Birmanites yunnanensis ( Reed, 1917 )

Fortey, Richard A., Wernette, Shelly J. & Hughes, Nigel C., 2022, Revision of F. R. C. Reed’s Ordovician trilobite types from Myanmar (Burma) and western Yunnan Province, China, Zootaxa 5162 (4), pp. 301-356 : 322-324

Birmanites yunnanensis ( Reed, 1917)

Figs 8.1–6

1917 Ogygites yunnanensis Reed , p. 42-45, pl. 6, figs 12–14; pl. 7, figs 1–8.

1951 Basiliella yunnanensis (Reed) ; Kobayashi, p. 33, pl. 4, figs 7,8.

1965 Basilicus (Basiliella) yunnanensis (Reed) ; Lu et al. p. 481–2, pl. 94, fig. 17, non figs 15,16.

2008 Birmanites yunnanensis (Reed) ; Zhou & Zhen, p. 236.

2014 Birmanites yunnanensis (Reed) ; Zhang et al., figs 5.41F,G.

Material. Lectotype (selected here): articulated thorax, pygidium and incomplete cephalon, Fig. 8.5 ( Reed, 1917, pl. 6, fig. 12 and pl. 7, fig. 4; refigured Lu et al. 1965, pl. 94, fig. 17), GSI 11887 and 11893. Assigned specimens: free cheeks, Figs 8.4, 8.6 ( Reed, 1917, pl. 7, fig. 5; pl. 6, figs 13/14; pl. 7, fig. 3, respectively), GSI 11894, 11889, 11892, respectively; pygidia, Figs 8.2, 8.3 ( Reed, 1917, pl. 7, figs 8, 6 respectively) GSI 11897, 11895, respectively. Reed (1917) also figured fragmentary thoracic segments and other pieces of cephalic exoskeleton that were not cast: thoracic segments pl. 7, figs 1, 2, GSI 11890, 11891, respectively; pygidium pl. 7, fig. 7, GSI 11896. All specimens from the Shihtien Formation (Darriwilian) at Pupiao, Baoshan Prefecture, western Yunnan.

Description. The original material of Reed (1917) is only slightly distorted and presumably also flattened to some degree. The most complete articulated specimen is not preserved at the front, but the exoskeleton must have been about 1.6-1.7 times longer than wide, and the sag. length of the thorax is close to that of the pygidium. Since there is no associated cranidium its shape has to be inferred from the outline of the facial sutures, which are strongly divergent in front of the eyes. We can estimate that the sag. length of the cranidium in front of the eyes was greater than its length behind them, but less so than on B. birmanicus , such that the anterior part of the cranidium was presumably wide and flattened. A well preserved cranidium from the Shihtien Formation figured by Zhang et al. (2014, fig. 5.41F) confirms these estimations and shows a prominent posteromedian glabellar tubercle. Smaller free cheeks show a lateral border and it was likely that these extended on to the cranidium at small size (e.g. Fig. 8.4 ) but probably effaced on larger individuals (e.g. Fig 8.1 ). Since the thoracic axis is well-defined it is probable that this continued into the axial furrows also defining the glabella. The strongly curved eyes are elevated on eye socles, length (exsag.) about 30% that of cranidium to judge from the unseparated free cheeks on Fig. 8.6 . Genal lateral border well-developed, particularly on smaller free cheeks, which are extended into stout genal spines, but a certain amount of crushing may have exaggerated the concavity of the border, which is hardly developed on the specimen in Fig. 8.4 . This specimen also shows the typically wide genal doublure of Birmanites , with sparse terrace ridges. Paradoublural line on the right-hand side of the specimen in Fig. 8.6 suggests that the doublure was less extensive at smaller size. The thorax shows somewhat zetoidal axial furrows and axial rings of uniform transverse width. Thoracic pleurae widen backwards. Left-hand side of thorax Fig. 8.5 has flaked off to show the dorsal surface of reflexed doublure which extends under distal parts of pleural furrows; distal tips of pleurae apparently blunt, or at most shortly spinose. Pygidium in range 1.3 to 1.6 times wider than long, with very weak, slightly flatter border most noticeable behind axis; the latter extending to 70-75% pygidial length. Narrow axis, axial furrows enclosing an angle of about 20 degrees, and three times or more as long as wide, with maximum width immediately behind thorax, tapering uniformly posteriorly. 5 or 6 clearly-defined axial rings slightly decreasing in width (sag.) posteriorly, and clear ring furrows a little wider and more diffuse medially. Articulating facet extends halfway or more across pleural field. Six pairs gently curved pleural furrows progressively more posteriorly directed and then shorter, the gently convex ribs between them also narrowing (exsag.). Ribs fade at the border. Doublure clearly seen on composite Fig. 8.2 , very broad, curving back almost to axis, and with series of widely spaced terrace ridges typical for genus. The lectotype shows evidence of fine raised lines running subparallel to the pygidial margin, but otherwise there is no evidence of sculpture.

