Cooperignathus aranda ( Cooper, 1981 )

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 : 188-190

Cooperignathus aranda ( Cooper, 1981)

Fig. 12A–P

New genus A Sweet et al., 1971: 168, 170, partim only pl. 1, fig. 22.

New genus A Sweet et al., 1971.– Repetski, 1982: 56, partim only pl. 27, figs. 1, 2, 5, 6.

Protoprioniodus aranda Cooper, 1981: 175 , pl. 29, figs. 1, 6, 7, 10, 12.

Protoprioniodus aranda .–Ethington & Clark, 1982: 86, partim only pl. 9, figs. 27–29.

Protoprioniodus aranda .–Fåhraeus & Roy, 1993: 30, text-figs.

5.21–5.23.

Protoprioniodus aranda .– Pohler, 1994: 27, partim only pl. 6, figs. 10–11,?12.

Protoprioniodus aranda .–Johnston & Barnes, 2000: 42, partim only pl. 6, figs. 17,?26, 30.

Protoprioniodus costatus .– An, 1987: 174, partim only pl. 14, fig. 5.

Protoprioniodus aff. simplicissimus .– An, 1987: 175, partim only pl. 16, fig. 18.

Protoprioniodus nyinti .–Stait & Druce, 1993: 317, partim only fig. 19M.

Protoprioniodus simplicissimus .–Ji & Barnes, 1994: 54, partim only pl. 16, figs. 10, 11.

Material. Two specimens (1 M, 1 Sa) from limestone nodules within shales of the upper Yandaminta Quartzite, and 25 specimens (5 Pa, 2 Pb, 10 M, 3 Sa, 4 Sb, 1 Sc) from the overlying Tabita Formation at Mount Arrowsmith ; four specimens (1 Pa, 2 M, 1 Sc) from unnamed dolomitic limestone unit at Koonenberry Gap .

Diagnosis. A ramiform-pectiniform species with a seximembrate apparatus, including albid, pastiniplanate P elements , makellate M element , alate Sa, and modified tertiopedate Sb and Sc elements ; S elements with a distinctive indentation at the conjunction of the posterior margin of the cusp with the upper margin of the posterior process.

Description. P elements bearing a longer anterior process, a posterior process and a shorter posterolaterally extended outer lateral process; each process with a wide platform and an adenticulate blade-like crest, which join at the centre, but without a recognizable cusp; basal cavity small, represented by a weak keel with a narrow groove underneath each process to join a shallow basal pit ( Fig. 12E ); zone of recessive basal margin not recognized on the basal surface; the Pa element has a wider platform, with anterior and posterior processes strongly curved inwards meeting at an angle of 90–120°; distal end of the posterior process bent sharply downwards; the inner lateral margin of the platform forming a discernible niche at the curvature ( Fig. 12B,D ). Pb element has a narrower platform, and with anterior and posterior processes much less inner laterally curved, meeting at an angle of about 150–160° ( Fig. 12F ). M element anteroposteriorly compressed, with a robust cusp and adenticulate inner and outer lateral processes; in posterior view, the outer lateral process bears a blade-like crest with strongly arched upper margin ( Fig. 12I,J ), and sharply pointed, inner lateral process ( Fig. 12I,K ); basal costa extends more or less parallel to the basal margin from distal end of the outer lateral process to the tip of inner lateral process on both anterior and posterior faces ( Fig. 12I–K ); cusp reclined outer laterally and also slightly curved posteriorly, with a ridge-like costa along the anterior and posterior faces; the costa on the anterior face extends basally to join the basal costa and then merges into a weakly developed basal buttress; the costa on the posterior face generally much stronger, truncated by the basal costa and merging with a more strongly developed basal buttress, with an angle of 50–65° between the posterior costa and the basal costa on the outer lateral process; in posterior view, the basal margin curves upwards distally, with the buttress at the lowest point. S elements ramiform, but adenticulate, with a prominent suberect or proclined cusp, a long posterior process, and a lateral process on each side along the anterolateral corner; a basal costa developed on each side of both processes, more or less parallel to the basal margin, so defining a shallow groove on each side just above the basal margin; posterior process gently curved downwards distally, bearing a blade-like crest, with gently arched upper margin forming a distinct cleft at its junction with the posterior margin of the cusp ( Fig. 12M,N ); lateral processes shorter, extending from tip of the cusp as a blade-like costa on each side and produced basally into a short adenticulate process ( Fig. 12 O,P ); in upper view, the three processes join the cusp to form a T- or Y-shaped junction; Sa element symmetrical (or nearly so) with straight posterior process and equally developed lateral processes; Sb element slightly asymmetrical with posterior process gently curved inner laterally; Sc element asymmetrical with more strongly inner laterally curved posterior and inner lateral processes.

Remarks. Cooper (1981), who described and illustrated only the M and S elements of this species, suggested the possible P element might be similar to its counterpart in the C. nyinti apparatus. Two types of pectiniform (pastiniplanate) elements recovered from the Tabita Formation are now designated as the Pa and Pb elements to complete the seximembrate apparatus of C. aranda . However, these pectiniform elements were not recorded from the Horn Valley Siltstone ( Cooper, 1981; Nicoll, pers. comm. 2002). Similar Pa elements were previously recorded from the El Paso Group of southern New Mexico ( Repetski, 1982) in association with S and M elements of C. aranda and C. nyinti , but one specimen ( Repetski, 1982, pl. 27, fig. 5) shows weak development of a secondary process on the distal end of the posterior process. A possible P element of C. aranda reported from Ibex area, Utah (Ethington & Clark, 1982: 87), was interpreted as a pathogenic development of the C. nyinti P element .

Johnston & Barnes (2000) regarded Protoprioniodus yapu Cooper, 1981 as a junior synonym of C. aranda . They suggested that P. yapu might represent variants of C. aranda M elements . However, based on Cooper’s definition (1981) of P. yapu , distinctive differences exist between the P elements of these two species, as well as in their M and S elements , which allow these two taxa to be readily differentiated.

Figures

Fig. 12. Cooperignathus aranda (Cooper, 1981): A–D, Pa element,AMF120333,Y4–4, A, close up showing reticulate surface structure, B, inner lateral view, C, outer lateral view, D, inner lateral, upper view; E, Pa element, AMF120334, C1611, basal view; F–H, Pb element, AMF120335, TAB1/39.5, F, upper view, G, inner lateral view, H, outer lateral, upper view; I, M element, AMF120336, W5, anterior view; J, M element, AMF120337, M/A7, posterior view; K, M element, AMF120338, C1612, basal posterior view; L, Sa element, AMF120339, TAB1/65.2, lateral view; M,N, Sb element, AMF120340, M/A7, M, lateral view, N, close up, lateral view; O,P, Sc element, AMF120341, Y4–4, O, lateral view, P, basal lateral view. Scale bars 100 µm, unless otherwise indicated.