Acodus sp.

Zhen, Yong Yi, Percival, Ian G. & Webby, Barry D., 2003, Early Ordovician Conodonts from Far Western New South Wales, Australia, Records of the Australian Museum 55 (2), pp. 169-220 : 180-182

Acodus sp. cf. emanuelensis McTavish, 1973

Fig. 7A–Y

Acodus emanuelensis .– Cooper, 1981: 158, pl. 28, figs. 1, 5, 6, 9, 10, 12.

Material. Forty-one specimens (7 Pa, 10 Pb, 9 M, 2 Sa, 3 Sb, 6 Sc, 4 Sd) from limestone nodules within shales of the upper Yandaminta Quartzite, and 1475 specimens (162 Pa, 322 Pb, 332 M, 179 Sa, 191 Sb, 95 Sc, 194 Sd) from the overlying Tabita Formation at Mount Arrowsmith; 40 specimens (2 Pa, 4 Pb, 14 M, 7 Sa, 2 Sb, 5 Sc, 6 Sd) from unnamed dolomitic limestone unit at Koonenberry Gap.

Diagnosis. A septimembrate species of Acodus , consisting of acodiform Pa and Pb, makellate M, and costate S elements bearing two (Sc), three (Sa and Sb), and four (Sd) strong, blade-like costae.

Description. Pa element with robust, laterally compressed, erect or slightly proclined cusp, anteroposteriorly extended base and moderately open basal cavity; cusp slightly curved inwards with straight, sharp anterior margin and nearly straight and sharp posterior margin; base triangular in outline in lateral view, formed by adenticulate posterior process and triangular anticusp; posterior process with straight or gently arched upper margin meeting straight posterior margin of the cusp at an angle of 90–115°; basal margin straight, forming an angle of about 50° with anterior margin; inner lateral face typically with a prominent, rounded-faced mid costa; outer lateral face smooth or with weak mid carina ( Fig. 7A–F). Pb element similar to the Pa, but with its cusp slightly reclined, and also more curved inwards ( Fig. 7G); in lateral view, curvature of anterior margin variable; posterior margin also curved, forming an angle of 50–90° with upper margin of the posterior process; anticusp less pointed, with an angle of 60–65° between basal and anterior margins ( Fig. 7I–L). M element makellate, anteroposteriorly compressed, with broad, smooth anterior face ( Fig. 7N,O), and a weak basal buttress on the posterior face ( Fig. 7H,M); anticusp triangular in outline; outer lateral process low, adenticulate with straight or slightly arched upper margin. Sa element symmetrical, triconodelliform, with broad anterior face, a blade-like costa on each lateral face and a third costa along the posterior margin; the three costae extended basally as short adenticulate processes ( Fig. 7P–R). Sb element like the Sa, but asymmetrical ( Fig. 7S,T). Sc element laterally compressed with a sharp costa along anterior and posterior margins, which extend basally merging into the upper margin of a low posterior process and an anticusp-like shorter anterior process; cusp slightly inner laterally curved, with smooth lateral faces ( Fig. 7U,V). Sd element asymmetrical with four blade-like costae, which extend basally into short adenticulate processes ( Fig. 7W– Y); costa along the posterior margin stronger than others; one costa on each lateral face, and the fourth costa along the anterior margin, but inner laterally curved ( Fig. 7W).

Remarks. Specimens from western New South Wales are identical with those described as A. emanuelensis from the Horn Valley Siltstone of central Australia ( Cooper, 1981). Although generally comparable with the type material from the Emanuel Formation of the Canning Basin, the New South Wales specimens do not develop the prominent thin blade-like arched upper margin of the posterior process displayed by the figured specimens from the Canning Basin. The Horn Valley material will be described as a new species by R.S. Nicoll (in prep.), to which the western New South Wales specimens would be assigned. Pending completion of that study, the present material is left in open nomenclature. Acodus sp. cf. emanuelensis , the most common species in the Mount Arrowsmith fauna, exhibits a wide range of variation. Specimens from samples with associated sponge spicules, such as M/A7 (inferred to represent relatively deeper water environments) are smaller, with costae extended basally into adenticulate processes in the S elements ( Fig. 7Y). Specimens from other, presumably shallower water, samples are larger with more weakly developed processes ( Fig. 7W).