Baltoniodus alobatus ( 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 : 214-216

publication ID	https://doi.org/ 10.3853/j.0067-1975.63.2011.1586
persistent identifier	https://treatment.plazi.org/id/03CCEB77-FFDB-637F-FC17-FC7296FBF943
treatment provided by	Felipe
scientific name	Baltoniodus alobatus ( Bergström, 1971 )
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Baltoniodus alobatus ( Bergström, 1971)

Figs 5L, Q View Figure 5 , 8M–N View Figure 8

Prioniodus alobatus Bergström, 1971: 145 , pl. 2, figs 4–5; Zeng et al., 1983: pl. 12, fig. 33; An, 1987: 169–170, pl. 25, figs 7–9 (cum syn.); Zhong, 1990: 150, pl. 17, figs 1–2; Wang & Zhou, 1998: pl. 1, figs 11–13.

Prioniodus lingulatus An, 1987: 170–171 , pl. 25, figs 10–17 (cum syn.).

Baltoniodus alobatus (Bergström) .– Dzik, 1994: 83, fig. 14d; Leslie, 2000: 1127, fig. 7.2–7.13 (cum syn.); Zhao et al., 2000: 189, pl. 34, figs 11–13, pl. 39, figs 16–20, 23 (cum syn.); Xiong et al., 2006: 368, pl. 1, figs 2–3; Agematsu et al., 2008a: 967, fig. 11.1–11.21; Ortega et al., 2008: fig. 6.1.

Prioniodus cf. P. alobatus Bergström. –An, 1981: pl. 4, fig. 9a–b.

Baltoniodus sp. cf. B. variabilis (Bergström) .– Agematsu et al., 2007: 33–35, fig. 10.1–10.17.

Material. Thirty-six specimens from two samples of the Kanling Formation (see Table 1).

Remarks. Bergström (1971, pl. 2, figs 4–5) described and illustrated only the amorphognathiform element of Prioniodus alobatus represented by the holotype, indicating that the other five or six types of elements comprising the apparatus were closely similar to those of B. variabilis ( Bergström, 1962) . According to the original description, the most distinctive feature of its amorphognathiform (Pa herein) element was “a wide, but low, platform-like posterior process” ( Bergström, 1971, p. 145). More recently, Leslie (2000) fully described and illustrated this species as consisting of a seximembrate apparatus including geniculate M, alate ramiform Sa, bipennate Sc, quadriramate Sd, pastinate Pa and Pb elements.

The illustrated Pa element ( Fig. 8N View Figure 8 ) from the upper part of the Kanling Formation is a broken specimen with only the posterior process preserved. Similar specimens with a wide platform and a centrally positioned row of denticles were also reported as B. alobatus from subsurface core samples of the Yingmai-10 well (Zhao et al., 2000, pl. 39, fig. 18), from the Tumuxieke Formation of Bachu (Wang & Zhou, 1998, pl. 1, fig. 11), from the Qiaerbake Formation in Bachu (Xiong et al., 2006, pl. 1, figs 2–3), and from the same sample (Nj406) of the Kanling Formation of the Dawangou section (Wang & Zhou, 1998, pl. 1, fig. 13). The illustrated Pb element ( Fig. 5Q View Figure 5 ) is identical with the specimen previously illustrated as B. alobatus by Wang & Zhou (1998, pl. 1, fig. 12) from a slightly higher level (sample Nj406) in the Kanling Formation of the same section, and also corresponds exactly to the ambalodiform (Pb herein) element of Prioniodus lingulatus ( An, 1987, pl. 25, fig. 12) from the Datianba Formation of Ganxi, Guizhou Province.

The illustrated Pa element ( Fig. 8N View Figure 8 ) is also identical with the holotype of B. lingulatus An, 1987 , which was defined as consisting of a seximembrate apparatus including amorphognathiform, ambalodiform, cordylodiform, tetraprioniodiform, trichonodelliform and oistodiform elements. An (1987) suggested that B. lingulatus was closely similar to B. alobatus , but the posterior process in the amorphognathiform element of the latter had a more convex upper surface, which was undulose in lateral view. One illustrated specimen of the Pa element of P. lingulatus ( An, 1987, pl. 25, fig. 15) from the Datianba Formation of Jiangsu is identical with the specimen illustrated by Leslie (2000, fig. 7.3) as the Pa element of B. alobatus in having a distinctive inner-lateral lobate expansion on the posterior process. Leslie (2000, p. 1127) indicated that this feature was seen in many of the specimens referred to as the Pa element of B. alobatus . Based on his description, one of the most distinctive features of B. alobatus is that its S elements have small closely spaced denticles intercalated with several much larger denticles, particularly on the posterior process of the Sc element ( Fig. 5L View Figure 5 ; see also Leslie, 2000, fig. 7.9). This feature is also developed in the type specimens of Prioniodus lingulatus An, 1987 (pl. 25, fig. 13), indicating that the latter, if not conspecific with B. alobatus , is certainly closely related to it. However, some specimens from a stratigraphically slightly lower level (Nj388, Fig. 5H–I View Figure 5 ) that are assigned to B. prevariabilis also show a similar denticle pattern on the posterior process.

Specimens representing the Pa element of B. alobatus recorded from the Tarim Basin ( Fig. 8N View Figure 8 ; also see Wang & Zhou, 1998; Xiong et al., 2006) are more comparable with the holotype of P. lingulatus in having a wider posterior platform than the holotype of B. alobatus . Considering that the width and the shape of the posterior platform varies significantly amongst the type material of B. lingulatus , that species is regarded herein as a junior synonym of B. alobatus as indicated by Zhao et al. (2000, p. 189). The Sc element of B. alobatus ( Fig. 5L View Figure 5 ) differs from the corresponding element of the stratigraphically slightly older species B. prevariabilis ( Fig. 5O–P View Figure 5 ) in having smaller denticles intercalated with a number of larger denticles on the posterior process, and the same feature is also observed in the Sd element ( Fig. 8M View Figure 8 ). The specimens of B. alobatus assigned by Agematsu et al. (2007, fig. 10) to B. sp. cf. B. variabilis are similar to the Tarim form. Their M element has an unusually long inner lateral process compared to the Swedish specimens (although those are usually broken).

The Pa element representing Baltoniodus sp. B ( Fig. 8I–L View Figure 8 ) that co-occurs with B. alobatus in sample Nj406 has a platform-like anterior process (rather than posterior process), which is narrower with the row of denticles near the outer lateral margin ( Fig. 8J–K View Figure 8 ). Furthermore, the anterior process is straight, extending downward in lateral view ( Fig. 8I View Figure 8 ) in comparison to the posterior process of B. alobatus . It shows some resemblance to B. variabilis , but the Pa element of the latter has a “triangular lateral expansion of the inner side of the posterior process” ( Bergström, 1971, p. 148, pl.2, fig. 2).

Kingdom	Animalia
Phylum	Chordata
Class	Conodonta
Order	Conodontophorida
Family	Balognathidae
Genus	Baltoniodus