Serratognathus diversus An, 1981

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 : 25-26

Serratognathus diversus An, 1981

Figs 12–14

Serratognathus sp. A An et al., 1981: pl. 1, fig. 10.

Serratognathus diversus An, 1981: 216 , pl. 2, figs 23, 27, 30; Zhen et al., in press a: figs 10A–K, 11A–K, 12D–M (cum syn.).

Serratognathus obliquidens Chen, Chen & Zhang, 1983: 136 , pl. 1, fig. 18.

Serratognathus tangshanensis Chen, Chen & Zhang, 1983: 136–137 , pl. 1, figs 14–17; Chen & Zhang, 1989: 223, pl.5, fig. 12.

Remarks. Both Serratognathus obliquidens and S. tangshanensis were erected as form species from the Honghuayuan Formation exposed in the Nanjing Hills ( Chen et al., 1983). We consider them to be junior synonyms of S. diversus , representing the asymmetrical elements (Sb + Sc) of our current notation.

Based on a large collection of S. diversus from the Honghuayuan Formation, Zhen et al. (in press a) recognized a trimembrate apparatus including symmetrical Sa, asymmetrical Sb and strongly asymmetrical Sc elements ( Fig. 12 ). Material of S. bilobatus from the Emanuel Formation shows the same apparatus. Morphologically, S. diversus and S. bilobatus are closely comparable except that the former has a much more prominent, laterally compressed cusp. Their elements are resembling a half-cut onion with a posteriorly located cusp enveloped anterolaterally by numerous overlapping laminar layers (varying from 6 to 20, see Fig. 13 ). The upper margins of these laminae are progressively lower anterolaterally. Laminae are ornamented with tooth-like denticles ( Fig. 13 ) along their anterior and lateral margins, and by blunt node-like denticles along the posterior margins ( Fig. 14D ). Underneath each specimen is a wide, smooth, distally arched basal face ( Fig. 12A, G ), which is crescentic in outline and divided anteroposteriorly into two lobes (lateral processes; Fig. 13 A–E, K ) by a broad, median groove. Typically a small ring-like node underneath the cusp represents the initial stage of the element ( Figs 13N , 14E ), without basal cavity.

Natural breakage surfaces on some of the specimens show these laminar layers to be tightly compacted ( Fig. 12G, H ) with partitions generally observed only near the edge of each layer ( Fig. 12E–H ). In most cases, no microstructure is observable on the surface of the breakage. However, some specimens show fine laminations (growth lamellae) formed by flattened crystallites ( Fig. 12I ).

Serratognathus extensus Yang in An et al., 1983 differs from S. diversus in having a more robust cusp and a long, laterally extended process on each side. This species is similar to S. diversus and S. bilobatus in that the anterior face bears 2–3 rows of denticles that extend continuously from the end of one lateral process to the other, and in lack of a basal cavity.Yang’s type material included 14 specimens from the Liangjiashan Formation in Hebei Province ( An et al., 1983, tables 6–7). An et al. (1983, p. 26) suggested that S. extensus might be directly evolved from S. bilobatus , and recognized the S. extensus Zone succeeding the S. bilobatus Zone in the Liangjiashan Formation.

Figures

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.