Acodus triangularis

Y Y Zhen, 2005, Revision of Two Prioniodontid Species (Conodonta) from the Early Ordovician Honghuayuan Formation of Guizhou, South China, Records of the Australian Museum 57, pp. 303-320 : 307-311

Acodus triangularis (Ding in Wang, 1993)

Figs 3–5

? Triangulodus cf. alatus Dzik. –An et al.: 1985: pl. 8, figs 1, 4, 8.? Tripodus alatus (Dzik) .– An, 1987: p. 193, pl. 16, figs 3–6. Oistodus triangularis Ding in Wang 1993: p. 185, pl. 19, figs 1, 2.

Material. 222 specimens from ten samples ( Table 1).

Diagnosis. A species of Acodus consisting of a septimembrate apparatus including pastinate Pa and Pb, makellate M, triform alate Sa, triform asymmetrical Sb, modified cordylodiform Sc, and tetra-costate Sd elements; elements mostly albid and small in size, all bearing short adenticulate processes; P elements with a triangular base bearing a large, open basal cavity; S elements bearing a longer posterior process typically with a thin, blade-like crest along the upper edge.

Description. The pastinate Pa element has a reclined to suberect cusp and a large, triangular base ( Fig. 3A–O). The cusp is triangular in cross section ( Fig. 3F) with a sharp costa along the anterior and posterior margins, a mid-costa on the convex outer lateral face, and a smooth, more or less flattened inner lateral face. Three costae extend basally to merge with the upper margin of the anterior, posterior and outer lateral processes respectively ( Figs 3A,G,K,I,M, 4 O). Fine striations are best developed in the area posterior to the outer lateral costa on the outer lateral face ( Fig. 3L). All three processes have a blade-like upper edge ( Fig. 3E,G) and an expanded base with posterior and anterior processes extending in the same plane ( Fig. 3H) or with anterior process slightly turned inner laterally ( Fig. 3M–O). The outer lateral process extends basally and outer laterally in a direction nearly normal to the anteroposterior plane in upper or basal view ( Fig. 3B,F), and is triangular in outline in anterior view ( Fig. 3C,I) or posterior view ( Fig. 3E). Posterior process extends posteriorly and typically longer than the other two ( Fig. 3G,K). The anterior process is anticusp-like, triangular in outline in the lateral view ( Fig. 3D,I– K). Basal cavity is pyramidal, large and moderately deep, with more or less straight basal margin or with both sides of the outer lateral process slightly restricted inward ( Fig. 3B,H).

The Pb element is similar to the Pa element, but with a slightly proclined cusp ( Fig. 4C,F,G,J,M), a shorter and less basally extended anterior process ( Fig. 4A,B,G), a shorter and anterolaterally located outer lateral process ( Fig. 4A,E,F,N), and a more open basal cavity ( Fig. 4D,H). The side between the anterior process and the outer lateral process is less inwardly restricted, and the basal margins on both sides of the outer lateral process are less restricted inward in upper or basal view ( Fig. 4B,D,E).

The M element is geniculate, anteroposteriorly compressed, with a low and well-developed outer lateral process and a short, anticusp-like inner lateral process, which is triangular in outline in anterior ( Fig. 4K) or posterior view ( Figs 3P, 4L). The anterior face is broadly convex and smooth, while the posterior face bears a broad carina and a weakly developed basal buttress. The basal margin is gently arched ( Figs 3P, 4K,L).

The Sa element is triform and symmetrical with a robust cusp, a short adenticulate posterior process, and a bladelike adenticulate lateral process on the anterolateral corner of each side ( Figs 4P–U, 5A). The cusp is isosceles triangular in cross section with a broad anterior face ( Fig. 5A), a sharp costa along the posterior margin, and a sharp costa on each lateral side ( Figs 4R,T, 5A). The three costae extend basally to form short, blade-like posterior and lateral processes ( Fig. 4R,Q). Basal cavity is triangular in outline with moderate depth ( Fig. 4R,U). Some specimens show a few small, rudimentary denticles along the thin upper edge of the posterior process ( Fig. 4S).

The Sb element resembles the Sa element in being tricostate, but is distinguished by being markedly asymmetrical ( Fig. 5B–J). The cusp is proclined, scalene triangular in cross section with a sharp costa along the anterior and posterior margins and a costa on the outer lateral face. The inner lateral face is concave and smooth or bearing a broad carina, with anterior margin prominently curved inwards ( Fig. 5B,C,G). The outer lateral face is divided into a broadly convex area defined by the anterior costa and the outer lateral costa, and a gently concave area between the outer lateral costa and the posterior costa ( Fig. 5D,F,H,I). The base is triangular in outline when posterior process is shorter ( Fig. 5J), but in more advanced forms, it is strongly laterally compressed ( Fig. 5E) as a long, adenticulate blade with a straight basal margin and a straight or weakly arched upper margin. The anterior process is anticusp-like, triangular in outline in the lateral view ( Fig. 5I). The outer

