Dionide hybrida 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 : 330

Dionide hybrida Reed, 1915

Fig. 10.4

1915 Dionide hybrida sp. nov.; Reed, p. 26–27, pl. 5, fig. 7.

1940 Digrypus hybridus (Reed); Kobayashi, p. 207, text-fig 2.

Material. Holotype: Cranidium part and counterpart from Hwe Mawng Beds (Upper Katian) of Hwe-hok, Myanmar, Fig. 10.4 , GSI 11509.

Description. The species is founded upon a single, reasonably well-preserved cranidium. The illustration herein is of a cast from the counterpart of the holotype figured by Reed (1915, pl. 5, fig. 7), the convex (tr.) cranidium slightly more than twice as wide as long, with the preglabellar field occupying 15% of the cranidial length (sag.). Glabella in front of the occipital ring with almost circular dimensions, but with gently constricted sides, transversely evenly convex. The prominent glabella tubercle is at glabellar mid-length (occipital ring included). The basal lateral glabellar lobes are more or less incorporated into the glabella, but their extent is revealed as slight bulges in the axial furrows, which are not apparent on the original drawing in Reed (1915). The posterior ends approach the lateral edges of the occipital ring. A small part of the otherwise effaced furrows defining their inner edges is deepened into a pair of small subcircular glabellar furrows. The depressed and narrow (sag.) occipital ring about two-thirds glabellar width, defined by an occipital furrow which is shallower and wider medially. Axial furrows narrow, shallowest around posterolateral glabellar lobes. Thin and deep posterior border furrow is probably incomplete as it extends to genal angle in Dionide (this seems to be shown on the right hand side of the 1915 illustration). Posterior border hardly convex, slightly wider (exsag.) than occipital ring. Fixed cheeks convex adjacent to glabella and downsloping to fairly narrow concave border that maintains nearly even width around the anterior perimeter, such that at the midline the preglabellar area is very short (sag.), about one-sixth of length of glabella behind (sag.). Left hand side suggests a prolonged genal extension posteriorly. The dorsal surface of cheeks is pitted, with the larger pits following the inner edge of the anterior border. Reed’s (1915) illustration emphasises this feature, but it is not so clearly marked on the cast, where more noticeable pits are concentrated in front of the glabella. Backwardly directed principal genal vein is not well developed, but a narrow ridge can be seen on the left-hand side of the cranidium Fig 10.4 , which follows a course similar to that of other species of Dionide .

Discussion. Among numerous Dionide species only a few have a preglabellar area well short of half the length of the glabella (sag.). One of them is the type species, D. formosa ( Barrande, 1846) the lectotype of which is illustrated in Horný & Bastl (1970, pl. 13, fig. 3, see also Šnajdr, 1990, p. 193). The main differences between the cranidium of D. hybrida and that of D. formosa are the wider fixed cheeks of the former and its more rectangular glabella, as well as more prominent surface sculpture of the latter. Dionide miaopoensis Lu, 1975 ) (also Peng et al., 1991) from the Upper Ordovician Maiopo Formation of Hubei Province also has a narrow preglabellar area but appears to resemble D. formosa rather than D. hybrida in the same differential features. A much older species with A cranidium like that of D. hybrida is D. levigena Fortey & Owens, 1987 , from the “late Arenig-early Llanvirn” (Dapingian to early Darriwilian) of South Wales (also Kennedy & Stammers, 2018, fig. 235) which has an almost circular glabella, and at least one example with cheeks as narrow (tr.). A clear difference is the separate convexity of the cheek lobe on D. levigena , which merges gently with the border in D. hybrida . A Chinese species of similar age to D. hybrida , D. regalis Lu & Zhou, 1981 (see Tripp et al., 1989, fig. 12) has a cranidium of which can hardly be distinguished from that of the Myanmar species; possibly, the gena is more convex and distinctly caecate. Kobayashi (1940) made Dionide hybrida the type species of a new genus, Digrypus, but we consider that there are not sufficient distinctions from Dionide formosa and other Dionide species to justify this taxon.

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.