Liostracina, Monke, 1903

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 : 45-46

Liostracina sp.

Fig. 22

Material. One cranidium figured, CPC42356. Nine cranidia not figured (mostly fragments).

Description. Cranidium up to 5 mm long (sag.), outline transversely subelliptical, maximum width across posterolateral projections of fixigenae, narrowest point at anterior tip of palpebral lobes (γ–γ); slightly convex (sag., tr.). Anterior margin strongly rounded. Posterior margin straight. Anterior branches of facial suture (γ-β section) diverge moderately from one another at 35°, then curve abruptly towards the midline from β level with the border furrow. Glabella anteriorly truncate, parallel-sided, oblong in outline; moderately convex, with maximum convexity across midwidth, lateral slopes gently convex; width:length ratio of 58%, occupying 61% of the cranidial length. Axial furrow narrow and deep. S1 short (tr.), deep and narrow (exsag.), intersecting axial furrow slightly anterior to ε, directed posteromedially for a short distance before fading. S2 faint, short (tr.), shallow, intersecting axial furrow at δ, directed transversely. S3 faint, short (tr.), shallow to almost completely effaced, intersecting axial furrow slightly anterior of γ, directed slightly anteromedially. Occipital ring of moderate length (sag.), posterior margin bowed slightly backwards, lacking axial node. Moderately shallow, narrow (sag.) SO slightly bowed anteromedially. Anterior cranidial border of moderate length (sag., exsag.), occupying about 12% of sagittal cranidial length, narrowing strongly abaxially. Anterior border furrow slightly narrow (sag., exsag.) and shallow. Preocular field moderately convex, gently downsloping toward the anterior border furrow. Preglabellar field with a narrow (tr.), faint and shallow preglabellar furrow, small pit at junction between the median preglabellar furrow and the anterior border furrow. Preglabellar field 24% of sagittal cranidial length. Palpebral lobes (tr.), 16% cranidial length, reniform in outline, defined by a wide (tr.), shallow palpebral furrow; anterior tip situated slightly anterior of S2, posterior tip level with the point between the midlength of S1 and SO. Eye ridge faint to almost completely effaced. Palpebral area of fixigena transverse, maximum width (tr.) is slightly greater than the adjacent glabellar width. Postocular field short (exsag.), and slightly downsloping toward the posterolateral projections; faint, elongate (exsag.) bacculae, near axial furrow, anterior tip situated opposite S1, posterior tip opposite anterior of SO. Posterolateral projections of fixigena narrow (exsag.), strongly downsloping towards lateral corners. Posterior border narrow (exsag.), separated from fixigenal field by deep, moderately narrow (exsag.) border furrow.

Prosopon over cranidium smooth with faint anastomosing network of genal caeca on exfoliated region of preglabellar field.

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

Discussion. This species is primarily represented by fragments. Only a single, nearly complete cranidium from AS 178 provides detailed morphology. This taxon was originally reported from Ross River Gorge (locality NT187) as Liostracina cf. volens by Öpik (1967), but was never figured nor described.

The best specimen from the Goyder Formation ( Fig. 22 ) resembles Liostracina volens Öpik, 1967 from the Mindyallan O’Hara Shale in the Georgina Basin , having a glabella of similar width and length, faint eye ridges, short (exsag.) palpebral lobes, and a smooth prosopon (cf. Öpik 1967, pl. 35, fig. 1–3). The Goyder Formation taxon differs in having a longer anterior cranidial border, more deeply incised glabellar furrows, a pit at the junction between the median preglabellar furrow and the anterior border furrow, an occipital ring that is not strongly arched backwards and lacks the occipital node, and a narrower posterior border furrow.

Occurrence. Only recovered from AS178 ( 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 22. Liostracina sp. Specimen comes from spot locality AS178. A, B, C, D, CPC42356, partial cranidium; A, dorsal view; B, oblique anterolateral view; C, lateral view; D, anterior view.All scale bars are 1 mm.