Prioniodus 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 : 204

Prioniodus sp. cf. P. amadeus Cooper, 1981

Fig. 21A–L

Material. One specimen (P) from limestone nodules within shales of the upper Yandaminta Quartzite and 47 specimens (27 P, 1 M, 19 S) from the overlying Tabita Formation at Mount Arrowsmith .

Description. The pastinate P elements are not differentiated into Pa and Pb due to limited material. They have a prominent, strongly laterally compressed, blade-like cusp with sharp anterior and posterior margins, and a sharp costa on the outer lateral face. The anterior and posterior margins, and the outer lateral costa extend basally into three processes ( Fig. 21A–D ). The posterior and outer lateral processes are longer and have several (up to seven) small denticles; anterior process shorter with a few small rudimentary denticles; cusp triangular in cross section with sharp anterior and posterior margins, and a sharp-edged costa on the outer lateral face; anterior process slightly curved inward, and outer lateral process extending anterolaterally, forming a more or less Y-shaped outline in upper view. M element makellate, with a long adenticulate outer lateral process, which has a straight basal margin and an arched upper margin, and a short inner lateral process, which is triangular in outline in anterior view ( Fig. 21L ); cusp anteroposteriorly compressed, with a broad costa on the anterior and posterior faces, and a straight inner lateral margin forming an acute angle (about 40–45°) with the basal margin of the outer lateral process. Sa element triform alate, symmetrical, with a long denticulate posterior process, a broad anterior face, and a sharp ridge-like costa on each lateral face, which extends basally into a short adenticulate process ( Fig. 21J,K ). Sb element modified bipennate, strongly asymmetrical with a slightly proclined cusp, inner laterally curved anterior margin, a straight denticulate posterior process and a short adenticulate anticusp-like anterior process ( Fig. 21E,F ). Sc element modified bipennate, similar to Sb but slightly asymmetrical with a long denticulate posterior process and a short, anticusp-like adenticulate anterior process; cusp erect, laterally compressed, with sharp anterior and posterior margins; anterior process slightly curved inward, and posterior process slightly arched with denticles of uniform size pointed posteriorly ( Fig. 21G ). Sd element tertiopedate, asymmetrical with a denticulate posterior process; cusp proclined, and inner laterally curved, with broad anterior margin and a blade-like costa on each lateral side, which extends basally into a lateral process; inner lateral process more prominent, with a few rudimentary denticles distally ( Fig. 21H,I ).

Remarks. This species, consisting possibly of a septimembrate apparatus, is relatively uncommon in the Mount Arrowsmith collections. Although the modified bipennate Sb and Sc elements resemble those of P. amadeus Cooper, 1981 from the Horn Valley Siltstone of the Amadeus Basin (see especially Cooper, 1981, pl. 31, fig. 6), that species differs in inclusion of quadriramate elements in the apparatus, and its P elements have a less laterally compressed cusp and an adenticulate anterior process.

Some broad similarities are apparent between P. sp. cf. amadeus and Oepikodus pincallyensis . However, S elements of the former (differentiated into a triform alate Sa, modified bipennate Sb and Sc, and tertiopedate Sd) are generally larger, with more robust posterior processes bearing posteriorly pointed denticles of uniform size. S elements of O. pincallyensis are quadriramate, or modified quadriramate. The P elements of P. sp. cf. amadeus have a more laterally compressed blade-like cusp compared to O. pincallyensis , and exhibit a shorter and less inner laterally curved anterior process, and a relatively larger and deeper triangular basal cavity.

Figures

Fig. 21. A–L, Prioniodus sp. cf. P. amadeus Cooper, 1981: A,B, P element, AMF120412, TAB1/78.2, A, outer lateral view, B, upper, inner lateral view; C,D, P element, AMF120413, TAB1/78.2, C, upper, antero-inner lateral view, D, postero-outer lateral view; E, Sb element, AMF120415, M/A7, inner lateral view; F, Sb element, AMF120416, M/A7, outer lateral view; G, Sc element, AMF120414, TAB1/78.2, lateral view;H,I, Sd element,AMF120417,TAB1/78.2, H, inner lateral view, I, posterior view; J,K, Sa element,AMF120418, M/A11-3, lateral views; L, M element, AMF124213, W/A11-5, anterior view. M–Q, Prioniodus sp.A: M,N, Pb element, AMF120420, M/A7, M, inner lateral view, N, anterior view; O, Pa element, AMF120419, Y4–2, outer lateral view; P, Pa element, AMF120422, Y4– 2, postero-outer lateral view; Q, Pa element, AMF120421, Y4–2, upper view. Scale bars 100 µm.