Trephina, Smith & Paterson & Brock, 2018

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 : 16-17

Trephina gen. nov.

Type species. Trephina ranfordi gen. et sp. nov.

Etymology. After Trephina Creek, near Ross River Gorge.

Diagnosis. Cranidium subtrapezoidal in outline, length:width ratio of 60%. Glabella anteriorly truncate, subquadrate in outline, width:length ratio of 60% to 68%. Anterior of glabella merges with anterior border, preglabellar furrow effaced; axial furrow narrow (tr.) and deep. S1 moderately well defined and bifurcated, with the anterior branch directed anteromedially and the posterior branch directed posteromedially; S2 faintly defined and bifurcated, with the anterior branch directed slightly anteromedially and the posterior branch directed slightly posteromedially; S3 relatively faint, directed anteromedially; S4 variably developed, directed anteromedially. SO bowed backwards. Anterior cranidial border narrow, 5% to 10% of sagittal cranidial length. Preocular field 28% of sagittal cranidial length between the anterior border and γ. Palpebral lobes 33% cranidial length, reniform in outline. Palpebral area of fixigena 60% of adjacent glabellar width. Posterolateral projections at posterior of palpebral lobe 13% total cranidial length. Genal spine blade-like, length 41% librigenal length. Pygidium up to 8 mm long (sag.), subtriangular in outline slightly convex, length:width ratio of 64% to 69% excluding spines. Axis prominent, moderately narrow (tr.), width:length ratio of 75% to 81%, occupying about 65% of sagittal length of pygidium. Three well-defined axial rings and a faint fourth present. Small terminal piece. A single pair of broadbased spines attached to pygidium anterolateral corners, spines are broad (tr.) and blade-like, curved posteriorly. Prosopon comprising closely spaced granules over most of the cephalon, except the posterolateral corners of the glabella, palpebral lobes and librigenal borders that are covered by small, closely spaced pits. Anterior cranidial border and the librigenal border also covered by well-defined horizontal terrace ridges. Pygidium covered by small, closely spaced pits over pygidial axis; pleural field with moderately sized pustules on pleural ribs. Spines and posterior margin covered with fine horizontal terrace ridges and evenly spaced pustules near the spine bases.

Discussion. This new genus has similarities to various leiostegioids, such as Pagodia Walcott, 1905 , Chuangia Walcott, 1911 and Lophoholcus Öpik, 1967. Cranidia of Trephina gen. nov. and some species of Pagodia (cf. Öpik 1967, pl. 17, figs 1, 2, 4; Shergold 1975, pl. 29, figs 5, 6; Shergold 1982, pl. 13, figs 1, 2, 4; Zhang & Jell 1987, pl. 94, figs 8, 10, 11, pl. 95, figs 2, 4, 5) possess an anteriorly truncate, subquadrate glabella that slightly constricts at or anterior to the mid-length, a short (sag., exsag.) anterior border, and prominent reniform palpebral lobes. Both genera also possess an anterior border furrow that is often obscured by the encroaching anterior glabellar lobe. However, the Goyder cranidia mainly differ in possessing medial effacement of the anterior border furrow through fusion of the anterior glabellar lobe with the anterior border, and in having distinctly incised glabellar furrows (particularly the bifid S1). Chuangia possesses some of the cranidial features mentioned above, although species typically have an effaced and more anteriorly tapered glabella (cf. Zhang & Jell 1987, pl. 89, figs 8, 9, pl. 90, figs 1, 3, 5, 12, pl. 91, figs 2, 6, 7, 8, pl. 92, figs 1, 3, 5, 7, 11, 12, pl. 93, figs 1–8; Peng et al. 2004a, pl. 20, figs 1–8; Choi et al. 2008, figs 15.1, 15.2, 15.4–15.7).

Trephina gen. nov. shares features with the monotypic genus Lophoholcus (type species: Lophoholcus asper Öpik, 1967) from the Mindyallan O’Hara Shale in the Georgina Basin. Both have similar cranidial and glabellar outlines, development of glabellar furrows, and shape of the anterior border. Lophoholcus differs from Trephina in having an anterior border separated from the glabella by a deep border furrow, lower glabellar convexity, presence of an occipital node, narrower (tr.) palpebral areas, longer (exsag.) and narrower (tr.) palpebral lobes, and a pygidium with a much wider (tr.) axis.

Pygidia of Trephina ranfordi were originally reported by Öpik (1967, appendix 2, p. 16) as ‘ Plectrifer? sp. nov. (pygidia)’, but they show little similarity to this genus. Plectrifer Öpik, 1967 differs in having a long axis that extends more than 90% the length of the pygidium and a distinct pygidial border furrow. The material from the Goyder Formation more closely resembles other leiostegioids, in possessing a single pair of posteriorly directed spines along the lateral border (e.g. Prochuangia Kobayashi, 1935 and Lotosoides Shergold, 1975). However, most of these taxa can be distinguished because their spines emerge from behind the anterolateral corners of the pygidium. Perischodory grandgei Raymond, 1937 from the Lower Ordovician Highgate Formation of the USA (see Raymond 1937, pl. 4 fig. 28; Shaw 1966, p1. 161, figs 3, 4) is similar to the new taxon in that it has a comparable subtriangular pygidium with a single pair of spines placed at the anterolateral extremities. However, the Goyder Formation material differs in having a shorter (sag.) axis, deeper pleural furrows, fainter posterior border, more posteriorly directed spines, and a pleural field with minute granules on the pleural ribs. Placement of spines in the anterolateral corners of Trephina gen. nov. is also comparable to material described as Pagodiinae gen. et sp. indet. from the Scopus Formation at Christmas Hills in Tasmania (Jago & Bentley 2007, p. 292, fig. 4D– J), which is Boomerangian in age (Jago et al. 2016). The Goyder Formation taxon differs in having a proportially shorter (sag.) axis and more posteriorly directed spines.