Pliomerina martellii ( 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 : 331

Pliomerina martellii ( Reed, 1917)

Fig. 10.7

1917 Pliomera martellii ; Reed, 1917, p. 55 –56, pl. 8, figs 15,16.

Material. Holotype: cranidium from Shihtien Formation (Darriwilian) of Pupiao, Fig. 10.7 ( Reed, 1917, pl. 8, figs 15,16) GSI 11915, Pupiao , western Yunnan, southwestern China.

Description. Cranidium twice as wide as long. Glabella with low transverse convexity, maximum inflation across frontal lobe, greatest width at L3 is 1.3 times that across occipital ring, and slightly less than sagittal length. Anterior outline of glabella at cranidial margin is gently and evenly arcuate about the mid-line. Deep glabellar furrows extend less than one-third across glabella: S1 slightly curving backwards, S2 hardly so, and S3 distinctly inward-backwardly directed making an angle of about 30 degrees to the transverse line. Since this specimen is not a true dorsal surface the furrows may well have been longer and narrower on the exterior surface. The latter is undoubtedly also true of the deep and wide axial furrows that expand in width towards the anterolateral glabellar corners. Of glabellar lobes L3 is most inflated and widens abaxially; its greatest width (exsag.) is twice that of narrow L1 and 1.5 that of L2. Occipital furrow curves forwards in a broad arc medially, such that the occipital ring is widest in its central two-thirds. The anterior cranidial border is developed as a narrow ridge anterolaterally, but narrows and merges with the frontal lobe of the glabella adaxially. Silicified material figured by Webby (1971) shows that it remains just distinct from the glabella on the exterior surface but this is not expressed on the internal mould. Fixed cheeks three-quarters as wide (tr.) as maximum width of glabella, narrow (exsag.) posterior limb bisected by deep, wide border furrow (as preserved). Medium sized palpebral lobe extends back as far as L2, the area inside it narrow (tr.) and inflated. There is some evidence of a reticulum in this area.

Discussion. Reed (1917, p. 56) referred to the type species being “represented by only two head-shields showing the cast and impression” and Webby (1971, p. 614) in turn referred to “two incomplete cranidia”. The type material consists of a single individual cranidium in part and counterpart, as Reed probably implied. Although it is preserved in full relief most of its features suggest that we are seeing the internal surface of this cranidium, and this should be taken into account in comparing with other species assigned to Pliomerina . Nonetheless, despite the lack of the associated pygidium from the type locality, with its characteristic well-defined elongate terminal piece, the species assigned to Pliomerina by Chugaeva (1958), Webby (1971) and listed in Zhou & Zhen (2008) form a tight group. A squat glabella with some lateral inflation of L3 is typical. However, P. tashanensis Lee, 2013 , from Jiangxe Province, China, has an elongate glabella with somewhat effaced furrows, no lateral prominence of L3, and the pygidial terminal piece is not inflated; it is not typical of the genus. Cranidia of P. australis, Webby (from Australia) of similar size to the holotype of P. martellii (e.g. Webby, 1971, pl. 115, fig. 1) have a more deeply arcuate or parabolic outline of the frontal glabellar lobe, although smaller individuals like the holotype ( Webby, 1971, pl. 114, figs 2-7) are more similar to P. martellii in this regard. However, the fixed cheeks are narrower (tr.) and the inflation of L3 more marked. Of three Katian species from Kazakhstan described by Chugaeva (1958) P. dulanensis is probably most similar to P. martellii , although its glabellar furrows are longer and more curved forwards. Possibly the most similar species to P. martellii is P. serrata described by Zhou & Zhou (2006) from Katian strata in Inner Mongolia; it may have longer glabellar furrows. An ‘outlier’ of the genus in Argentina ( Edgecombe et al., 1999) provides good evidence that a segment of the Precordillera was part of Gondwana by the later Ordovician. It should be noted that Reed (1917) reported this species from several localities, but this is not reflected in the collections.

Figures

FIGURE 10. (1, 2) Lonchodamas? sp. (Reed) from the Li-Lu Formation (Katian), Lilu, northern Shan State, Myanmar. 1a, b. Pygidium (original of Reed 1915, pl. 5, fig. 6), GSI 11508, original and retrodeformation. 2. Cranidium (refigured from Reed 1915, pl. 5, fig. 5), GSI 11507. (3, 6) Lonchodamas shanensis (Reed) from the Hwe Mawng Beds (uppermost Katian), northern Shan State, Myanmar. 3. Pygidium (original of Reed 1915, pl. 5, fig. 3), GSI 11505, Hwe-hok. 6. Cranidium (original of Reed 1915, pl. 5, fig. 2), GSI 11504, Mong Ha. (4) Dionide hybrida (Reed), Cephalon (original of Reed 1915, pl. 5, fig. 7), GSI 11509, Hwe Mawng Beds (uppermost Katian), Hwe-hok, northern Shan State, Myanmar. (5, 8) Ovalocephalus? sp. indet. (Reed) from the Li-Lu Formation (Katian), Mân-ngai, northern part of Shan State, Myanmar. 5. Cranidium (original of Reed 1915, pl. 8, fig. 17), GSI 11563. 8. Thorax (original of Reed 1915, pl. 8, fig. 21), GSI 11567. (7) Pliomerina martellii (Reed), Cranidium (original of Reed 1917, pl. 8, fig. 15), GSI 11915, Shihtien Formation (Darriwilian), Pupiao, Baoshan Prefecture, western Yunnan, China. (9) Prionocheilus liluensis (Reed), Cranidium (original of Reed 1915, pl. 8 figs 6 and 7), GSI 11554, Li-Lu Formation (Katian), Lilu, northern Shan State, Myanmar. (10, 13) Ovalocephalus sp. (Reed) from the Li-Lu Formation (Katian), Lilu, northern Shan State, Myanmar. 10. Pygidium (original of Reed 1915, pl. 8, fig. 9), GSI 11556. 13. Cephalon; not previously figured; no GSI catalogue number. (11, 12) “Calymene” oldhami (Reed) from Lower Naungkangyi Beds (probable Darriwilian), Mâ Shio, northern Shan State, Myanmar. 11. Cephalon and thorax (original of Reed 1915, pl. 8, fig. 11), GSI 11558. 12. Cephalon and partial thorax (original of Reed 1915, pl. 8, fig. 10), GSI 11557. Scale bars = 2 mm for 1–3, 6, 8–13; = 5 mm for 4, 5; = 1 mm for 7.