Cooperignathus nyinti ( 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 : 184-188

Cooperignathus nyinti ( Cooper, 1981)

Figs. 10A–N , 11A–O

New genus A Sweet et al., 1971: partim only pl. 1, fig. 19.

New genus A Sweet et al., 1971.– Repetski, 1982: 56, partim only pl. 27, figs. 3, 4.

Protoprioniodus nyinti Cooper, 1981: 176 , pl. 29, figs. 1–8, 11, 12.

Protoprioniodus nyinti .–Stait & Druce, 1993: partim only fig. 19N,O.

Protoprioniodus aranda .–Ethington & Clark, 1982: 87, partim only pl. 9, figs. 24–26, 30.

Protoprioniodus aranda .–Johnston & Barnes, 2000: partim only pl. 6, figs. 23, 24.

Material. Six specimens (3 M, 1 Sa, 1 Sb, 1 Sc) from limestone nodules within shales of the upper Yandaminta Quartzite , and 62 specimens (6 Pa, 7 Pb, 19 M, 7 Sa, 11 Sb, 12 Sc) from the overlying Tabita Formation at Mount Arrowsmith; one additional specimen (M) from unnamed dolomitic limestone unit at Koonenberry Gap .

Diagnosis. A ramiform-pectiniform species consisting of a seximembrate apparatus including modified anguliplanate P elements , makellate M element , alate Sa, and modified tertiopedate Sb and Sc elements ; all elements albid, with adenticulate processes, and a ledge-like costa lying parallel to and slightly above the basal margin to define the basal surface, which is represented by a shallow groove between this costa and the basal margin.

Description. P elements are modified anguliplanate units, arched and crescentic in outline in lateral view, with convex outer lateral face and concave inner lateral face; upper surface with a thin anteroposteriorly extended blade-like crest which lacks recognizable cusp and denticles and curves inwards, and also with its upper margin gently bowed inward; ledge-like costa on each side extending throughout the whole unit, more or less parallel to the basal margin, to define a groove-like basal surface on each side between costa and the basal margin in lateral view, and a narrow platform in the upper view; on the concave side, the groovelike basal surface is more strongly developed and extends continuously from anterior to posterior extremities; underneath the element, the basal margins on both sides narrow to define a small, shallow basal pit at mid curvature and a narrow groove extending towards the distal ends of the anterior and posterior processes; on the convex side, the Pa element displays a short ridge-like outer lateral process ( Fig. 10G ) developed between the horizontal, ledgelike costa and the basal margin to divide this groove-like area into a slightly shorter anterior part and a longer posterior part, and basally this short process forms a prominent basal buttress ( Fig. 10A–C,E ); in the Pb element this outer lateral process is much weaker or even absent ( Fig. 10I ); surface of the P elements ornamented by reticulation which is best developed near the ledge-like costa ( Fig. 10L,N ). M element anteroposteriorly compressed, with a robust outer laterally strongly recurved cusp, an adenticulate outer lateral process and an adenticulate, anticusp-like inner lateral process; cusp also slightly curved posteriorly, and inner- and outer-lateral processes curved slightly anteriorly; anterior and posterior faces bearing a round-faced costa; basally, the costa on the posterior face is truncated by a strong, ledge-like horizontal costa extending from the outer lateral process to the inner lateral process, with an acute angle (about 30°) between them; posteriorly, the broad basal buttress defining a more or less triangular opening of the basal cavity ( Fig. 11B ). S elements bearing a tricostate cusp, a sharp costa along the posterior margin, and a costa along the anterolateral corner on each lateral face, and with a broad anterior face; three costae extending basally into three adenticulate processes, which are tripod-like in lateral view; posterior process long, straight or inner laterally curved; lateral processes extending basally and laterally, ornamented with fine striae which are vertically arranged more or less parallel to the axis of the cusp in anterior view ( Fig. 11J ); in upper view, the three processes meet the cusp in a T- or Y-shaped junction; three processes with a prominent costa on each side more or less parallel the basal margin to define the basal surface. Three morphotypes are recognized, based mainly on curvature of the posterior process; Sa element symmetrical with straight posterior process; Sb element slightly asymmetrical, with posterior process slightly curved inward and a more strongly downwardly extended inner lateral process; Sc like the Sb, but asymmetrical with a more strongly inwardly curved posterior process.

