Cooperignathus Zhen, 2003

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

Cooperignathus Zhen n.gen.

Type species. Protoprioniodus nyinti Cooper, 1981 .

Etymology. For Dr Barry J. Cooper who described the type species from central Australia.

Diagnosis. Conodonts of ramiform-pectiniform configuration consisting of a seximembrate apparatus; all elements albid, with adenticulate processes, and a prominent basal surface defined by a ledge-like costa parallel to and slightly above the basal margin; P elements pectiniform with a variably developed platform, and adenticulate anterior, posterior, and outer lateral processes; cusp lacking, with the upper surface of the unit occupied by thin blade-like crest(s); basal cavity shallow, represented by a small basal pit and narrow basal grooves extending to tips of the processes; M element makellate, with adenticulate outer lateral and inner lateral processes, basal buttress variably developed; S elements tertiopedate, consisting of nearly symmetrical Sa, slightly asymmetrical Sb, and asymmetrical Sc element .

Remarks. Cooperignathus appears closely related to Protoprioniodus McTavish , as indicated by the general similarity of their S and M elements . The two genera are distinguished by their P elements , those of Cooperignathus being planate, lacking a cusp, but exhibiting a variably developed platform. Species of Protoprioniodus have pastinate P elements with a prominent cusp, and a more open basal cavity.

One of the most distinctive characters of Cooperignathus is the development of a ledge-like costa to define the basal surface. A comparable structure is also developed in the elements of Protoprioniodus yapu , and some specimens referable to Acodus sp. cf. emanuelensis ( Fig. 9A ) from western New South Wales. Evolution of Cooperignathus from Protoprioniodus may have proceeded firstly by reduction and subsequent total loss of the cusp, accompanied by an increase in the area of the basal surface, and secondly by the displacement of this area from the basal lateral side to the underside of the P elements ( Fig. 9 ). The Cooperignathus apparatus appears to be the earliest form with planate elements, and phylogenetically might represent an early stage of evolutionary development of the Polyplacognathidae clade.

Two species, C. aranda and C. nyinti , are included in Cooperignathus . They are widely distributed in the Early Ordovician ( O. evae Zone ) of Australia, South China and North America. Because the diagnostic P elements are relatively rare, both species were previously ascribed to Protoprioniodus based on the M and S elements .

Occurrence. Horn Valley Siltstone, Amadeus Basin ( Cooper, 1981), Coolibah Formation, Georgina Basin (Stait & Druce, 1993) in central Australia; Tabita Formation at Mount Arrowsmith, and unnamed dolomitic limestone unit at Koonenberry Gap, western New South Wales (herein); Dawan Formation, Hubei Province, South China ( An, 1987); Wah Wah Limestone, Ibex area of western Utah (Ethington & Clark, 1982), Juab Limestone, Utah ( Sweet et al., 1971; Ethington & Clark, 1982); E1 Paso Group, western Texas and southern New Mexico ( Repetski, 1982); Broken Skull and Sunblood formations, southern District of Franklin, Canada ( Tipnis et al., 1978); upper part of the Eleanor River and Ship Point formations, Arctic Canada ( Barnes, 1974); Cow Head Group ( Pohler, 1994; Johnston & Barnes, 2000), and St. George Group ( Stouge, 1982; Ji & Barnes, 1994), western Newfoundland, Canada.

Figures

Fig. 9. Schematic diagram to show possible evolutionary relationships of Cooperignathus. A, Acodus sp. cf. emanuelensis,?M element, AMF120702, TAB1/39.5, anterior view. B, Protoprioniodus yapu, Pb element, AMF120705, outer lateral view; C, P. yapu, Pa element,AMF120706, outer lateral view. D, C.nyinti, Pb element,AMF120703, outer lateral view; E, C.nyinti, Pa element, AMF120704, outer lateral view. F, C. aranda, Pb element AMF120335, TAB1/39.5, basal view; G, C. aranda, Pa element,AMF120334, C1611, upper view. B–E, from EC-5, Horn Valley Siltstone, Ellery Creek, Amadeus Basin. Scale bar 100µm. Bs, basal surface; Bc, basal cavity; Co, ledge-like costa defining the margin of the basal surface; Ou, outer lateral process.