Ammagnostus antarcticus Bentley, Jago & Cooper, 2009

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 : 12-14

Ammagnostus antarcticus Bentley, Jago & Cooper, 2009

Fig. 5

?1967 Agnostoglossa bassa ; Öpik, p. 146, pl. 60, figs 6–14, text-fig. 44.

?1989 Ammagnostus sp. indet.; Wang, Mills, Webby & Shergold, p. 111, fig. 4A–D. 2009 Ammagnostus antarcticus ; Bentley, Jago & Cooper, p. 171, 172, fig. 4A–I.

Material. Nineteen cephala and eight pygidia figured, CPC42177–CPC42203. 26 cephala and eleven pygidia not figured (mostly fragments).

Description. Cephalon and pygidium each attaining about 2.6 mm in length; nonscrobiculate; surface smooth. Cephalon subquadrate in outline, about 94% as long as wide, nonspinose, moderately convex; border reasonably wide (sag., exsag.), flattened; border furrow well developed; preglabellar median furrow faintly developed or absent. Glabella 70–73% (mean 71%, n = 4) as long (sag.) as cephalon. Small anterior lobe clearly defined, semiovate in shape, obtusely angular to rounded anteriorly, occupying 29–30% (mean 30%, n = 4) of glabellar length. F3 deeply incised, strongly bowed rearward in a V-shape, bowing is weaker in smaller specimens. Posterior glabellar lobe wider (tr.) than anterior lobe, slightly tapers forward, convexity similar in both lobes. F2 furrows at glabellar mid-length (sag.), moderately developed as distinct notches, gently directed anteriorly for a short distance. F1 furrows effaced. Glabellar node indistinct, located slightly posterior of F2 furrows; basal lobes small, subtriangular in outline.

Pygidium rounded to weakly quadrate in outline, 90% as long as wide, moderately convex, with short (exsag.) posterolateral spines, situated level with widest point in pygidial axis; border flattened, wide (sag., exsag.) and narrows distinctly toward anterior; border furrow moderately deep and wide (sag., exsag.). Axis wide (tr.) and long (sag., exsag.), reaching posterior border furrow; about 64 to 70% (mean 67%, n = 5) as wide as long and occupying 73% of pygidial length (sag.); slightly constricted across M2 and expanding to become wider behind F2; posteriorly rounded to obtusely rounded and wide (tr.), moderately convex. F1 and F2 furrows effaced. M1 lobe broad (tr.), lateral margins slightly convex. M2 lobe only distinguished from M1 by constriction. Axial node well developed, of moderate size, extending a short distance onto posteroaxis. Posteroaxis occupying about 58 to 63% (mean 60%, n = 3) of axial length, with maximum width slightly posterior of lobe mid-length. Small secondary axial node developed on some specimens, located a short distance posterior to the maximum width of the posteroaxis.

Hypostome and thorax unknown.

Discussion. The larger cephala and pygidia from the Goyder Formation most closely resemble those of Ammagnostus antarcticus Bentley, Jago & Cooper, 2009 described from the Mindyallan Spurs Formation in Antarctica. Similarities include: a long (sag.) anterior border with a shallow border furrow; a glabella that is approximately 70% the length (sag.) of the cephalon; F3 that is strongly bowed rearward in a V-shape; and an indistinct glabellar node. They also share a pygidium that is wider than long with a well developed axial node that encroaches onto the posteroaxis, and a posterior border that is very wide (sag.), with small posterolateral spines. The holotype cephalon of A. antarcticus (Bentley et al. 2009, fig. 4E) is 8.1 mm in length (sag.), which is more than 3 times larger than the largest specimen from the Goyder Formation (at 2.6 mm; Fig. 5A ). The other cephala figured by Bentley et al. (2009, fig. 4A–C, F) are closer in size to the specimens reported here. Given the close morphological similarity to A. antarcticus , it is considered likely that the Goyder Formation material represents a collection of mostly juvenile specimens or perhaps a geographic variant. Smaller cephala from the Goyder Formation are reminiscent of Ammagnostus laiwuensis (Lorenz, 1906) and Ammagnostus quadratiformis (Guo & Luo in Guo et al., 1996) —both of which are synonyms according to Peng & Robison (2000) —based on the presence of a shorter (sag.) anterior border and a transverse F3 furrow (e.g. compare Fig. 5K, P with Peng & Robison 2000, fig. 20.1–20.5 and Yuan et al. 2012, pl. 2, figs. 7, 12). This suggests it is difficult to distinguish species of Ammagnostus Öpik, 1967 based on small specimens, prompting taxonomic caution with juvenile material.

Ammagnostus bassa (= Agnostoglossa bassa ) Öpik, 1967 from the Mindyallan Mungerebar Limestone of the Georgina Basin resembles the Goyder Formation material in sharing a moderately long (sag.) anterior border, F3 that is slightly bowed rearward, an indistinct glabellar node, and a pygidium with a posterior border that is relatively long (sag.). However, the coarsely silicified material of A. bassa makes it otherwise difficult to determine if A. antarcticus is synonymous (Öpik 1967, pl. 60, fig. 6–14).

Ammagnostus sp. indet. from the Mindyallan Boshy Formation of N.S.W. ( Wang et al. 1989, fig. 4A-D) also resembles A. antarcticus in having a posterior glabellar lobe that is wider (tr.) than the anterior lobe, F3 that is slightly bowed rearward, an indistinct glabellar node, and a long (sag.) posterior border on the pygidium. The limited amount of poorly preserved material from the Boshy Formation only permits a tentative assignment to A. antarcticus . All other known species of Ammagnostus differ from A. antarcticus in having a shorter (sag.) anterior border, more transverse (or only slightly posteriorly bowed) F3, and a shorter (sag.) posterior pygidial border.

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

Distribution. Goyder Formation, Amadeus Basin, Northern Territory; Spurs Formation, Northern Victoria Land, Antarctica. Possibly the Mungerebar Limestone, Georgina Basin , Northern Territory and Queensland, and 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 5. Ammagnostus antarcticus Bentley, Jago & Cooper, 2009. All specimens come from sample GOY/173.6. A, S, T, U, CPC42177, partial cephalon; A, dorsal view; S, oblique anterolateral view; T, lateral view; U, anterior view.B, CPC42178, partial cephalon. C, CPC42179, partial cephalon. D, CPC42180, partial cephalon. E, CPC42181, partial cephalon. F, CPC42182, partial cephalon.G, CPC42183, partial cephalon.H, CPC42184, partial cephalon.I, CPC42185,partial cephalon. J, CPC42186, partial cephalon. K, CPC42187, partial cephalon.L, CPC42188, partial cephalon. M, CPC42189, partial cephalon. N, CPC42190, partial cephalon. O, CPC42191, partial cephalon. P, CPC42192, partial cephalon. Q, CPC42193, partial cephalon. R, CPC42194, partial cephalon. V, CPC42195, partial cephalon. W, Z, A 1, CPC42196, partial pygidium; W, dorsal view; Z, lateral view; A 1, posterior view. X, CPC42197, partial pygidium. Y, CPC42198, partial pygidium. B 1, CPC42199, partial pygidium. C 1, CPC42200, partial pygidium. D 1, CPC42201, partial pygidium. E 1, CPC42202, partial pygidium. F 1, CPC42203, partial pygidium.All scale bars are 1 mm.