Hebeia helixensis, Jago & Cooper, 2005

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 : 14-16

Hadragnostus helixensis Jago & Cooper, 2005

Fig. 6

2005 Hadragnostus helixensis ; Jago & Cooper, p. 665, fig. 2A–L.

Material. Eleven cephala and nine pygidia figured, CPC42204–CPC42223. 53 cephala and 36 pygidia not figured (mostly fragments).

Description. Cephalon and pygidium each attaining about 3.5 mm in length; nonscrobiculate; surface finely granulose. Cephalon subquadrate to subcircular in outline, about 87% as long as wide, nonspinose, strongly convex; border wide (sag., exsag.), flattened; border furrow well developed, moderately narrow (sag., exsag.); preglabellar median furrow shallow, short (sag.), moderately narrow (tr.). Glabella 71 to 75% (mean 72%, n = 5) as long (sag.) as cephalon. Small anterior lobe clearly defined, subpentagonal to semiovate in shape, obtusely angular anteriorly, occupying 32 to 38% (mean 36%, n = 6) of glabellar length. F3 shallowly incised, bowed moderately rearward; posterior glabellar lobe weakly tapered, strongly convex in its posterior half, less so anteriorly. F2 furrows slightly anterior of glabellar mid-length (sag.), weakly developed as faint notches, gently directed anteriorly for a short distance. F1 furrows effaced to weakly developed; glabellar node indistinct, located slightly posterior of F2 furrows; basal lobes small, subtriangular in outline.

Pygidium rounded to weakly quadrate in outline, 80% as long as wide, strongly convex, short (exsag.) posterolateral spines; border flattened, wide (sag., exsag.) and narrows distinctly toward anterior; border furrow deep and wide (sag., exsag.). Axis wide (tr.) and long (sag.), reaching posterior border furrow; about 68 to 72% (mean 72%, n = 3) as wide as long and occupying 81 to 88% (mean 86%, n = 3) of pygidial length (sag.); slightly constricted across M2 and expanding to become much wider behind F2; posteriorly rounded and wide (tr.); strongly convex. F1 furrow only faintly developed as weak lateral indentations in axial margin; where visible, furrow is bent strongly forward. M1 lobe broad (tr.) being 38% of pygidial width, lateral margins convex, converging posteriorly. F2 furrow effaced to weakly developed in a few individuals, indentations in axial margin develop into a faint furrow crossing the axis; where visible, F2 slightly curved posteriorly behind axial node. M2 lobe constricted, margins variably convex and converging posteriorly. Axial node well developed, of moderate size, elongate, crossing slightly over F2. Posteroaxis occupying about 63% of axial length, with maximum width near posteroaxis mid-length. Proportions of posterior lobe highly variable. Secondary axial node absent.

Hypostome and thorax unknown.

Discussion. The Goyder Formation specimens are considered conspecific with Hadragnostus helixensis Jago & Cooper, 2005 from the Mindyallan Spur Formation in Antarctica. Shared features include: a wide anterior border furrow; an effaced to faintly developed median preglabellar furrow; a large anteroglabella; F3 that is slightly curved posteriorly; a wide (tr.) pygidial axis greater than 50% of pygidial width (tr.); F1 and F2 on the pygidium that are essentially effaced except in exfoliated specimens (e.g. Fig. 6P, X ); a pygidial axis that is constricted (tr.) around M2 and expands (tr.) in the posteroaxis; and a wide posterior border furrow. Only the absence of a node just posterior to F2 on the posteroglabella and the presence of finely granulose ornament differentiate the Goyder Formation material. As the posteroglabellar node and ornament is often difficult to discern in the considerably smaller Antarctic specimens (Jago & Cooper 2005, fig. 2A, B, E, F, H, I), it is likely these differences represent ontogenetic variation. The posteroaxis on the pygidium in the holotype and a paratype specimen (Jago & Cooper 2005, fig. 2C, D) appears less rounded posteriorly than those of the Goyder Formation material ( Fig. 6Q, S, T, U, W, X, Y ). Two smaller pygidia figured by Jago & Cooper (2005, fig. 2G, K) demonstrate this feature is variably developed.

Hadragnostus helixensis differs from the type species, H. las Öpik, 1967, in having a much wider anterior border, a narrower (tr.) median preglabellar furrow, a pygidium that is longer (sag.) than wide (tr.), a wider (tr.) posteroaxis, and a much wider posterior border. Likewise, H. helixensis differs from H. edax Fortey & Rushton, 1976 , described from the Guzhangian of Iran, in having a much wider anterior border, a narrower (tr.) pygidium that is longer sagittally, a greater expansion (tr.) behind M2 of the posteroaxis, and a much wider posterior border. Hadragnostus helixensis is closest to H. modestus from the Guzhangian of South China ( Peng & Robison 2000, p. 32, fig. 23), but H. helixensis differs in possessing a moderately wider anterior border, a narrower (tr.) pygidial axis, and a slightly more posteriorly situated pygidial axial node.

Occurrence. GOY section horizons 49.4, 73.2, 83.9 and 91.9 m ( Fig. 3 ). Also recovered at AS 167, AS 168, AS 178, AS 256 and GOYWEST.

Distribution. Goyder Formation, Amadeus Basin, Northern Territory; Spurs Formation, Northern Victoria Land, Antarctica. 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 6. Hadragnostus helixensis Jago & Cooper, 2005. All specimens come from sample GOY/97. A, L, CPC42204, partial cephalon; A, dorsal view; L, oblique anterolateral view. B, M, O, CPC42205, partial cephalon; B, dorsal view; M, anterior view; O, lateral view. C, CPC42206, partial cephalon. D, CPC42207, partial cephalon. E, CPC442208, partial cephalon.F,CPC42209, partialcephalon.G,CPC42210,partial cephalon.H, CPC42211, partial cephalon.I, CPC42212, partial cephalon. J, CPC42213, partial cephalon. K, CPC42214, partial cephalon. N, P, X CPC42215, partial pygidium; N, posterior view; P, lateral view; X, dorsal view. Q, CPC42216, partial pygidium. R, CPC42217, partial pygidium. S, CPC42218, partial pygidium. T, CPC42219, partial pygidium. U, CPC42220, partial pygidium. V, CPC42221, partial pygidium. W, CPC42222, partial pygidium.Y, CPC42223, partial pygidium. All scale bars are 1 mm.