EOPHILOBRYOIDELLA SINOANISICA NEW GENUS AND SPECIES, AN EARLY PHILOBRYID BIVALVE FROM THE UPPER ANISIAN (MIDDLE TRIASSIC) OF QINGYAN, SOUTHWESTERN CHINA

Journal of Paleontology, Mar 2004 by Stiller, Frank, Jinhua, Chen

INTRODUCTION

THE BIVALVES of early late Anisian (Middle Triassic) age from Leidapo (also Bangtoupo) in the northeastern vicinity of Qingyan in the south-central part of Guizhou Province, southwestern China (Fig. 1), are highly diverse and fairly well studied (e.g., Chen et al., 1974; Yin, 1974; Nanjing Institute of Geology and Palaeontology, 1976; Gan and Yin, 1978; also Stiller, 2001). The study of these Anisian fossil assemblages began with Koken (1900) and Hsu and Chen ( 1943). The renewed study of this site and its fossil assemblages by Stiller (1997, 2001) and Chen et al. (2001) yielded a bivalve taxon with unusual and, for an early Mesozoic species, unexpected hinge features. The new taxon is fairly common at Leidapo/Bangtoupo; nevertheless it was only recently recognized (Stiller, 2001) and has not heretofore been described. The only bivalve group having comparable hinge features is the Philobryidae (Arcoida, Limopsoidea). This small bivalve family has only been described from Eocene and more recent rocks (Keen, 1969; Tevesz, 1977), but Skelton and Benton (1993, p. 246) mentioned a yet undescribed philobryid from the Kimmeridgian (Upper jurassic) without giving any further information. The features of the new taxon from southwestern China, particularly those of the hinge region, strongly suggest an assignment to the Philobryidae. The finding of the new taxon described herein thus significantly extends the stratigraphie range of this family to the lower upper Anisian (lower Middle Triassic).

The specimens are from the highly fossiliferous section at the hillock called Leidapo (or Bangtoupo) (Fig. 1), which comprises a succession of generally thinly bedded, more or less marly and mostly slightly silty mudstones with thin, more calcareous intercalations of marls to marly limestones. This section is situated stratigraphically in the lower middle part of the Leidapo Member of the Qingyan Formation. The Leidapo Member is equivalent to the lower upper Anisian, and the Leidapo/Bangtoupo section is situated approximately at the transition from the Schreyerites binodoxus (Hauer, 1851) to the Paraceratites trinodosus (Mojsisovics, 1882) Zone, i.e., at the transition from the Pelsonian to the Illyrian. This designation is mainly based on the occurrence of Schreyerites binodosus, Paraceratites brembanus (Mojsisovics, 1882), and some other cephalopod taxa typical for the higher Anisian at Leidapo/Bangtoupo. Paraceratites trinodosus occurs in the strata overlying this section. For more detailed information on the locality, its stratigraphy, and its fossil assemblages see Stiller (2001).

There is some confusion about the correct name of this site (compare Stiller, 2001, p. 14). Leidapo has been used in the geological literature for some time, but the local people in 1994 and the official Chinese topographical map of the region refer to it as Bangtoupo (Stiller, 2001). However, according to renewed, recent investigations among local people by the second author, Leidapo seems to be correct after all. In order to avoid misunderstandings, Leidapo/Bangtoupo is used herein.

SYSTEMATIC PALEONTOLOGY

The Philobryidae are placed in the Superfamily Limopsoidea of the Order Arcoida. Nicol (1966) placed the family in the Mytiloidea, but Keen (1969) tentatively included them in the Limopsoidea. Tevesz (1977) indicated that the Philobryidae certainly are Arcoida and that they show strong affinities with the Limopsidae. Carter (1990) placed the Philobryidae in the Limopsoidea and Skelton and Benton (1993) also accepted an assignment to the Arcoida.

The type and additional specimens figured herein are deposited in the collections of the Nanjing Institute of Geology and Palaeontology, Academia Sinica, in Nanjing, P. R. China (NIGP); other specimens that were examined are housed in the collections of the Geologisch-Palaontologisches Institut und Museum, Universitat Munster, Germany (GPIMMS).

Class BIVALVIA Linne, 1758

Order ARCOIDA Stoliczka, 1871

Superfamily LIMOPSOIDEA Dall, 1895

Family PHILOBRYIDAE Bernard, 1897

Genus EOPHILOBRYOIDELLA new genus

Type species.-Eophilobryoidella sinoanisica n. gen. and sp., by monotypy.

Diagnosis.-Shell small, equivalved, inequilateral; outline obliquely oval; umbones prosogyrate, prominent, beaks situated anterior but not terminal; outer surface smooth except for fine growth lines. Hinge margin edentulous; hinge plate long, rather broad; ligament internal, located in a narrow, long groove running from below the beak obliquely backwards and curving across the hinge plate, thereby dividing the hinge plate into two portions; anterior portion larger than posterior portion; both portions with numerous crenulations directed about perpendicular to the dorsal shell margin.

Etymology.-From the ancient Greek eox (dawn), referring to the new genus's early position in the evolution of philobryids.

Occurrence.-Middle Triassic, lower upper Anisian (Leidapo Member); Qingyan, Guizhou Province, southwestern China.

Discussion.- see discussion of the type species.