Desquamatia cf. kimberleyensis ( Coleman, 1951 )

Ma, Xue-Ping, Becker, Ralph Thomas, Li, Hua & Sun, Yuan-Yuan, 2006, Early and Middle Frasnian brachiopod faunas and turnover on the South China shelf, Acta Palaeontologica Polonica 51 (4), pp. 789-812 : 796-799

Desquamatia cf. kimberleyensis ( Coleman, 1951)

Figs. 5–7.

1951 cf. Atrypa desquamata kimberleyensis Coleman , n. subsp.; Coleman 1951: 683–684, pl. 101: 7–19.

1971 cf. Desquamatia (Synatrypa) kimberleyensis (Coleman) ; Roberts 1971: 168–171, text−figs. 40, 41, pl. 38: 1–22.

1978 cf. Desquamatia (Synatrypa) kimberleyensis ( Coleman, 1951) ; Grey 1978: 20–33, text−figs. 10–16, pl. 2: 1 (specimens of figs. 2, 3 need restudy); in addition, Veevers (1959a: 121) and Grey (1978: 20) also regarded Atrypa multimoda Coleman as a synonym of D. kimberleyensis .

Material.— 28 specimens from eastern Yunnan (most are well preserved) and northern Guangxi (poorly preserved). Samples : PY4 (10 specimens), PY5 (8 specimens) ; 10 specimens from the Caiziyan section of Guilin .

Description.—Shell medium−sized for genus, generally 25– 30 mm wide in adults; transversely to slightly elongatedly oval; strongly to medium dorsi−biconvex in lateral profile. Surface covered with fine costae, about 7–8 costae per 5 mm from mid−length anteriorly of adult shells; costa number increases by bifurcation chiefly on ventral valve and intercalation on dorsal valve; micro−growth lines interrupted by variously spaced growth lamellae, about 9 growth lamellae per 10 mm in the middle part of shell, becoming denser to very dense in the anterior part. Ventral beak normally slightly incurved to adpressed in adult shells, with a minute area and small deltidial plates below the pedicle opening.

Internally secondary shell thickening prominent ( Fig. 6 ). Ventral valve with deeply incised muscle field; with dental nuclei; tooth of normal size, with well−developed accessory lobe. Dorsally, muscle field slightly incised, with a low, centrally grooved median ridge; crural base subrounded ( Fig. 7C ), crurelatively large; disjunct jugal process tipped by small jugal plate; spiralia with about 14–15 whorls for sectioned specimen (about 26 mm wide and long), directed dorso−medianly.

Discussion.—When Coleman (1951) described the Atrypa fauna from the Devonian of western Australia, he did not designate a holotype for Atrypa desquamata kimberleyensis . Grey (1978) subsequently designated specimen UWA 26247a as the holotype for Desquamatia (Synatrypa) kimberleyensis and UWA 26273c as the holotype for Atrypa reticularis teicherti Coleman, 1951 (elevated to species level and assigned to Kyrtatrypa by Grey 1978). Unfortunately both holotypes are not well preserved in terms of micro−ornament. Both holotype and paratype specimens of the two species have about the same number of costae: 6–7 costae per 5 mm in the anterior part of shell, which is confirmed by our own counting on their figures ( Coleman 1951: pl. 100: 1–10; pl. 101: 7–11, 18, 19). Kyrtatrypa teicherti has been based on only five specimens and there were no young forms in the collection ( Coleman 1951: 681) at that time. According to Coleman (1951) and Grey (1978), D. kimberleyensis is distinct from Kyrtatrypa teicherti chiefly by the presence of an area and deltidial plates. However, it is not easy to differentiate bigger specimens of D. kimberleyensis from Kyrtatrypa teicherti as they are similar in the nature of the area. The two forms are also similar in internal structures of adult specimens except that the latter has a prominent robust dorsal median septum as demonstrated by Veevers (1959a) and Grey (1978); the number of spires is also similar (6–9 whorls; see of Veevers 1959a: text−fig. 70 and Roberts 1971: fig. 41).

The studied Yunnan specimens are similar to but larger than both Australian K. teicherti and D. kimberleyensis , and seemingly have more numerous and finer costae. Internally they are different from K. teicherti in their less prominent dorsal median septum and less deeply impressed dorsal muscle field; they differ from D. kimberleyensis type specimens by usually having a slightly incurved to adpressed beak, but this difference could well be a result of older shells. The most significant difference from the Australian material lies in the number of spiral whorls (about 14–15 whorls revealed by the near 26 mm wide specimen of the Chinese form versus about 8–9 whorls in 21–22 mm wide specimens of the two Australian forms). Further study is required to demonstrate whether there is a difference in the number of spires for similar sized specimens from South China and western Australia.