Remarks. In the Horn Valley Siltstone, C. nyinti is very common while C. aranda is less so, but always occurs with C. nyinti except in the lowest stratigraphic horizons ( Cooper, 1981). Morphologically, C. nyinti and C. aranda are very similar. The latter was originally defined only on S and M elements . No P elements were formally recognized previously for this species, although Cooper (1981: 175) stated that the P element of C. aranda might be very similar to its counterpart in C. nyinti with differences “solely in size, degree of thickening, and robustness”. The S elements of C. nyinti and C. aranda are similar, except that those of C. aranda have an indentation at the junction between the posterior margin of the cusp and the upper margin of the posterior process. The M elements of these two species are also very similar. Both have a strong posterior costa, a basal buttress, and a strong costa paralleling the basal margin to define a basal groove on the posterior face. However, the M element of C. nyinti has a more strongly outer-laterally reclined cusp, and a straight or slightly arched basal margin, whereas that of C. aranda has a longer inner lateral process with the lowest point of the basal margin underneath the basal buttress, and with a more strongly arched upper margin to the outer lateral process.

The P elements illustrated from the Horn Valley Siltstone ( Cooper, 1981) show a distinctive short outer lateral process on the convex outer lateral side that links the horizontal ledge-like costa and the basal margin to divide the groove into two parts. These specimens are referred herein to the Pa position. Re-examination of a large topotype collection of this species confirms that Pa and Pb elements of C. nyinti are also represented at the type locality. Like the Horn Valley Siltstone samples, P elements are relatively rare in the Mount Arrowsmith samples, with a similar situation noted in the Wah Wah and Juab formations of the Ibex area, Utah (Ethington & Clark, 1982). These latter authors recorded a ratio of about 8(S):11(M):1(P) for the species ( C. nyinti + C. aranda ), and argued that the P elements must have been very minor constituents of the apparatus. Hence they regarded P. nyinti as a probable junior synonym of P. aranda , and suggested that the differences on which Cooper (1981) distinguished these two species might merely represent a broad spectrum of morphologic variation within the one species apparatus. Repetski (1982), and Johnston & Barnes (2000) expressed similar views. However, recovery of two additional types of pectiniform elements from the Mount Arrowsmith samples, which are now referred to as the Pa and Pb elements of C. aranda , strongly suggests the coexistence of the two species of Cooperignathus .

Figures

Fig. 10. Cooperignathus nyinti (Cooper, 1981): A–C, Pa element, AMF120319, M/A7, A, basal, outer lateral view, B, basal view, C, antero-outer lateral view; D, Pa element, AMF120320, M/A7, inner lateral view; E–G, Pa element, AMF120321, M/A7, E, close up showing the basal cavity and outer lateral process, F, outer lateral view, G, upper, outer lateral view; H, Pb element, AMF120322, M/ A7, outer lateral view; I–K, Pb element, AMF120323, Y4–2, I, outer lateral view, J, basal view, K, upper, outer lateral view; L–N, Pb element, AMF120324, M/A7, L, inner lateral view, M, inner lateral-basal view, N, showing reticulate surface structure. Scale bars 100 µm, unless otherwise indicated.

Fig. 11. A–O, Cooperignathus nyinti (Cooper, 1981): A, M element, AMF120325, M/A7, anterior view; B, M element, AMF120326, TAB1/86.7, basal view; C, Sa element, AMF120327, M/A7, lateral view; D, Sb element, AMF120328, TAB1/39.5, outer lateral view; E–J, Sb element,AMF120329, TAB1/8.1, E, upper view, F, close up of the cusp, upper view, G, outer lateral view, H, inner lateral view, I, anterior view, J, close up showing striation and basal surface, anterior view; K–M, Sc element, AMF120330, M/A7, K, outer lateral view, L, upper inner-lateral view, M, upper view; N, Sc element,AMF120331,TAB1/39.5, outer lateral view; O, Sc element,AMF120332, M/A7, inner lateral view. P–X, Baltoniodus sp. A; P,Q, M element, AMF120425, Y4–2, P, close up showing the surface sculpture, Q, anterior view; R, M element, AMF120426, Y4–2, anterior view; S, P element, AMF120423, Y4–7, inner lateral view; T,U, P element, AMF120424, M/A7, T, upper view, U, upper, outer lateral view; V, P element, AMF120284, M/A11-5, basal view; W,X, P element, AMF120285, M/A11-5, W, outer lateral view, X, upper view. Scale bars 100 µm, unless otherwise indicated.