摘要
一、前言特列奇阶作为一个阶,国际地层委员会业已批准下列界定:以turriculatus带之底作下界,以crenulata之顶作上界(Holland,1985)。在以笔石占绝对优势的斜坡相区,如陕南紫阳地区,界定特列奇阶并无困难(傅力浦、宋礼生,1986)。而在以介壳相为主的陆棚相区,尤其在内陆棚相区,如何界定特列奇阶确非易事。林宝玉等(1982)所定义的石牛栏阶即是一例。石牛栏阶的命名地点和层型剖面在四川綦江观音桥。按照他们的意见,石牛栏阶的下界“大致相当于笔石相的Spirograptus turriculatus带”(林宝玉等,1982,145页)。由于并无可靠的、足以确证属于Spirograptus turriculatus带的笔石作为依据,这一结论似乎虚悬无着。
The Leijiatun section is located at Leijiatun, a small village about 6 km north of Shiqian County town, N. Guizhou (Text-fig. 1), with rocks from Late Ashgillian to Llandoverian in age exposed along both sides of the road from Leijiatun to Raochaguan. Here the studied part of this section contains three entirely conformable formations in ascending order, namely, Leijiatun Formation, Majiaochong Formation and Rongxi Formation. The Leijiatun Formation belongs to the shelly facies, yielding brachiopods, corals, trilobites, echinoderms, nautiloids, and occasionally graptolites, with chitinozoans (Nos. TT 800—804) found from the yellowish-green shale in the basal part, including Conochitina leijiatunensis sp. nov., C. cf. iklaensis, and Ancyrochitina shiqianensis sp. nov.. The Majiaochong Formation is composed of yellowish-green to greyish-green shale, yielding only a few brachiopods, with chitinozoans (No. TT 783) obtained from the basal part, which are identified as Conochitina sp. in addition to the three species shared with the Leijiatun Formation. The Rongxi Formation consists mainly of purplish red, yellowish green to greyish green silty mudstone and sandy shale intercalated with siltstone, yielding rather scarce fossils, with the chitinizian Conochitina daozhenesis collected from the lower part, but very few in number (Text-fig. 2).
It is noteworthy that Ancyrochitina cf. sp. A reported by Nestor in 1984 has been transferred to A. shiqianensis sp. nov. in the present paper. A. cf. sp. A is restricted to the Rumba Formation of Estonia, which is mainly correlated with the sedguickii Biozone according to Nestor (1984). It seems to the author that the presence of Conochitina cf. iklaensis along with Ancyrochitina shiqianensis provides a cogent evidence for the C. iklaensis-C. emmastensis Biozone. As previously mentioned, this biozone is assuredly correlated with the sedgwickii Biozone (Geng and Cai, 1988). This is also borne out by the fact that the Xiangshuyuan Formation, a unit lying immediately beneath the Leijiatun Formation, at Tudiao of Yanhe, N. Guizhou yields graptolites (Pristiograptus gregarius, Pseudoclimacograptus hubeiensis, Petalolitus sp., and Monograptus aduncus) which are said to span the triangulatus to convolutus Biozones (Zhou et al., 1985, p. 38).
Conochitina daozhenensis was first described by Geng (1986) from the Hanchiatien Formation at Bayu of Daozhen, N. Guizhou together with Ancyrochitina brevicollis and the graptolite Streptograptus plumosus. Recently, the author discovers that in the Nanjiang Formation at Qiaoting of Nanjiang, N. Sichuan C. daozhenensis makes its first appearance in the turriculatus Biozone yielding the graptolites Spirograptus turriculatus, Streptograptus runcinatus and others, and extends to the drepanoformis Biozone (equivalent to the crispus Biozone) which contains the graptolites Monograptus drepanoformis, Oktavites cf. intermedius and others. So far as the author knows, its upper range is within the spiralis-grandis Biozone. This implies a Telychian age of the species.
The chitinozoan-based Aeronian-Telychian boundary at the Leijiatun section is arbitrarily placed at the last occurrences of C. cf. iklaensis and A. shiqianensis, because the samples from an interval between the beds bearing C. daozhenensis (No. TT 762) and C. cf. iklaensis and A. shiqianensis (No. TT 783) are unfortunately barren of chitinozoans. The newly-defined top of Aeronian is traditionally regarded as wholly Telychian.
Both the lower and uppermost parts of the Rongxi Formation are characterized by red beds which are customarily called the Lower Red beds among the Chinese geologists and palaeontologists. In the publicated literature and stratigraphic correlation charts in China, the Lower Red beds have widely been believed to be synchronous throughout the Yangzi Region for years. In this paper the author demonstrates for the first time that the Lower Red beds could not be regarded as wholly Telychian in age in terms of chitinozoan Biozone-based age determination. For example, at the Dazhongba section of Yichang, W. Hubei, the Lower Red beds are lying within the C. iklaensis-C, emmastensis and A. brevicollis Biozones, which are partly of the Late Aeronian age, while at the Leijiatun and Hanjiadian sections the Lower R d beds are lying only within the Ancyrochitina brevicollis Biozone, indicating a Telychian age (Text-fig. 3).
