Pagodia flexuosa, Opik, 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 : 38-39

Polycyrtaspis cf. flexuosa Öpik, 1967

Fig. 18

Material. Two partial cranidia figured, CPC42328, CPC42329.

Description. Cranidium up to 7 mm long (sag.) outline incompletely preserved; strongly convex (sag., tr.). Anterior margin a slightly rounded obtuse angle. Posterior margin incompletely preserved. Anterior branches of facial suture (γ-β section) diverge moderately from one another at 36°, then curve abruptly towards the midline from β before they reach the anterior margin. Glabella strongly tapered at 47°, pyriform in outline; width (tr.) across anterior 29% maximum width; moderately convex; maximum convexity across midwidth, lateral slopes gently convex; width:length ratio of 96%, occupying 72% of the cranidial length; anterior axial furrow wide (sag., exsag.) and shallow, disappearing around median preglabellar ridge; lateral axial furrow narrow (tr.) and moderately deep. S1 well defined, deep and narrow (exsag.), intersecting axial furrow approximately level with the anterior of the palpebral lobe, directed posteromedially. S2 and S3 much fainter; S 2 in level with ε and directed posteromedially. S3 reduced to a slight indentation in glabellar margin, positioned slightly anterior to ε. Occipital ring short (sag.), posterior margin incompletely preserved; surmounted by a small occipital node medially. SO moderately deep and seems to be widest (sag.) medially, with the lateral corners being slightly narrower and deeper; bowed backwards laterally and strongly forward medially. Anterior cranidial border moderately narrow (sag., exsag.), occupying about 10% of sagittal cranidial length, narrowing abaxially; outline a wide (tr.) based, broadly triangular shape; slightly convex. Anterior border furrow narrow (sag., exsag.) and deep, disappears in front of median preglabellar ridge. Preocular field slightly convex, moderately closely spaced toward the anterior border furrow. Preglabellar field with narrow (tr.) ridge crossing sagittally the preglabellar field, ridge traces the sagittal line between the glabellar anterior and anterior border furrow, nested within a preglabellar furrow. Preocular field and preglabellar field elongate (sag., exsag.), 18% of sagittal cranidial length. Palpebral lobes long, of moderate width (tr.), 30% cranidial length, reniform in outline; anterior tip opposite S1, posterior tip just posterior of SO. Eye ridge incompletely preserved. Palpebral area of fixigena slightly downsloping towards the axial furrow, maximum width (tr.) is 44% the adjacent glabellar width; a pair of thin ridges run along the inner margin of the eye ridges and palpebral lobes. Postocular field, posterolateral projections and posterior border all incompletely preserved.

Prosopon over most of cranidium smooth. Anterior border covered by fine horizontal terrace ridges, and frontal area possibly covered by very faint caeca.

Hypostome, rostral plate, librigena, thorax and pygidium unknown.

Discussion. This taxon is represented by two fragmentary cranidia from the upper sandy limestone beds of the GOY section ( Fig. 3 ). These specimens compare closely with cranidia of Polycyrtaspis flexuosa Öpik, 1967 from the Mindyallan O’Hara Shale in the Georgina Basin. In the Goyder Formation cranidia, the broadly triangular anterior cranidial border, preglabellar field with weak median preglabellar ridge, pyriform glabella with welldefined furrows, long palpebral lobes, and a pair of thin ridges running along the inner margin of the eye ridges and palpebral lobes all conform with features in the holotype (cf. Öpik 1967, pl. 9, fig. 1). The Goyder cranidia are slightly wider (tr.) with a more strongly tapered glabella, but without more specimens, it is difficult to assess whether these differences are ontogenetic features or genuine taxonomic distinctions. A second, much smaller cranidium of P. flexuosa illustrated by Öpik (1967, pl. 9, fig. 2) varies slightly compared to the holotype, displaying a less tapered glabella and broader (tr.), more strongly curved palpebral lobes. The size and morphology of the Goyder Formation specimens do not appear to contradict these ontogenetic trends, so it is possible they represent the same species.

Occurrence. GOY section horizon 132.7 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 18. Polycyrtaspis cf. flexuosa Öpik, 1967. A, B, C, E come from sample GOY/173.6; and D, F come from sample GOY/168.9.A, C, E, F,CPC42328, partialcranidium; A, dorsal view;B,anterior view; C, oblique anterolateral view; E, lateral view.D, F, CPC42329, partial cranidium; D, dorsal view; F, oblique anterolateral view. All scalebars are 2 mm.