Lonchodomas shanensis

Lonchodomas shanensis (Reed, 1915)

Fig. 10.3, 6

1915 Ampyx rostratus var. shanensis; Reed 1915, p. 24 –25, pl. 5, figs 2,3.

Material. Lectotype (selected herein): disarticulated, incomplete dorsal exoskeleton from Mong Ha, Fig. 10.6 (Reed, 1915, pl. 5, fig. 2), GSI 11504. Other material: cranidium from Hwe-hok, Fig. 10.3 (Reed, 1915, pl. 5, fig. 2), GSI 11505. Both specimens from the Hwe Mawng Beds (uppermost Katian).

Description. The disassociated exoskeleton indicates that the sag. length of the cephalon (to the base of the anterior spine) is similar in length to the rest of the thorax + pygidium. Glabella with the shape of a narrow and elongate rhomb. There is no indication of lateral glabellar muscle impressions, nor of the lateral lobes developed by some species attributed to Lonchodomas. The maximum glabellar width is probably slightly less than half its length. The transverse convexity of the glabella is very low posteriorly, hardly elevated above the cheeks, and the occipital ring is hardly defined. An indistinct median crest runs along the anterior half of the glabella and is extended into a stout frontal spine of unknown length, which from its base probably had a prismatic cross section. Posterior border furrow shallows towards glabella; so far as it can be observed posterior border arches slightly forwards. Free cheeks not observed. Four thoracic segments clearly shown, and traces of the fifth, posterior segment to the left of the pygidium; thorax is probably subparallel sided, or with a gentle posterior taper. Transverse width of pleurae similar to that of axis, which is weakly convex; long (sag.) articulating half rings present. Pleural tips truncate; weak pleural furrows just posterior to median line and gently concave. Pygidium distinctive, just over twice as wide as long, with a moderately well-defined axis initially just over one-third anterior pygidial width, and this similar to its length, making a neat isosceles triangle as it tapers to border, axial furrows enclosing an angle of 50 degrees. Apart from half-ring, ring furrows not expressed. Distinct anterior pleural furrow elegantly concave laterally, and behind it one shallow but straight pleural furrow making a near right angle to the axial furrow and extending to border. Border itself is steeply downturned and of similar height along its length.

Discussion. The most distinctive specific characters of this raphiophorid are on the pygidium, with its axis making an almost equilateral triangle, and only two well marked pleural furrows, the second quite different from the first. It differs from L. rostratus (Sars, 1835) (e.g. Whittington, 1959) in these characters, and in having a narrower and less carinate glabella. Hence Reed’s “var.” is employed as a specific name. Although it cannot be proved that there was no sixth thoracic segment, the evidence we have supports the presence of five segments, typical of Lonchodomas. Reed (1915, p. 25) noted only four segments, presumably not recognising the fragmentary fifth segment on the left-hand side. A number of Lonchodomas species have been described from China, but none has the peculiar pygidial structure of L. shanensis. Curiously, a similar pygidium is present on Maiopopsis whittardi (Yi, 1957) (e.g. Lu, 1975, pl. 42, fig. 4), but Maiopopsis has a completely different cephalic structure to that of Lonchodomas, and the pygidial similarities are surely a matter of convergence. Cephalic features of Lonchodomas remain relatively conservative from early in the history of the genus (Nielsen, 1995).

Ampyx aff. macullumi from the Upper Naungkangyi Beds figured by Reed (1915, pl. 5, figs 4-6) and refigured here (Figs 10.1,2) shows a carinate glabella and apparently prismatic frontal spine, and may be referable to Lonchodomas. However, the pygidum referred by Reed to this species is unlike that of L. shanenis and probably does not belong with the cranidium. It is here retained under open nomenclature as Lonchodomas ? sp.