SINOLEPERDITIINI (OSTRACODA) FROM THE LOWER EMSIAN SHANGLUN FORMATION AT THE SHANGLUN VILLAGE, CENTRAL GUANGXI, CHINA

Journal of Paleontology, Mar 2004 by Shang-Qi, Wang, Lundin, Robert F

ABSTRACT-The Shanglun Formation at the Shanglun locality contains a sinoleperditiine fauna of five species of which two are new [Sinoleperditia (Sinoleperditia) shanglunensis and Paramoelleritia (Paraleperditia) wangchengyuani] and one is left in open nomenclature because of inadequate materials. In this fauna, the ratio of the vertical height of the trailing chevron muscle scar to that of the adductor muscle scar (th:ah ratio) ranges from 0.85 to 0.95 with a mean value of 0.90. On the basis of the sinoleperditiines and a beyrichiid ostracode, the upper part of the Shanglun Formation is correlated with the middle-to-late early Emsian Moding Formation. The sinoleperditiines and microfacies analysis indicates that the sediments bearing the fauna were deposited in a low energy restricted carbonate platform environment.

INTRODUCTION

TUB RAPID evolutionary development of the trailing chevron muscle scars (Wang and McKenzie, 2000) in Devonian sinoleperditiines, a group of leperditicope arthropods (see Vannier, Wang, and Coen, 2001), of China provides a potentially useful biostratigraphic sequence for establishing the age and correlation of rocks that, in some cases, contain few other fossil groups. The height of the trailing chevron muscle scar (th) increases relative to the height of the adductor muscle scar (ah) through the Devonian. The work of Wang and McKenzie (2000) has demonstrated most of this Devonian biostratigraphic sequence, but virtually no information on early Emsian sinoleperditiines has been published in the primary studies of this group (Wang and Liu, 1994; Wang, 1994, 1995, 1996, 1997; Wang and McKenzie, 2000). The purpose of this paper is to describe the sinoleperditiine fauna from the early Emsian Shanglun Formation and, particularly, to report on the th:ah ratios of these sinoleperditiines. This is significant because the new data provided herein fills an early Emsian biostratigrapical gap in the th:ah values, and supports the evolutionary trend demonstrated by Wang and McKenzie (2000, text-fig. 5). Sinoleperditiines are presently known only from China, but this study and that of Wang and McKenzie (2000) point out the potential value of investigating the relative sizes of adductor muscle scars and ordinary chevron muscle scars, from which trailing chevron muscle scars evolved (Wang and McKenzie, 2000, p. 594; Vannier, Wang, and Cocn, 2001, p. 80), of leperditicopes from North America, Siberia, Europe and other parts of the world.

THE SHANGLUN IORMATlON

Introduction.- -The Shanglun Formation is distributed mainly in Xiangzhou and Wuxuan counties in an outcrop area about 50 km long from south to north. It is about 103 m thick in an exposure on a hillside at Shanglun village, about 3 km northeast of the town of Miaohuang, Xiangzhou, Guangxi (Fig. 1). At this section the formation is composed mainly of a set of grey and dark grey dolomites (Fig. 2) that contain very few megafossils. Some brachiopods, the tabulate Favosites sp., and the rugosan Heterophrentis sp. (Yu and Kuang, 1980) are the only megafossil species recorded to date. Except for the sinoleperditiines and a beyrichiid ostracode species reported herein, microfossils have not been found in the Shanglun Formation.

As shown in Figure 2, the Shanglun Formation conformably overlies the Tonggeng Formation and underlies the Ertang Formation (Zhong et al, 1992) at the Shanglun section. Liu (1978) first named the unit as the Shanglun Member of the Yukiang Formation. Yu and Yin (1978) named it the Shanglun Dolomite and referred it to the Ertang Formation. Zhong and others (1992) separated the unit from the Erlang Formation and referred to il as the Shanglun Formation. This usage is followed herein.

Depositional environment.-Zhong et al. (1992, (ig. 2.43) studied the sedimentary environment of the Shanglun Formation and noted that it was characterized by a predominance of line-grained dolomites and that it may represent a carbonate platform environment with restricted circulation of seawater. For interpreting the sedimentary environment of the sinoleperditiine-bearing rocks, some lithological thin sections, including samples Sh-I-I, Sh-I-2, Sh-2 and Sh-3 (Fig. 2), were made and preliminarily described by Prof. Zhang jun-ming as follows:

Sh-I-I: Dark-grey micropelletal limestone with bioclasts, mainly yielding sinolepcrditiines and calcispheres. Bioclasts are dominantly leperditiid fragments but the section also contains a few possible foraminifer fragments. Micropellets, 0.04-0.55 mm in diameter, are composed of carbonate muds. Sinoleperditia (Sinoleperdltia) miaohuangensis (Sun, 1978), S. (S.) sp., S. (S.) shanglunensis new species, Paramoelleritia (Paraleperdilia) wangchengyuani new species, and Eocarinabeyrichia carincita Wang, 1983 have been found in this sample.

Sh-1-2: Dark-grey pelletai micrite with bioclasts, mainly yielding sinoleperditiines, some beyrichiid specimens and possible calcispheres. Leperditiid shells are the dominant bioclasts and trilobite fragments are rare. Carbonate muds show pelletai texture. S. (S.) miaohuangensis and E. carincita are known from this sample.