Triangulodus larapintinensis ( Crespin, 1943 )

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 : 212-216

Triangulodus larapintinensis ( Crespin, 1943) Fig. 28A–V

Oistodus larapintinensis Crespin, 1943: 231 , partim only pl. 31, figs. 1–6, 9, 12, 13.

Trigonodus triangularius Nieper , in Hill et al., 1969: O.14, pl. O 7, fig. 22.

Trigonodus larapintinensis .– Cooper, 1981: 180, pl. 27, figs. 5, 6, 11, 12, 16, 17 (emend.).

? Trigonodus larapintinensis .– Watson, 1988: 129, pl. 2, figs. 12– 14, 18–20, 22, 23.

Triangulodus larapintinensis .–Stait & Druce, 1993: 315, figs. 14A–C, 21D–F,H–J.

Material. 259 specimens (41 Pa, 31 Pb, 42 M, 31 Sa, 57 Sb, 41 Sc, 16 Sd) from the Tabita Formation at Mount Arrowsmith, and 43 specimens (6 Pb, 10 M, 2 Sa, 10 Sb, 15 Sc) from unnamed dolomitic limestone unit at Koonenberry Gap .

Description. Apparatus septimembrate; the Pa element scandodiform with erect cusp, and sharp anterior and posterior margins ( Fig. 28A,B ). Pb element similar to the Pa, but with cusp curved inward and inner laterally twisted and the posterior margin showing a rounded gradational curvature towards the base (proclined near the base, Fig. 28C,D ). M element geniculate with a short adenticulate outer lateral process ( Fig. 28E,F ). Sa element symmetrical,

with a broad anterior face, a posterior costa, and a lateral costa on each side ( Fig. 28G–K ). Sb element asymmetrical with a broad anterior face, a weak costa along the posterior margin and a sharp costa on the inner lateral face near the anterior margin, and with a smooth outer lateral face ( Fig. 28L–O ). Sc element asymmetrical with a suberect cusp, having a sharp costa along the posterior and anterior margins; base more inflated on the outer side with anterior margin inner laterally curved ( Fig. 28P–S ). Sd element similar to Sc, but strongly asymmetrical and less laterally compressed with a twisted cusp ( Fig. 28T–V ).

Remarks. The type material from the Waterhouse Range, central Australia, which was poorly illustrated by Crespin (1943), was re-examined by Cooper (1981) who assigned the species to Trigonodus Nieper , in Hill et al., 1969. However, as C.Y. Wang (1992) pointed out, that name is pre-occupied by the Triassic bivalve Trigonodus Sand- berger, in Alberti, 1864, leaving Triangulodus as the valid genus. A brief discussion of the complex nomenclatural history of Crespin’s species, provided by Strusz (1994: 132, 252), suggests further study (probably involving recollec- tion at the type locality) is needed to fully resolve the species concept. As this is beyond the scope of the present work, we are reliant on Cooper’s concept of the species, together with comments kindly provided by R.S. Nicoll who is in the process of reviewing Crespin’s and Cooper’s types.

The material from western New South Wales is identical with that of the species illustrated from the Horn Valley Siltstone, except that the Pb element from central Australia has a much more strongly twisted cusp ( Cooper, 1981, pl. 27, fig. 11). Cooper (1981) recognized a quadricostate asymmetrical Sb element , which he did not illustrate. The asymmetrical Sc element ( Cooper, 1981, pl. 27, fig. 5), described as tricostate (anterior, posterior, and one lateral), is here regarded as the Sb element . Cooper’s Sd element (1981, pl. 27, fig. 12) is the most laterally compressed specimen, with a sharp costa along the posterior margin, and an inner laterally curved costa along the anterior margin. It is now reassigned to the Sc position. A asymmetrical element like Sc but with a twisted cusp from the western New South Wales fauna is interpreted as the Sd element ( Fig. 28T–V ); it is also distinguished by its less laterally compressed base.

Figures

Fig. 28. Triangulodus larapintinensis (Crespin, 1943): A,B, Pa element, AMF120476, TAB1/85.7, A, inner lateral view, B, outer lateral view; C,D, Pb element, AMF120477, Y4–6, C, inner lateral view, D, outer lateral view; E, M element, AMF120478, Y4–6, anterior view; F, M element,AMF120479, M/A7, posterior view; G, Sa element,AMF120480, M/A7, anterior view; H, Sa element,AMF120481, Y4–6, posterolateral view; I, Sa element,AMF120482, M/A4, lateral view; J,K, Sa element,AMF124214, Y4–8, lateral views; L,M, Sb element,AMF120484,Y4–7, L, inner lateral view, M, outer lateral view; N,O, Sb element, AMF124215,Y4–8, N, inner lateral view, O, basal view; P,Q, Sc element, AMF124216, Y4–8, P, basal view, Q, inner lateral view; R,S, Sc element, AMF124217, Y4–7, R, outer lateral view, S, inner lateral view; T, Sd element, AMF124218, M/A11-3, outer lateral view; U,V, Sd element, AMF124219, Y4–8, U, posterior view, V, outer lateral view. Scale bars 100 µm.