Drepanoistodus costatus ( Abaimova, 1971 )

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 : 191-194

Drepanoistodus costatus ( Abaimova, 1971)

Fig. 15A–R

Drepanodus costatus Abaimova, 1971: 490 , pl. 10, fig. 6, text-fig. 3.

Scolopodus cornutiformis Lee, 1976: 172 , pl. 2, fig. 18.

Drepanodus pitjanti Cooper, 1981: 162 , pl. 26, figs. 3–5, 7, 8.

Scolopodus flexilis An, 1981: 216 , pl. 3, figs. 1, 2.

Scolopodus flexilis .– An et al., 1983: 142, pl. 14, figs. 13–18, pl.

33, fig. 4.

Scolopodus ordosensis Z.H. Wang & Luo, 1984: 284 , pl. 4, figs. 22, 23.

Drepanodus pitjanti .– Watson, 1988: 111, pl. 3, figs. 14, 16, 17, pl. 5, fig. 15.

Scolopodus flexilis .–An & Zheng, 1990: 173, pl. 5, figs. 1–6 (cum syn.).

Drepanodus ? costatus .– Smith, 1991: 30, fig. 17k.

Drepanoistodus costatus Stait & Druce, 1993: 303 , figs. 12L,M, 17J,K,M,N,?L (cum syn.).

Drepanodus pitjanti .–Albanesi, in Albanesi et al., 1998: 136, pl. 4, figs. 1–7, text-fig. 16.

Material. One specimen (Sa) from limestone nodules within shales of the upper Yandaminta Quartzite, and 71 specimens (8 M, 9 Sa, 21 Sb, 20 Sc, 13 Sd) from the overlying Tabita Formation at Mount Arrowsmith ; 20 specimens (6 M, 4 Sa, 6 Sb, 3 Sc, 1 Sd) from unnamed dolomitic limestone unit at Koonenberry Gap .

Remarks. Specimens from western New South Wales appear identical to the type material of D. pitjanti , from the Horn Valley Siltstone, for which Cooper (1981) recognized a quinquimembrate apparatus consisting of an M and four S elements .Albanesi (in Albanesi et al., 1998) reinterpreted the Sb element originally defined by Cooper (1981), as occupying the P position of the apparatus.

Cooper (1981) suggested that D. pitjanti and Drepanodus costatus are closely related, but that the latter lacks a geniculate M element . In material from the Coolibah Formation of the Georgina Basin, central Australia, Stait & Druce (1993) also recognized a quinquimembrate (M, Sa to Sd) apparatus for Drepanodus costatus , and referred this species to Drepanoistodus . They suggested that D. costatus could be differentiated from D. pitjanti mainly by its more strongly costate cusp. Stait & Druce (1993) correctly regarded Scolopodus flexilis An —widely distributed in the Liangjiashan Formation and the Beianzhuang Formation (Lower Majiagou Formation) of North China —as a junior synonym of D. costatus . With its less strongly developed costa, S. flexilis is very similar morphologically to the type material of D. pitjanti . The M element of the Chinese species (An & Zheng, 1990, pl. 5, fig. 2; referred to as a graciliform element) is almost identical with the M element of D. costatus illustrated from the Coolibah Formation (Stait & Druce, 1993), except that only one strong costa and an additional much weaker costa are developed on the posterior face, whereas the illustrated Coolibah specimen (Stait & Druce, 1993, fig. 17J) shows three much less strongly developed costae. The M element ( Fig. 15E–H ) of the Mount Arrowsmith and Koonenberry Gap material is identical to that of D. pitjanti from the Horn Valley Siltstone, both forms being characterized by a single strong costa on the posterior face. They also have a cusp more strongly outer laterally curved than do corresponding elements illustrated from the Georgina Basin and North China. The complex variations in the number of costae and other morphological details of M elements in the taxa discussed above suggest that such characters are of little or no use in differentiating these species.

Figures

Fig. 15. Drepanoistodus costatus (Abaimova, 1971): A–C, Sa element, AMF120361, TAB1/78.2, A, anterolateral view, B, lateral view, C, basal view; D, Sa element,AMF120362, C1612, basal view; E–G, M element, AMF120369, C1611, E, basal view, F, posterior view, G, anterior view; H, M element,AMF120368, TAB1/86.7, anterior view; I, Sb element,AMF120363, C1612, outer lateral view; J,K, Sb element, AMF120364, M/A11-5, J, basal view, K, inner lateral view; L, Sc element, AMF120365, M/A7, inner lateral view; M–O, Sc element, AMF120366, Y4–8, M, inner lateral view, N, basal view, O, outer lateral view; P–R, Sd element, AMF120367, Y4–7, P,Q, lateral views, R, basal view. Scale bars 100 µm.