[ Fig.4 caption continued] … (K,L) M element, AMF126740, AFI993, (K) anterior view (IY64009), (L) posterior view (IY52017). (M,N) Pb element, AMF126741, THH3, (M) outer lateral view (IY66032), (N) antero-outer lateral view (IY66034). (O) Pa element, AMF126742, THH10, outer lateral view (IY66037). (P–R) Sa element,AMF126743,AFI993, (P,Q) posterolateral views (IY66013, IY66014), (R) basal-posterior view (IY66012). (S–U) Sa element, AMF126744, AFI997, (S) lateral view (IY66010), (T) posterior view (IY66009), (U) basal view (IY66011). Scale bars 100 µm.

lateral process is also anticusp-like extending basally and outer-laterally with blade-like upper margin curved posteriorly. Basal cavity is moderately deep and triangular in outline and extends as narrow grooves underneath the posterior and anterior processes ( Fig. 5B).

The Sc element is asymmetrical, strongly compressed laterally with a robust, proclined to suberect cusp, a posterior process varying from short ( Fig. 5K,L) to relatively long ( Fig. 5 O), and a short, anticusp-like anterior process ( Fig. 5K–O). The cusp is convex lens-like in cross section, slightly bent inward with a sharp costa along its anterior and posterior margins, and with smooth outer and inner lateral faces, or with a weakly developed broad carina on the inner lateral face ( Fig. 5 O). The inner lateral face is less convex with anterior margin gently curved inward. Basal cavity is convex lens-like in outline with gently arched basal margin in lateral view ( Fig. 5M).

The Sd element is tetra-costate and asymmetrical, with a sharp costa along the anterior and posterior margins, and a sharp costa on each lateral side ( Fig. 5P –AA). The outer lateral side is more convex with a broad convex area between the anterior process and the outer lateral process, and a less convex to concave area defined by the outer lateral process and the posterior process ( Fig. 5P,U). The inner lateral side is less convex to concave with an inner laterally curved anterior margin ( Fig. 5R,T,Y). Costae extend basally to merge with the upper margin of four corresponding short, blade-like processes. Small rudimentary denticles may occur along the upper edge of the posterior process of some specimens ( Fig. 5P). However, the broken upper edge in some specimens superficially exhibits a denticulate appearance ( Fig. 5U).

Remarks. Oistodus triangularis was originally proposed as a form species with the type material from the Honghuayuan Formation of Anhui Province (Ding in Wang, 1993). The two figured specimens are both pastinate with a large, triangular base and a robust cusp which bears a sharp costa along its anterior and posterior margins, and a third costa on the outer lateral face (Ding in Wang 1993, p. 185). The holotype has a reclined cusp and less downwardly extending anterior process ( Wang, 1993, pl. 19, fig. 1), whereas the other figured specimen ( Wang, 1993, pl. 19, fig. 2), with a suberect cusp and a longer and more downwardly extending anterior process, is identical with the Pa element described herein from Guizhou.

Specimens previously referred to Triangulodus alatus Dzik, 1976 from the lower Dawan Formation of South China (An et al., 1985; An, 1987) possibly represent the S elements of Acodus triangularis . The type material of T. alatus , consisting of large-sized hyaline elements, occurs in late Darriwilian erratic boulders of NW Poland. The holotype ( Dzik, 1976, p. 433, fig. 20k) is a scandodiform P element with an adenticulate posterior process and a short adenticulate anticusp-like anterior process. Dzik (1983, 1994) re-assigned this Baltic species to Eoneoprioniodus Mound, 1965 , which is likely a senior synonym of Triangulodus van Wamel, 1974 (see Clark et al., 1981). Triangulodus alatus was also reported from the upper Darriwilian (serra Zone) of the Tarim Basin ( Zhao et al., 2000), and from the upper Darriwilian (serra Zone) of Oklahoma ( Bauer, 1987).

Specimens referred to Acodus deltatus Lindström, 1955 from the Emanuel Formation of Western Australia ( McTavish, 1973) are reassigned herein to A. longibasis which differs from A. deltatus mainly in having a more extended posterior process. This species from the Canning Basin shows some resemblance to A. triangularis from South China, in particular the Pa and S elements. Based mainly on the length of posterior process of the S elements in the Emanuel Formation, McTavish (1973) split this species into several subspecies of A. deltatus including A. deltatus longibasis McTavish with long posterior process, A. deltatus deltatus with a shortest posterior process, and A. deltatus tortus McTavish with an inwardly twisted cusp of the S elements. Similar variations are also observed in the species from South China with the posterior process of the S elements varying from short ( Fig. 5J,L,K) to relatively long ( Fig. 5B–H,M,O).