Discussion. The individual selected here as the lectotype has a part and counterpart with two different GSI designations (11887 and 11893, respectively). GSI 11887 is a more complete specimen containing almost all of the thorax and all of the pygidium. GSI 11893 is only a small portion of thorax and the free cheek. Identical deformation signatures leave little doubt that these are the same individual though Reed (1917) did not recognize them as such. Although quite well preserved, the type material does not include a cranidium. However, Zhang et al. (2014, fig. 5.41F) illustrated a cranidium from western Yunnan associated with a pygidium (fig. 7.41G) identical to those of B. yunnanensis from the type collection that confirms an assignment to Birmanites . Kobayashi (1951) had assigned B. yunnanensis to Basiliella , a genus clarified by Zhou & Fortey (1986) and not close to Birmanites , and more typical of lower palaeolatitude limestone facies of North China. The smaller free cheeks of B. yunnanensis have lateral borders which are better defined than on most illustrated species of Birmanites , but this is likely to be an earlier ontogenetic feature, since small birmantines can have well-defined genal borders (e.g. Birmanites hupeiensis Zhou, Yin & Tripp, 1984 , fig. 3e; Opsimasaphus jaanussoni Kielan, 1960 , pl. 7, fig. 2). Reed (1917, pl. 7, fig. 3) illustrated a larger free cheek with the doublure extending to the eye lobe and carrying widely spaced terrace ridges, typically birmanitine. The cast from this specimen ( Fig. 8.4 ) has a feeble lateral border and shows much of the course and extent of the facial suture, which is compatible with the cranidium figured by Zhang et al. (2014). Similarly, wide doublure with terrace ridges is seen on the pygidium illustrated in Reed (1917, pl. 7, fig. 6) but the impression of the ridges is less clear on the cast from the counterpart of that specimen used here ( Fig. 8.2 ). Birmanites yunnanensis differs from the type species and many others in having relatively straight (not distally backwardly curved) pygidial pleural furrows, while the axis is about 70% total pygidial length. Generally similar pygidia are displayed by the Sandbian-Katian B. qilangensis Zhang, 1981 , from Xinjiang (Tarim) (see Zhou et al., 2014, fig. 29 A-C) and by B. hupeiensis Yi 1957 , from the Darriwilian Shihtzupu Formation of Guizhou Province. The latter had been erroneously identified with B. yunnanensis by Sun (1931) and Kobayashi (1951) and placed in the genus Basiliella by the latter (see Zhou et al., 1984, p. 17). The free cheeks illustrated by Zhou et al. (1984, fig. 3e) are very similar to Reed’s illustrated here in Fig. 8.6 , with regard to development of the border. However, if the cranidium assigned to B. yunnanensis in Zhang et al. (2014) is correct, then the preglabellar area of B. hupeiensis is relatively much longer (sag.).

Figures

FIGURE 8. (1–6) Birmanites yunnanensis (Reed) from the Shihtien Formation (Darriwilian), Pupiao, Baoshan Prefecture, western Yunnan, China. 1. Librigena (original of Reed 1917, pl. 7, fig. 3), GSI 11892. 2. Pygidium (original of Reed 1917, pl. 7, fig. 6), GSI 11895. 3. Pygidium (original of Reed 1917, pl. 7, fig. 8), GSI 11897. 4. Librigena (original of Reed 1917, pl. 7, fig. 5), GSI 11894. 5. Articulated specimen (original of Reed 1917, pl. 6, fig. 12), GSI 11887. 6. Cephalon (original of Reed 1917, pl. 6, fig. 14), GSI 11889. (7, 10) Remopleuriidae gen. et sp. indet. (Reed) from the Shihtien Formation (Darriwilian), Pupiao, Baoshan Prefecture, western Yunnan, China. 7. Cranidium, (original of Reed 1917, pl. 6, fig. 9), GSI 11884. 10. Librigena (original of Reed 1917, pl. 6, fig. 10), GSI 11885. (8) Dolerobasilicus? sp. (Reed), Pygidium (original of Reed 1915, pl. 7, fig. 3), GSI 11535, Lower Naungkangyi Beds, Nawa, northern Shan State, Myanmar. (9) Nileus sp., Hypostome (original of Reed 1917, pl. 8, fig. 5), GSI 11904, Shihtien Formation (Darriwilian), Pupiao, Baoshan Prefecture, western Yunnan, China. (11, 12) Niobe sp. (Reed). 11a, b. Pygidium (original of Reed 1915, pl. 5, fig. 14), GSI 11516, Hwe Mawng Beds (uppermost Katian), Nati, northern part of Shan State, Myanmar, original and retrodeformation. 12. Pygidium (original of Reed 1917, pl. 6, fig. 11), GSI 11886, Shihtien Formation (Darriwilian), Shidian, Baoshan Prefecture, western Yunnan, China. Scale bars = 5 mm for 1–6, 8, 9; = 2 mm for 7, 10–12)