河北秦皇岛石门寨地区晚石炭世本溪组下部的腕足动物及其古环境

BRACHIOPODS FROM LOWER BENXI FORMATION OF SHIMEN-ZHAI REGION, QINHUANGDAO, HEBEI AND THEIR PALAEOENVIRONMENTS

本文描述了采自石门寨地区本溪组下段地层(含(竹蜓)灰岩之下)中的腕足动物化石并讨论了它的古生态环境。这个腕足动物群具有极大的丰度和很低的分异度,共3属5种,95%以上的个体为Derbyia,只有少数Meekella和Orbiculoidea。这种特殊的腕足动物组合反映当时本区处于非正常浅海环境,使得大多数底栖腕足动物无法生存。Derbyia和Meekella因具长的铰合线,外套膜能充分扩张、能吸收较多氧气,故能够生存、繁衍并形成极大丰度。这一腕足动物群未见于上部层位和其他地区,它的消失是古地理环境激剧变化引起的,是群落演替的结果。

The Benxi Formation of the Shimenzhai Region can be divided into two parts. The upper part consists of fine-grained sandstones, shales and limestone intercalations, with Fusulinella, Choristites, etc. found in the limestones, indicating that it is equivalent to the Benxi Formation of the Benxi Region in Liaoning and corresponding to the Moscovian of USSR. The lower part is mostly composed of terrestrial deposits including sandstones, siltstones and shales with abundant fossil plants at the base, and yellowish green shales and mudstones, with fossil brachiopods at the top; it corresponds to the Tianshifu Formation of the Benxi Region in Liaoning, which is equivalent to the Ba shkirian of USSR. This paper describes the fossils collected from the lower part of the Benxi Formation of the Shimenzhai Region. The fauna with a high abundance and a low diversity includes 3 genera and 5 species, among which over 95% individuals are composed of Derbyia with only a few Meekella and Orbiculoidea. This particular brachiopod assemblage indicates the abnormal palaeoenvironments and the unsuitable habitat for most benthic brachiopods of the region in that period, in which only a few elements of Orbiculoidea and long-hinged orthotetids such as Derbyia, Meekella could exist. The Shimenzhai Region is situated on the southeastern margin of the Shanhaiguan uplift in the Yan-Liao subsiding belt caused by the uplift when the water invaded from northeast into this region. The sea water first reached some subsiding areas, forming quiet water bays with muddy bottoms. In such environments, due to the shortage of oxygen and the desalted sea water, most sessile benthic brachiopods could not exist; only the eurytropic elements such as Orbiculoidea and the long hinged Derbyia and Meekella could survive, because the long hinge made their mantle fully expand to take in more oxygen from the sea water. Such a niche was favourable for Derbyia to flourish in great abundance. This fauna has not been found in the upper part and in other regions. Their disapearance was caused by the rapid change of palaeoenvironment as a result of community succession. Description of new species Derbyia shimenzhaiensis sp. nov. (Pl. Ⅰ, figs. 10, 13—22; Pl. Ⅱ, figs 1—9, 24, 25) Shell medium to large, transversely subcircular in outline, convexo-concave to unequally biconvex, slightly wider than long. Ventral valve concave to slightly convex; interaea high, triangular, apsacline; hinge straight, shorter than the widest part; maximum width at midvalve; cardinal extremities rounded. No sulcus or fold present. Dorsal valve more convex than ventral valve, with greatest convexity at posterior part. Surface ornamented with fine costae, increasing in number by bifurcations and intercalations, usually forming fascicostae; concentric lamellae or wrinkles well-developed. Ventral interior with strong median septum, about 1/3 to 1/2 as long as the shell. Dorsal interior with diverged brachiophore supports and prominent median ridge; muscle scarsubcircular. Comparison The new species is similar to Derbyia regularis Waagen, but differs from the latter in its transversely subcircular outline. Orthotetes regularis (Waagen) described by Sarytcheva et al. (1952) closely resembles the new species, but the Russian specimen is more transverse with a transversely quadrate outline; Orthotetes radiata (Fish.) (Yang, 1964) also closely resembles the new species in outline, but differs in its interior structures. The new species shows great variations in outline and convexity; most individuals are subcircular to subqudarate and slightly biconvex, but some are subtransversely circular in outline and a few are convexo-concave, while some possess nodular interior surfaces. Meekella shilingensis sp. nov. (Pl. Ⅱ, figs. 10—20, 23) Shell medium to large, subquadrate to subcircular in outiline, moderately biconvex. Ventral valve gently convex; apex pointed; interarea triangular, apsacline to orthocline; delthyrium covered with convex pseudodeltidium. Hinge straight, long, slightly shorter than wide; cardinal extremities rounded; maximum width situated imteriorly. Dorsal valve convex posteriorly, gently concave at anterior half. Surface ornamented with fine costae and strong plications originating from middle part; concentric wrinkles well-developed. Ventral interior with parallel dental plates, about 1/4 to 1/2 as long as the shell; no median septum or ridge. Dorsal interior with diverged brachiophore supports and low median ridge. Comparison The new species closely resembles Meekella venusta (Trs.) and M. eximia (Eichw.). but differs from M. venusta in its longer hinge and prominent concentric lamellae, and from M. eximia in its low, regular interarea, well-developed concentric lamellae and plications developed at anterior. The species is close to Meekella striatocosta (Cox), but-diffeds in its subquadrate outline and longer hinge.