Tropodus Kennedy, 1980

Zhen, Yong Yi & Nicoll, Robert S., 2009, Biogeographic and Biostratigraphic Implications of the Serratognathus bilobatus Fauna (Conodonta) from the Emanuel Formation (Early Ordovician) of the Canning Basin, Western Australia, Records of the Australian Museum 61 (1), pp. 1-30 : 26

Tropodus Kennedy, 1980

Chionoconus Smith, 1991: 22 .

Type species. Tropodus comptus (Branson & Mehl, 1933) .

Remarks. Tropodus Kennedy, 1980 is treated herein as a valid genus, having a rather different species apparatus as that of Acodus , particularly its S elements that exhibit a much wider variation characterized by the occurrence of multi-costate elements, and the P elements typically with weaker development of a lateral costa that may be represented by a broad carina. Tropodus was proposed to consist of a bimembrate apparatus (comprising comptiform and pseudoquadratiform elements) with Paltodus comptus Branson & Mehl, 1933 as the type species ( Kennedy, 1980). Although the genus was originally defined as consisting of a symmetry transition series of elements bearing “three or more, prominent keel-like costae” ( Kennedy, 1980, p. 65), both elements of his revised T. comptus have five costae, likely representing only part of a species apparatus. The comptiform element (= Sc herein) was represented by the form species P. comptus , an asymmetrical element with costate anterior and posterior margins, and with two costae on the outer face and one on the inner face ( Kennedy, 1980, pl. 2, figs 21–24), and the pseudoquadratiform element (= Sd herein) by the form species Scolopodus pseudoquadratus Branson & Mehl, 1933 , a symmetrical quinquicostate element with a broad anterior face and a costate posterior margin ( Kennedy, 1980, pl. 2, figs 25–27).

The other species originally included in Tropodus was Walliserodus australis which was defined as consisting of a transitional series from tricostate to multicostate elements ( Serpagli, 1974, p. 89). Kennedy (1980) admitted that “the two types of elements in T. comptus are very similar to two of the many morphologies of elements in T. australis ( Serpagli, 1974) ” although the latter presented “a multitude of variably costate forms.” Serpagli (1974) originally defined a quinquimembrate apparatus for T. australis including asymmetrical tricostate (= Sb1 herein), asymmetrical quadricostate (= Sb2 herein), strongly asymmetrical, laterally compressed multicostate (= Sc herein) and a nearly symmetrical quinquicostate (= Sd) elements; Sa, P and M elements were originally not recognized in either species.

Based on material from the Cow Head Group of Newfoundland, Bagnoli et al. (1988) and Stouge & Bagnoli (1988) included P and M elements in the species apparatus of Tropodus . Stouge & Bagnoli (1988) also revised both T. comptus and T. sweeti ( Serpagli, 1974) , and suggested that W. australis proposed by Serpagli (1974) comprised elements belonging to both T. comptus and T. sweeti . As Smith (1991) correctly pointed out, distinguishing between S elements belonging to these species is rather uncertain, as Stouge & Bagnoli (1988) had included the holotype (tricostate element) of W. australis in the synonymy lists of both T. comptus and T. sweeti . However, the P elements they defined for T. comptus and T. sweeti show remarkable differences. The P elements of T. comptus illustrated from the Cow Head Group ( Stouge & Bagnoli, 1988, pl. 16, fig. 2) and also from Utah (Ethington & Clark, 1982, text-fig. 22, pl. 11, figs 6, 7) are more or less scandodiform bearing a smooth, convex outer face and a concave inner face with a broad, prominent mid carina, while the P element of T. sweeti is a typical acodiform element with a prominent costa on one side ( Serpagli, 1974, pl. 14, figs 13, 14, pl. 24, figs 8–10). Smith (1991) restricted Tropodus to Kennedy’s original definition by including only costate elements in the species apparatus and proposed Chionoconus to accommodate those elements which Stouge & Bagnoli (1988) defined as the P elements of T. comptus .

Zhen et al. (2004) reported the occurrence of T. Australis (assigned to T. comptus ) and T. sweeti from two samples within the Early Ordovician Hensleigh Siltstone in central New South Wales. The P and M elements of both these species are comparable with those described by Serpagli (1974) from the San Juan Formation of Precordilleran Argentina and by Stouge & Bagnoli (1988) from Newfoundland. However, Zhen et al. (2004), following Ji & Barnes (1994), applied a rather different concept for the S elements by including a symmetrical Sa element in the apparatuses. These studies also raised uncertainties about the definition of the constituent species of Tropodus . Firstly, if P elements of Tropodus are confined to “scandodiform” elements as Stouge & Bagnoli (1988) originally proposed, T. sweeti should be excluded from it. Secondly, although Kennedy (1980) suggested that Triangulodus consisted of hyaline elements, if Tropodus is considered as bearing “scandodiform” P elements, it would be very similar to Triangulodus , if it is not considered as a junior synonym of the latter. Among the specimens from the Hensleigh Siltstone, two types of P elements were recognized for T. australis ( Zhen et al., 2004) . Both Pa and Pb elements have a broad convex outer face and a concave inner face with a thin anterior margin curved inward. The lateral costa is weak, and may even be represented only as a broad carina on the inner side of the Pa element ( Zhen et al., 2004, pl. 1, figs 1, 3) which is comparable with those illustrated by Stouge & Bagnoli (1988, pl. 16, fig. 2)and those from the Emanuel Formation ( Fig. 16L–P ). However, the lateral costa is plainly evident in the Pb element ( Zhen et al., 2004, pl. 1, fig. 4). Similar variations among the P elements of T. comptus were also observed in the material from the St. George Group (Ji & Barnes, 1994). Therefore, Tropodus is defined herein as consisting of a geniculate M element, a series of highly variable S elements (in respect to the number of the costae), and acodiform P elements with a lateral costa on the inner side varying from weak to well developed.

Figures

Fig.16. Tropodus australis (Serpagli, 1974). A,B, M element; (A), CPC39914, WCB705/243, posterior view (IY128-032); (B), CPC39915, WCB705/243, anterior view (IY129-007).(C), Sa element, CPC39916, WCB705/243, posterior view (IY128-034).(D), Sb1 element (tricostate), CPC39917, WCB705/243, outer lateral view (IY128-033). E,F, Sb2 element (four costate); (E), CPC39918,WCB705/243, outer lateral view (IY128-038); (F), CPC39919,WCB705/243, inner lateral view (IY128-036).G–I, Sc element; (G), CPC39920,WCB705/243, outer lateral view (IY129-003); (H), CPC39921, WCB705/243, inner lateral view (IY129-022); (I), CPC39922, WCB705/243, inner lateral view (IY118-032). J,K, Sd element, CPC39923, WCB705/243, (J), basal view (IY118-033), (K), basal view close up showing the lamellar structure in the basal cavity (IY118-034). L–N, Pa element; (L), CPC39924, WCB705/243, outer lateral view (IY128-028); (M), CPC39925, WCB705/243, inner lateral view (IY128-022); (N), CPC39926, WCB705/243, inner lateral view (IY128-029). O,P, Pb element, CPC39927, WCB705/243; (O), outer lateral view (IY129-031), (P), basal view (IY129-030). Scale bars 100 µm.