The Chinese specimens are also similar to Desquamatia (Seratrypa) typica ( Chen, 1983) , for example, in rib number, nature of ventral beak, etc. They can be distinguished by their smaller size, well developed concentric lamellae (versus absence of concentric ornament in D. typica according to its original description and illustrations), the presence of dental nuclei (versus none in D. typica ) and fewer spiralia whorls (14–15 versus about 20 in D. typica ). D. cf. kimberleyensis differs from varieties and mutations of “ Atrypa desquamata ” described by Grabau (1931) in having finer and denser costae at various degrees (e.g., mutation alpha, mutation beta, var. magna , mutation hunanensis, and mutation kansuensis). The Chinese specimens also differ from Desquamatia richthofeni ( Kayser, 1883) in shell shape (dorsibiconvex and with a uniplicate anterior commissure); the ventral valve of the latter species is convex only in the umbonal region and concave in the remaining part of the shell and has no sinus or fold. Our material is also distinct from Desquamatia peshiensis ( Grabau, 1931) in having a small incurved ventral beak, dorsibiconvex lateral profile, and shell outline of about equal width and length (versus highly protruding ventral beak, about equibiconvex lateral profile, and elongated shell outline in the latter species). D. peshiensis also has a robust dorsal median septum and prominent dental cavities ( Chen 1983: 277, text−fig. 12). Of the Frasnian atrypids described by Copper (1967) from the Bergisches Land, Germany, only the finely ribbed variety of Desquamatia (Seratrypa) pectinata (his pl. 25: 1–3) and Desquamatia schroeteri have a similar number of ribs near the anterior margin, comparable to that of our studied species. It differs from the two German species in having much denser growth lamellae and from the former by having a much deeper ventral muscle field and more whorls in the spiralia (14–15 versus 8–10 for similar sized specimens); the internal structure of D. schroeteri is not known.

Specimens from the Early Frasnian of the Caiziyan section are similar, but have slightly coarser ribs (about 5–6 costae per 5 mm at anterior margins). As they are generally not as well preserved and no internal structures have been revealed, they are provisionally assigned to Desquamatia cf. kimberleyensis (e.g., Fig. 5F).

Occurrence.—The studied specimens were collected in the Panxi section of eastern Yunnan and the Caiziyan section of northeastern Guangxi, approximately the Frasnian Palmatolepis transitans – Pa. punctata zones.

Figures

Frasnian, lateral (B1), ventral (B2), anterior (B3), posterior (B4), and dorsal (B5) views of a rounded exfoliated shell, 27.7 mm wide, 25.6 mm long, and about 14.5 mm thick. C. PUM05008, sample PY4, Panxi section, probably Middle Frasnian, lateral (C1), dorsal (C2), and ventral (C3) views of the sectioned specimen (Fig. 6). D. PUM05009, sample PY5, Panxi section, probably Middle Frasnian, ventral beak broken, showing small conjunct deltidial plates (note that true foramen (approximately dashed line) takes up only a small part at the bottom of the seen later enlarged hole). E. PUM05010, sample PY5, Panxi section, probably Middle Frasnian, posterior (E1), lateral (E2), anterior (E3), ventral (E4), and dorsal (E5) views, 26.7 mm wide, 27.7 mm long, 18.5 mm thick, adpressed ventral beak. F. PUM05011, sample GC22, Caiziyan section, Early Frasnian, dorsal view.

Fig. 7. Photographs of acetate peels of Desquamatia cf. kimberleyensis (Coleman, 1951) from the Middle Frasnian of the Panxi section, PUM05008, sample PY4 (same specimen as Fig. 5C). A. Left side at 0.8 mm. B. Left inner socket ridge at 2.2 mm showing combed knots (traces of muscle attachment?). C. Left tooth and socket at 3.2 mm.

Fig. 6. Transverse serial sections of Desquamatia cf. kimberleyensis (Coleman, 1951), PUM05008, sample PY4, Panxi section, probably Middle Frasnian, specimen figured in Fig. 5C. Acetate peel number: 05A. Numbers refer to distance in mm from the ventral apex.