. Long (sag.), postaxial ridge continues from the terminal piece until just before the posterior border, ridge the same width (tr.) as the terminal piece. Axial furrow deep and narrow (tr.), fading around terminal piece. Pleural regions slightly convex, with five narrow (exsag.), shallow pleural furrows that terminate just before reaching margin; pleural furrows directed posterolaterally; five distinct, shallow, narrow (exsag.) interpleural furrows present, following the same course as pleural furrows, dividing the pleura into a slightly narrower (exsag.) anterior band and a wider (exsag.) posterior band. Border not well defined.

Prosopon over pygidium of fine pustules.

Cephalon, hypostome, rostral plate and thorax unknown.

Discussion. The pygidia from the Goyder Formation are almost identical to those reported as Saukia ? priscilla by Öpik (1967) from the Mindyallan O’Hara Shale and Georgina Limestone in the Georgina Basin. Both show a semicircular outline, a short axis, five axial rings (see Opik 1967, pl. 30, fig. 7), a long (sag.) postaxial ridge, five narrow (exsag.) pleural and interpleural furrows, pleurae divided into a narrow anterior and wide posterior bands, no pygidial border, and a fine pustulose ornament. The only minor difference is that the Georgina Basin pygidia lack the subtle indentation in the posteromedial margin, immediately behind the postaxial ridge. These slight variations are more likely to be preservational, since two of the three Georgina Basin specimens do not have a complete posterior margin.

Öpik (1967) tentatively assigned the material from the Georgina Basin to Saukia Walcott, 1914 . This assignment was largely based on the presence of five axial rings and five pleural furrows, which Öpik (1967) suggested was similar to both Saukia and Tellerina Ulrich & Resser, 1933 . The pygidia illustrated here and by Öpik (1967) are unlikely to belong to either of these genera, since Saukia and Tellerina typically have a shorter postaxial ridge, and shorter (tr.) pleural and interpleural furrows. A more compelling comparison can be made with Paraacidaspis , especially P. hunanica Egorova in Poletaeva, 1960 and Paraacidaspis sp. (Peng et al. 2004a) from the Guzhangian of South China, and P. ultima Shergold, Feist & Vizcaino, 2000 from the Guzhangian–Paibian of southern France. Paraacidaspis hunanica and P. ultima have essentially identical pygidia and both are similar to those from the Goyder Formation and Öpik’s (1967) Georgina Basin material. Paraacidaspis sp. described by Peng et al. (2004a) also has a similar pygidium, although it has a more coarsely granulose prosopon making it immediately distinguishable from the material reported herein. The Goyder Formation and Georgina Basin material differs from these previously described species in possessing the following features: a slightly longer (sag.) axis, narrower anterior pleural bands, and wider posterior pleural bands. These minor differences are likely of only interspecific significance, hence, Saukia ? priscilla Öpik, 1967 is questionably reassigned to Paraacidaspis .

Similar pygidia have been described from the Boomerangian to Idamean Spurs Formation in Antarctica. The pygidium assigned to Nganasanella? sp. by Cooper et al. (1996, fig. 5U) is small and incomplete, but is comparable to the Goyder Formation material. Only minor differences are apparent, including a shorter (sag.) axis and a relatively wider anterior pleural band. Coosia ? sp. 2 illustrated by Jago & Cooper (2005, fig. 6S) has a similar outline to the Goyder Formation species, but possesses more pleural and interpleural furrows than the Goyder Formation pygidia. The pygidium of Coosia ? sp. 1 illustrated by Jago & Cooper (2005, fig. 6R) differs from the Goyder Formation specimens in having a more subcircular outline, narrower (tr.) axis, wider (exsag.) anterior pleural band, and a longer (sag.) terminal area. Material described by Bentley et al. (2009, figs 7K, L, 8U) as Coosia ? sp. also differs in having wider anterior pleural bands and a longer (sag.) terminal area. Given these relatively minor differences, it is possible that the Antarctic taxon—following the synonymy of Bentley et al. (2009, p. 183)—represents a new species of Paraacidaspis .

Another similar pygidium, identified as? Paraacidaspis sp., was described from the Guzhangian of Sweden by Żylińska et al. (2015). This specimen differs from previously described taxa, including Paraacidaspis ? priscilla (Öpik, 1967), in possessing a subrectangular outline, only four pleural furrows, and a narrower (tr.) postaxial ridge.

Occurrence. GOY section horizons 73.2 and 83.9 m ( Fig. 3 ) and at AS 168.

Distribution. Goyder Formation, Amadeus Basin, Northern Territory. O’Hara Shale and Georgina Limestone, Georgina Basin , Northern Territory and Queensland. 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 19. Paraacidaspis? priscilla (Öpik, 1967). A comes from sample GOY/109.2; and B–F come from spot locality AS168. A, CPC42330, partial pygidium. B, CPC42331, partial pygidium, latex cast. C, CPC42332, partial pygidium. D, CPC42333, partial pygidium. E, CPC42334, partial pygidium, latex cast. F, CPC42335, partial pygidium. All scale bars are 2 mm, except C = 1 mm.