Birmanites birmanicus ( Reed, 1915 )

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 : 319-321

Birmanites birmanicus ( Reed, 1915)

Figs 6 , 7 .

1915 Ogygites birmanicus Reed , p. 30-32, pl. 5, figs 15–18; pl. 6, figs 1–4.

1915 Megalaspis aff. Hyorhina Leuchtenberg in Schmidt, 1906, Reed, p. 35–36, pl. 7, fig. 2.

Material Holotype (selected Reed, 1915, p.32): incomplete cephalon and first few thoracic segments from the Hwe Mawng Beds at Hwe Mawng, Fig. 6.2 , ( Reed, 1915, pl. 6, figs 1–2), GSI 11522. Paratype material: incomplete dorsal exoskeleton from Hpakhi, Fig. 6.6 ( Reed, 1915, pl. 5, figs 15–16), GSI 11518; incomplete dorsal exoskeleton from Hpakhi, Fig. 6.3 ( Reed, 1915, pl. 6, fig. 3), GSI 11523; incomplete dorsal exoskeleton from Hwe Mawng, Fig. 6.4 ( Reed, 1915, pl. 6, fig. 4), GSI 11524; hypostome in life position from Hwe Mawng, Fig. 6.1 ( Reed, 1915, pl. 5, fig. 17), GSI 11519; cranidium from Hwe Mawng ( Reed 1915, pl. 5, fig. 18), GSI 11520. Additional material: cranidium from Hpakhi, Fig. 6.5 ( Reed, 1915, pl. 7, fig. 2), GSI 11534. All from Hwe Mawng Beds (uppermost Katian) at Hapakhi or Hwe Mawng, northern Shan State.

Descriptive remarks. Reed (1915) recognised distortion in the specimen figured here as Fig. 6.4a , but did not seem to acknowledge distortion more generally, regarding the variation in relative widths from one specimen to another as variation within the species. The incomplete dorsal shield ( Fig. 6.6 herein) does not show evidence of distortion and compares in its proportions with subsequently described species. However, the holotype ( Fig. 6.2a ) is very likely transversely extended. An attempt at its retrodeformation restoring to similar proportions to the undistorted specimen ( Fig. 6.2b ) produces a more plausible, semicircular outline for the eyes. The glabella occupies very slightly more than half the cephalic length (sag.) in both. A large cranidium in Reed’s illustrations may be lengthened (1915, pl. 7, fig.2), but we regard it as probably also belonging to B. birmanicus , rather than ‘ Megalaspis ’. We were not able successfully to cast the original cranidium of Reed’s pl. 5, fig. 18, which shows similar extension, and this extension may apply also to the original of Fig. 6.3 . The preservation of the glabella is not good, but Reed correctly described the pyriform frontal lobe extending far back, with a single pair of glabellar furrows defining a weak triangular lobe adjacent to the palpebral area, best shown on the left-hand side of Fig. 6.6 . The cephalic doublure extends almost to the eye, and most specimens showing it are composite, i.e. the impression of the strong, widely spaced terrace ridges on the doublure is marked on the dorsal surface. Not all material displays the weak ‘radiating folds’ ( Reed, 1915, p. 31) crossing the free cheeks and preglabellar field and it may be that the flattening also exaggerates them. Reed noted up to 40 such folds, but there are about half that number. The hypostome ( Fig. 6.1 ) is slightly twisted and cannot really be retrodeformed; it does show the elongate oval middle body and wide fork we regard as typical for Birmanitinae . The retrodeformed Fig. 6.4b shows the similarly wide pygidial doublure approaching the axis, with about eight very strong terrace ridges shallower exteriorly and converging around the tip of the axis. This pygidium shows up to eight ribs; a gentle backward curvature is seen on the least distorted specimens, which is one reason to suspect that the original of Fig. 6.3 has been stretched lengthwise to produce steeper curvature and a deeper pygidial margin. Seven axial rings are visible on Fig. 6.6 , which also shows the transverse ring furrows slightly shallowing medially.

Discussion. The type series is imperfect, but sufficient to support the diagnosis given by Lu (1975). Well-preserved material of Birmanites in full relief is rare. Where it is known (e.g. Birmanites politus Lu, 1975 , pl. 8, fig. 8; also see Zhou & Dean 1986, pl. 59. fig. 14; Chugaeva 1958, pl. 4, fig. 1; Ghobadi Pour et al., 2011) the glabella shows more details than we have available on the type series of B. birmanicus , notably a posterior median glabellar tubercle, and two closely spaced, narrow and short glabellar furrows on the flanks of the frontal glabellar lobe. The least distorted of Reed’s specimens ( Fig. 6.6 ) shows the posterior glabellar furrow opposite the palpebral lobe. The radially disposed ridges on cheeks and preglabellar area are variably developed and may be exaggerated by flattening. They are, however, also shown on Birmanites yangtzeensis Lu (1975 , pl. 8, figs 9,11) and ‘ Ogygites ‘ almatyensis Chugaeva (1958 , pl. 4, fig.1). Placing Reed’s (1915, pl. 7, fig.2; Fig. 6.5 herein). Megalaspis aff. hyorhina ( Schmidt,1906) into Birmanites birmanicus leaves no doubt about the high divergence of the facial sutures in front of the eye (we were not able to cast the whole of this cranidium from its counterpart), and this specimen also shows faint radial ridges supporting our determination. Zhou & Zhen (2008) show Birmanites ranging through much of the Chinese Ordovician. While B. yangtzeensis is similar in cephalic characters to B. birmanicus , its pygidium shows interpleural furrows and has a relatively wide border.

Figures

FIGURE 6. Birmanites birmanicus (Reed) from the Hwe Mawng Beds (uppermost Katian), northern Shan State, Myanmar. 1. Hypostome (original of Reed 1915, pl. 5, fig. 17), GSI 11519, Hwe Mawng. 2a, b. Cephalon (original of Reed 1915, pl. 6, fig. 2), GSI 11522, Hwe Mawng, original and retrodeformation. 3. Partial specimen (original of Reed 1915, pl. 6, fig. 3), GSI 11523, Hpakhi. 4a, b. Pygidium (original of Reed 1915, pl. 6, fig. 4), GSI 11524, Hwe Mawng, original and retrodeformation. 5. Cranidium (original of Reed 1915, pl. 7, fig. 2), GSI 11534, Hpakhi. 6. Partial specimen (original of Reed 1915, pl. 5, figs 15,16), GSI 11518, Hpakhi. Scale bars = 2 mm for 1; = 5 mm for 2–6.

FIGURE 7. Reconstruction of Birmanites birmanicus (Reed). A) Dorsal skeleton; B) Hypostome.