Systematic Description Genus Ancyrochitina Eisenack, 1955 Ancyrochitina shiqianensis sp. nov. (P1. Ⅰ, figs. 5—7, 9)
1984 Ancyrochitina cf. sp. A, Nestor, pl. Ⅰ, fig. 5.
Description Test cylindroconical. Chamber occupying half or more than half of total length. Flexure gentle. Shoulder less pronounced. Neck cylindrical. Collarette more or less flaring, but never tapering with fringe of fine terminal hairs. Neck occupying half or less than half of total length. Base convex. Basal margin distinct. Six or more stout appendices branching in three to four orders: first-order branching up to 29μm in length and 9μm in width; second-order branching up to 17μm in length and 5μm in width. Test surface smooth.
Dimensions Total length 158—181μm; length of neck 58—72μm. Max. width 115—120μm, cor. width 81—84μm (cor. coef. 0.8). Width of aperture 56—60μm, cor. width 39—42μm (cor. coef. 0.8). Remarks The present new species is allied with Ancyrochitina cf. sp. 1 figured by Achab (1981) from the Gun River Formation of Anticosti Island, Canada in shape and dimensions, but differs from the latter in the absence of fine spines on the neck. It is also superficially similar to A. laevaensis Nestor, 1980, but differ from the latter in possessing a smooth rather than a fine granulate test surface. It is obviously close to A. cf. sp. A figured by Nestor (1980) from the Rumba Formation of Estonia and in all probability the two species are the same, although the latter shows some vague character in the details.
Occurrence Leijiatun and Majiaochong Formations.
Genus Conochitina Eisenack, 1931, restrict Paris, 1981.. Conochitina daozhenensis (Geng, 1968) (P1. Ⅰ, fig, 1)
1986 Eisenackitina daozhenensis, Geng, pp. 121—122, pl. Ⅰ,figs. 1—2; pl. Ⅱ, figs. 1, 2, 4; pl. Ⅲ, figs. 2—6.
1986 Eisenackitina bayuensis, Geng, p. 122, pl. Ⅰ, fig. 8; pl. Ⅱ, fig. 3.
Dimensions Total length 162μm. Max. width 134μm, cor. width 94μm (cor. coef. 0.7). Width of aperture 88μm, cor. width 62μm.
Remarks According to Paris' (1981, p. 155) emendation of Eisenackina, the essentially important diagnostic features of this genus lie in the possession of a constantly reduced collarette and the surface ornamentation. The author agrees to the emended diagnosis for Eisenackitina, with the transfer of E. daozhenensis and E. bayuensis to Conochitina. C. daozhenensis and C. bayuensis established by the author (1986) virtually have the same charateristics and the same stratigraphic range; the only distinction lies in their sizes, which are 133—158μm and 100—115μm in length respectively, However, an intermediate form between both in size range occurs also in quantities, making species discrimination more difficult. The author now considers this gradation in size as the normal variation of a natural species population, and therefore these two species are merged into a single species bearing the name of Conochitina daozhenensis.
Occurrence Rongxi Formation.
Conochitina cf. iklaensis Nestor, 1980 (Pl. Ⅰ, figs. 2, 3)
Description Test slender, cylindrical. Flanks parallel or subparallel. Basal margin sharply or broadly rounded. Base flat or convex. Test surface smooth.
Dimensions Total length more than 172μm. Max. width 57—59μm, cor. width 40—41μm (cor. coef. 0.7).
Remarks Since the specimens are mechanically distorted, the occurrence of a sharply rounded basal margin seen on figure 3 might not be inherent. The poor preservation of our specimens precludes a firm determination to this species.
Occurrence Leijiatun and Majiaochong Formations.
Conochitina leijiatunensis sp. nov. (P1. Ⅰ, fig. 8)
Description Test small, subquadrangular, somewhat longer than wide or approximately as long as wide. Basal margin broadly rounded. Base flat or slightly convex. Shoulder and flexure absent. Aperture straight. Test surface smooth. Dimensions Total length 70—80μm. Max. width 70—75μm, cor. width 56—60μm (cor. coef. 9.8). Width of aperture 55—60μm, cor. width 44—52μm (cor. coef. 0.8).
Remarks In shape and size the new species is distinct from any previously established species of this genus.
Occurrence Leijiatun and Majiaochong Formations.
Conochitina sp. (P1. Ⅰ, fig. 4)
Description Test conical. Flanks somewhat convex. Flexure present, but less conspicuous. Neck cylindrical, occupying one-fourth or more of total length. Basal margin broadly rounded. Base flat.
Dimensions Total length 188—210μm. Max. width 103—117μm, cor. width 82—94μm (cor. coef. 0.8). Width of aperture 84—90μm, cor. width 67—72μm (cor. coef. 0.8).
Occurrence Majiaochong Formation.
出处
《古生物学报》
CAS
CSCD
北大核心
1990年第5期623-636,667,共14页
Acta Palaeontologica Sinica
基金
国家自然科学基金4870090项目资助
为中英志留系研究项目的阶段成果之九
