Serratognathus bilobatus Lee, 1970

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 : 22

Serratognathus bilobatus Lee, 1970

Fig. 11A–L

Serratognathus bilobatus Lee, 1970: 336 , pl. 8, figs 6, 7; Metcalfe, 1980: pl. 1, figs 16–19; An, 1981: pl. 2, fig. 26; An & Ding, 1982: pl. 5, fig. 25; An et al., 1983: 149, pl. 16, figs 20–22, pl. 17, figs 1, 2; An, 1987: 189, 190, pl. 18, fig. 11; Ding, 1987: pl. 6, fig. 16; An & Zheng, 1990: pl. 7, fig. 13; Ding et al. in Wang, 1993: 207, pl. 20, fig. 5; Chen & Wang, 1993: fig. 2Q, 2U; Wang et al., 1996: pl. 2, figs 1–7, 9; Nicoll & Metcalfe, 2001: fig. 6.19–6.22; Metcalfe, 2004: pl. 2, figs 9–10.

Material. Seven specimens from three samples ( Table 1).

Diagnosis. Species of Serratognathus with a trimembrate apparatus, including symmetrical Sa, asymmetrical Sb, and strongly asymmetrical Sc elements; all elements semiconical in outline with fan-shaped array of small, closely spaced denticles along anterior and lateral edges of overlapping laminar layers; cusp small, posteriorly positioned and anterolaterally enclosed by up to 16 vertically overlapping laminar layers, which are anterobasally divided by a broad median groove into two lobe-like lateral processes; basal cavity absent.

Description. Trimembrate apparatus, including symmetrical Sa, asymmetrical Sb, and strongly asymmetrical Sc elements which form a symmetry transition series; each element semi-conical in outline, formed by upwardly overlapping layers, resembling a half-cut onion; composed of the cusp and a gently posterolaterally extended lobe-like lateral process on each side. Cusp small or indistinctive, weakly compressed laterally with a broad posterior face, and the anterior margin embedded in up to 16 surrounding overlapping layers, which are bordered by small, closely spaced denticles along the anterior and lateral margins. Discrete denticles surround the cusp in a semicircle, representing the anterolateral edge of each overlapping layer. Node-like denticles weakly developed on posterior face, may be absent towards the base ( Fig. 11B, E, G, I ); fine striae microstructure best developed on the posterior face in the area above the basal margin ( Fig. 11 I, J ). Denticulate anterior margin of each layer turned upwards. Basal face smooth, wide, and distally arched; crescentic in outline in basal view; bisected by anteroposteriorly directed median groove extending anterobasally to separate into two lobes underneath each lateral process. Basal cavity absent; basal end of the cusp represented by a small ring-like node ( Fig. 11K ), but in one specimen basal cavity represented by a small, shallow pit underneath the cusp ( Fig. 11B ).

Sa element symmetrical, outline crescentic in upper view with convex anterior face and concave posterior face, and tower-like in anterior view ( Fig. 11A ). Cusp small, located posteromedially with a short lobe-like process on each side, which extends posterolaterally; in anterior view, two processes separated anterobasally by a prominent rather deep median groove ( Fig. 11A ). Basal face smooth, bisected by the median groove into two symmetrical lobes.

Sb element ( Fig. 11B–F ) like Sa, but asymmetrical; inner lateral process shorter in posterior view ( Fig. 11B ), extending laterally with basal margin nearly horizontal; outer lateral process longer extending posterolaterally ( Fig. 11C ). Basal margin of the two processes forming an angle of 125° or more in the upper view ( Fig. 11E ). Basal face asymmetrical, with a longer lobe under outer lateral process and a shorter lobe under inner lateral process. One specimen ( Fig. 11B ) exhibits a small and shallow basal cavity.

Sc element ( Fig. 11G–L ) similar to Sb, but strongly asymmetrical with longer and more strongly posteriorly extended lateral processes. In upper view, the basal margins of the two lateral processes form a rather narrower angle of about 70–80° ( Fig. 11G, L ); outer lateral process longer ( Fig. 11G, L ).

