Birmanitinae Kobayashi, 1960

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 : 317-318

Subfamily Birmanitinae Kobayashi, 1960

Discussion. Birmanites Sheng, 1934 , with type species Ogygites birmanicus Reed, 1915 , was originally described as an asaphid, but was placed in a separate family Birmanitidae by Kobayashi (1960, p. 254) and in Dikelokephalinidae in the 1959 Treatise on Invertebrate Paleontology (Harrington et al. in Moore, 1959). Lu (1975, p. 319) regarded Birmanites as no more than an unusually flat asaphid, with a relatively small glabella, an opinion with which we concur; the taxon Birmanitidae has disappeared from the literature. It is, however, still available as a subfamily name within Asaphidae . Division of Asaphidae into subfamilies is itself contentious; a relatively conservative approach was used by Jaanusson (in Moore, 1959), who employed seven subfamilies. The opposite approach was that of Balashova (1976) who elevated Asaphidae to subordinal level and divided the group into numerous families and subfamilies. Balashova nominated subfamilies centred on well-known Baltic endemic genera such as Megistaspis Jaanuson, 1956 , Pseudoasaphus Schmidt, 1901 and Ptychopyge Angelin, 1854 , to each of which she consigned a number of her own, finely divided genera. This is not an appropriate place to review this work of prolific taxonomic ‘splitting’, although the majority of Balashova’s taxa are confined to the Ordovician Baltic Shield, and her scheme has not been widely adopted. However, Balashova’s (1976) Subfamily Nobiliasaphinae (placed in Family Pseudoasaphidae Balashova, 1969 , by Balashova [1976]) includes the genera Opsimasaphus Kielan, 1960 , and Nobiliasaphus Přibyl & Vaněk, 1965 , both of which are closely similar to Birmanites , and must surely be included within the same subfamily, whatever taxonomy is eventually adopted. The name Birmanitinae Kobayashi, 1960 is the older name for this group even though it was not recognised by Balashova (1976).

An objective appraisal of asaphid trilobites is overdue. An unpublished PhD thesis by Mark Bell (2009) under the supervision of RAF attempted a cladistic analysis of Asaphidae . One group that comprised a clade with moderate support included Birmanites , and the name Birmanitinae is appropriate for it. The most important characters uniting the group are displayed by the hypostome, which has a characteristic rounded to oval middle body with prominent maculae at the rear, and a broad, U-shaped fork, and hypostome widest at, or in front of the maculae. The characteristic hypostome is illustrated by a specimen from the type series of Birmanites birmanicus figured herein ( Fig. 6.1 ) and an illustrative reconstruction ( Fig. 7.b ), and by the Barrandian lectotype (selected Kielan, 1960, p. 76) of Nobiliasaphus nobilis (see Horný & Bastl, 1970, pl. 6 fig. 1; Rabáno, 1989, pl. 3, fig. 6; also, Hughes, 1979, fig. 11 for a similar hypostome, and Hammann & Leone 1997, pl. 6, fig. 3 and Turvey, 2007 for further examples). Dorsal characters in the group are variable with regard to width of pleural areas, preglabellar field, dorsal effacement and the like, but a glabella with an inflated, often pyriform frontal lobe and with a pair of posteriorly inwardly sloping, and often deepened axial/glabellar furrows which constrict a narrow median glabellar lobe in front of the occipital ring, is also typical. Where known, the medial tubercle is immediately pre-occipital ( Birmanites —see Lu, 1975, pl. 8, fig. 8), and the dorsal facial sutures run at, or at least very close to the anterior margin before they meet medially. A distinctive genus from central Australia, Norasaphus Fortey & Shergold, 1984 , which includes the only tuberculate asaphids, also belongs within the same group, showing similar hypostome construction and glabella furrows, even though it is much more convex and less flattened peripherally than Birmanites . Turvey (2007) added further Chinese taxa to this group. Nobiliasaphus is distinguished from Birmanites in having a curious structure of the pygidial axis, with chevron-like medial backward deflections of the ring furrows, the ring itself bisected transversely by fainter extra furrows and ridges of similar form.

This asaphid group is peri-Gondwanan, with scattered occurrences from France, Iberian Peninsula and Wales eastwards to southwest China. The genus Pamirotetchites Balashova, 1966, from the Pamirs in central Asia is a junior synonym of Nobiliasaphus as Balashova (1976, p. 59) realised, and displays the same distinctive pygidial structure as N. nobilis (Barrande) (see also Opsimasaphus pseudodawanicus Lu, 1975 , in Turvey, 2007). Norasaphus is so far confined to Australia, while Nobiliasaphus does not extend east of the Pamirs. Birmanites is more widespread. Whether this group should be included within a more inclusive clade based on Pseudoasaphus (as claimed by Balashova, 1976) is uncertain. Many genera of Asaphidae are known to be endemic to Baltica, such as those placed in Megistaspidinae by Balashova (1976), and Pseudoasaphus may be no exception. Pseudoasaphus spp. have anterior branches of the facial sutures which cross the fontal area well away from the anterior cephalic margin before meeting in an acute point (e.g. Jaanusson, 1953, pl. 4, fig. 3), and hypostomes attributed to related genera are relatively long and narrow, with more extended ‘forks’ subtending an almost v-shaped outline ( Balashova, 1976, pl. 3).

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.