Cahabagnathus sweeti ( Bergström, 1971 )

Zhen, Yong Yi, 2011, Middle to Late Ordovician (Darriwilian-Sandbian) Conodonts from the Dawangou Section, Kalpin Area of the Tarim Basin, Northwestern China, Records of the Australian Museum 63 (3), pp. 203-266 : 218-221

Cahabagnathus sweeti ( Bergström, 1971)

Fig. 8A–H

Polyplacognathus sweeti Bergström, 1971: 143–144 , pl. 1, figs 1–2, text-fig. 14C–D; Harris et al., 1979: pl. 2, figs 12–13; Wang & Lou, 1984: 257, pl. 11, fig. 20, pl. 12, figs 5–6.

Petalognathus bergstroemi Drygant, 1974b: 54 , pl. 1, figs 1–2.

Cahabagnathus sweeti (Bergström) .– Bergström, 1983: 51, fig. 6I–J; Bauer, 1990: pl. 1, figs 16–17; Bergström, 1990: pl. 1, fig. 18; Gao, 1991: 128, pl. 8, figs 4–6; Bauer, 1994: pl. 4, figs 1–3; Wang et al., 1996: pl. 1, figs 24, 27, pl. 4, fig. 11; Zhang, 1998b: fig. 9G–H; Wang & Zhou, 1998: pl. 1, fig. 1; Lehnert et al., 1999: pl. 3, fig. 11; Zhao et al., 2000: 192, pl. 29, figs 7–10; Leslie & Lehnert, 2005: figs 1–2; Bergström, 2007: fig. 3C.

Material. Eight specimens from two samples of the lower Kanling Formation (see Tables 1–2).

Description. This species is rare in the Dawangou samples, being represented by only a few specimens. It has a bimembrate apparatus consisting of paired (sinistral and dextral) stelliplanate Pa and pastiniplanate Pb elements. Both elements bear an indistinct cusp and four processes (anterior, posterior, antero-lateral and postero-lateral) with a central row of nodes on each one. The sinistral and dextral elements form mirror-images of each other ( Fig. 8C–D ). Pa element has a short, blade-like anterior process nearly linear with the longer posterior process; antero-lateral process narrower and longer, being nearly perpendicular to the anterior and posterior processes; postero-lateral process shorter with a wider platform, forming an obtuse angle (about 110°) with the anterior process ( Fig. 8A–B ). A couple of specimens from the same sample have a rather prominent cusp, and their postero-lateral process has a narrower platform and forms a narrower angle (about 60°) with the anterior process ( Fig. 8F–G ), hence they are only questionably included in this species. Pb element with a short anterior process and a long posterior process represented by a row of nodes arranged more or less in straight line across the centre of the platform; posterior process with a wide and rounded platform in upper view; antero-lateral process long and tapering distally with the mid-row of nodes approximately perpendicular to the row of nodes on the anterior or posterior process; postero-lateral process shorter, with central row of nodes forming an angle of about 130° with the row of the nodes on the posterior process; nodes also developed along the platform margins of posterior and postero-lateral processes ( Fig. 8C–E ).

Remarks. Bergström (1983) suggested that five distinctive species of Cahabagnathus occurring in successive stratigraphic order in North America formed an evolutionary lineage from the oldest C. sp.A (= C. directus Bauer, 1987 ) of mid-late Darriwilian age (early P. serra Zone ) to the youngest C. carnesi in the early Sandbian (early A. tvaerensis Zone ). He proposed the generic name Cahabagnathus for this group and indicated that its direct ancestor might be a species of Eoplacognathus , such as E. foliaceus and E. reclinatus . Bergström (1983, p. 41) and Zhang (1998b, p. 14–15) also discussed in detail the evolutionary trends and character transformations through several recognized stages of the Cahabagnathus lineage. Within the eight known species of Cahabagnathus, Leslie & Lehnert (2005) recognized two lineages that shared an unknown common ancestor, and suggested that their dispersals and speciation were largely influenced by the transgressive and regressive events of sea level changes.

Cahabagnathus sweeti differs from C. carnesi Bergström, 1983 in having the short anterior process in the Pb element more or less extending straight, rather than bending towards the antero-lateral process as in the Pb element of C. carnesi . In China, C. sweeti has only been recorded from the Kanling Formation ( Gao, 1991; Wang et al., 1996; Wang & Zhou, 1998) and subsurface age equivalents (Zhao et al., 2000) of the Tarim Basin, and from the Sandaogou Formation of Longxian County, Shaanxi Province in North China (Wang & Lou, 1984; Wang et al., 1996).

Figures

Figure 8. A–H, Cahabagnathus sweeti (Bergström, 1971). A–B, sinistral Pa (stelliplanate) element, NIGP 152884, Nj384,A, upper view (IY154-019), B, upper-lateral view (IY154-020); C, dextral Pb (pastiniplanate) element, NIGP 152885,AFT-X-K13/43, upper view (IY164- 019); D, sinistral Pb (pastiniplanate) element, NIGP 152886, AFT-X-K13/43, upper view (IY164-020); E, sinistral Pb (pastiniplanate) element, NIGP 152887, AFT-X-K13/43, upper view (IY164-021); F, dextral Pa (stelliplanate) element, NIGP 152888, AFT-X-K13/43, upper view (IY164-022); G–H, dextral Pa (stelliplanate) element, NIGP 152889, AFT-X-K13/43, G, upper view (IY164-024), H, lateral view (IY164-026). I–L, Baltoniodus sp. B. I–J, L, Pa element, NIGP 152890, Nj406, I, outer lateral view (IY155-011), J, upper view (IY155-012), L, upper view, close up showing fine striation on the upper surface (IY155-013); K, Pa element, NIGP 152891, Nj406, antero-outer lateral view (IY161-012).M–N,Baltoniodus alobatus (Bergström, 1971). M, Sd element,NIGP 152892, Nj406, postero-lateral view (IY161-010); N, Pa element, NIGP 152893, Nj406, upper view (IY161-009). O–P, Cornuodus longibasis (Lindström, 1955). O, Sb element, NIGP 152894,AFT-X-K13/13, outer lateral view (IY167-001); P, Sa element, NIGP 152895, Nj291, lateral view (IY158-004). Scale bars 100 µm, unless otherwise indicated.