Remarks. Lee (1970) illustrated two specimens of Serratognathus bilobatus from the Dumugol Formation of South Korea, with the figured holotype ( Lee, 1970, pl. 8, fig. 7a–d) assignable to the asymmetrical Sb element defined herein and the other figured paratype ( Lee, 1970, pl. 8, fig. 6) is a symmetrical Sa element. Both specimens are identical with those from the Emanuel Formation. Serratognathus bilobatus differs from S. diversus An, 1981 ( Figs 12–14 ) from the Honghuayuan Formation of South China mainly in having a smaller, often indistinct, cusp ( Fig. 11G, H ).

Figures

Fig. 11. Serratognathus bilobatus Lee, 1970. (A), Sa element, CPC39898, 161–166 m, anterior view (IY116-041). B–F, Sb element; B–E, CPC39899, WCB705/133, (B), posterior view (IY116-002), (C), outer lateral view (IY116-006), (D), upper view showing the tip of the cusp (IY116-012), (E), upper view (IY116-011); (F), CPC39900,WCB705/133, anterior view (IY116-028). G–L, Sc element, CPC39901, WCB705/133, (G), posterior view (IY116-013), L, upper view (IY116-023), (H), basal-posterior view (IY116018), (I), posterior view of the cusp (IY116-014), (J), posterior view, close up showing fine striae (IY116-015), (K), basal view showing ring-like basal node (IY116-022). Scale bars 100 µm unless otherwise indicated.

Fig. 12. Serratognathus diversus An, 1981. All from the Honghuayuan Formation in Guizhou. A–C, Sa element; (A), AM F.135048, THH7, anterobasal view (IY119-004); (B), AM F.135044, THH7, lateral view (IY119-002); (C), AM F.135047, THH10, posterior view (IY119-040). (D), Sb element, AM F.135049, THH7, outer lateral view (IY119014); E–H, broken specimens showing overlapping laminar layers on the surface of breakage. E,F, Sb element, AM F.135815, THH7, (E), postro-lateral view (IY133-035), (F), upper view (IY133-037); G,H, Sc element,AM F.135816, THH9, (G), inner lateral view (IY133-040), (H), close up showing the surface of breakage (IY133-041); (I), Sc element, AM F.135817, YTH10, showing the partition between laminar layers and the fine laminar structure within each laminar layer (IY133-033). Scale bars 100 µm unless otherwise indicated.

Fig. 13. Serratognathus diversus An, 1981. All from the Honghuayuan Formation in Guizhou. Showing size variation; A–E,G,I,K and M are magnified equally. (A), Sb element, AM F.135818, AFI983A, anterior view, bearing six laminar layers (IY133-046). (B), Sb element, AM F.135819, THH10, anterior view, bearing six laminar layers (IY133-038). C,M, Sc element,AM F.135820,YTH5, (C), anterior view, bearing eight laminar layers (IY133-024), (M), posterior view (IY134-019). (D), Sa element,AM F.135821,AFI986, upper-anterior view, bearing eight laminar layers (IY133-050). E,F, Sc element,AMF.135822,YTH4, (E), anterior view, bearing 10 laminar layers (IY133-021), (F), close up showing denticles along anterior margin of each laminar layer (IY133-022). …[continued on facing page]

Fig. 14. Serratognathus diversus An, 1981. All from the Honghuayuan Formation in Guizhou. (A), Sb element, AM F.135050, THH7, basal-posterior view, showing fine growth laminar preserved on the lateral and posterior surfaces of the cusp (IY119-022). (B), Sc element, AM F.135825 (same specimen as Fig. 13K,L), THH12, close up showing growth laminar along the posterobasal margin (IY134-037). (C), Sc element, AM F.135816 (same specimen as Fig. 12G, H), THH9, closing up showing fine growth laminar on the posterior face (IY134-049). D,E, Sa element, AM F.135048, THH7, (D), close up showing fine growth laminar on the posterior face (IY119-006), (E), close up showing the small ring-like node representing the initial stage of the element (IY119-012). Scale bars 10 µm.