Emanuella aff. takwanensis ( Kayser, 1883 )

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 : 807-809

Emanuella aff. takwanensis ( Kayser, 1883)

Figs. 15, 16 .

1883 aff. Nucleospira takwanensis n. sp.; Kayser 1883: 84–85, pl. 10: 2, 2a–h.

1959 aff. Emanuella takwanensis Kayser 1883 ; Veevers 1959b:903–906, text−figs. 3, 4A, 5.

1970 aff. Emanuella takwanensis Kayser 1883 ; Dürkoop 1970, text−fig. 56−2, pl. 17: 1a–e (photographs of lectotype).

Material.—Sample PY5, Panxi section, probably Middle Frasnian (65 specimens, most articulated, silicified) and a few slabs with abundant silicified specimens about 50 m upwards; sample DS59, Dushan section, Early Frasnian (10 specimens).

Description.—Shell medium to small sized (around 10 mm in width, generally not greater than 12 mm for adults); suboval in outline, ventribiconvex in lateral profile, anterior commissure rectimarginate to uniplicate; hinge line straight but shorter than greatest width; cardinal extremities rounded. Ventral area apsacline, slightly curved dorsally, delthyrium generally open (with traces of thin pseudodeltidial plates seen from both sides in rare specimens). Dorsal interarea low, anacline, notothyrium open. Surface smooth; concentric “growth lines” and radial “costellae” present on exfoliated shell surface. Micro−ornament of short discontinuous radial spines (or spine bases) arranged roughly in concentric bands and lying on shell surface within the primary layer on part of a small poorly preserved ventral valve (sample DS59), with a density about 10 spines per 1 mm within the same band.

Internally, ventral valve without dental plates (occasionally a dental plate may be seen, see Fig. 16D ), instead with strong dental ridges (terms used as in Veevers 1959b), teeth relatively strong; thickened infilling present in posterior part of ventral valve, becoming thinner anteriorly to form a structure similar to the delthyrial plate ( Figs. 15B, 16A ) (= pedicle collar of Veevers 1959b; the function of this structure is not certain at present as the position of the foramen can not be determined so that its relation with the pedicle is not known, therefore the name of the structure is left open for further study).

Dorsally, cardinal process low, with some vertical lamellae; sockets relatively deep; inner socket ridges high and strong, outer socket ridges fused to shell wall; socket plates (= outer hinge plates) fused to shell wall posteriorly, but becoming free and oblique anteriorly; crural plates (= inner hinge plates) extending ventro−laterally from shell wall, very slender; crural bases small and low, giving rise to relatively strong crura anteriorly. Spiralia with 7–8 whorls, directed laterally.

Discussion.—Externally shells are similar to species of Ladjia Veevers, 1959a in the presence of radial “costellae”. However, Givetian Emanuella from the Panxi section also possess radial “costellae”, and are still assigned in the genus Emanuella , which seems to disappear mostly in the late Middle Devonian, e.g., in the Guangxi and Hunan provinces ( Hou 2000). Late Devonian Emanuella have been reported from South China, but without illustrations and solid biostratigraphic data, in the Panxi section of eastern Yunnan. Chen (1984) studied similar brachiopods from the Tuqiaozi Formation (Early Frasnian) of the Longmenshan section and redescribed Zhonghuacoelia , which is characterized by the presence of a pair of dental plates and small size. Veevers (1959a: 124) stated that the development of dental plates is transitional as he found that three of 100 dorsal and ventral valves from the same locality possessed variously developed dental plates. The presence of this structure in one of the Yunnan specimens ( Fig. 16D ) confirms that point of view.

The shell is similar to Emanuella takwanensis from the Middle Devonian (Givetian) of eastern Yunnan, so that in the description of the Panxi section (Yunnan compiling group 1978: 50), it is listed as E. takwanensis . It differs in having a smaller shell size, strong dental ridges, and outer socket ridges completely fused to the shell wall versus a free socket raised above shell wall by socket plate in Givetian E. takwanensis (compare Fig. 16C with Veevers 1959b: text−fig. 4 A and Dürkoop 1970: text−fig. 56−1). This was supported by Xu et al. (1978: 359, text−fig. 46), and our own unpublished data. The nature of the socket relative to the shell wall in the studied species is identical with that of Emanuella torrida Veevers, 1959a (i.e., sessile sockets of Veevers) and E. laskarewi Kelus, 1939 , as figured by Dürkoop (1970: text−fig. 56−2), but the studied species differs from E. torrida in its spiralia with more whorls (7–8 in the studied species versus 6 at most according to Veevers’ description). Spines or spine bases (micro−ornament) of the present species as seen from only one specimen are longer and arranged roughly in concentric bands. In addition, concentric “growth lines” and radial “costellae” can be seen on the exfoliated shell surface, not mentioned by Veevers (1959a) for his specimens. It is distinct form Ladjia saltica Veevers, 1959a in having a shorter hinge, and a smaller, narrower delthyrium.

Occurrence.—The species occurs in the Early and Middle Frasnian of South China (approximately the Frasnian part of Early Mesotaxis falsiovalis Zone through to the lower part of the Palmatolepis hassi Zone ).

Figures

Fig. 16. Transverse sections of Emanuella aff. takwanensis (Kayser, 1883); all specimens are from sample PY5, Panxi section, probably Middle Frasnian. A–D. Sections at various distances from the ventral apex of specimens PUM05042~PUM05045; acetate peel numbers: 05J, 05G, 05I, and 05H respectively. E. Transverse serial sections of specimen PUM05046, acetate peel number: 05F; numbers refer to approximate distance in mm from the ventral apex.