摘要
前言澄江海绵化石群的系统分类学、生态学、埋藏学和演化生物学等问题将分为不同专题进行研究。本文是关于澄江海绵化石的第一篇报道,研究内容侧重于细丝海绵科系统分类学。细丝海绵科属于普通海绵类,为澄江海绵群体中的优势类型,也是最为原始的普通海绵类之一。它们的骨骼甚薄,主要由席状外骨层所组成。外骨层由大小不相同的两套单轴骨针纵向排列而成;大骨针互相叠接成骨棒状或互相交错排列成网状,以便起着主要的支撑作用。
The sponges of the Lower Cambrian Chiungchussu Formation in Chengjiang, Yunnan are of unique importance in early history of sponges. At least 11 genera and 20 species have been found, forming the second most diversified metazoan group in Chengjiang. A majority of the sponges belong to the Demospongea with a broad spectrum of morphological variation and sizes. The present paper is the first report on the Chengjiang sponge fauna with its focus laid on the Leptomitidae which are a dominant group of the Demospongea, All the fossils reported in this paper were collected from Quarry M_2 on the western slope, and Quarry CF_5 on the northwestern slope of the Maotian hill, about five km SE of Chengjiang. They were embedded in the mudstone layers of the lower part of the Yuanshan Member. This member is the unit representing the upper part of the Chiungchussu Formation. Stratigraphically the fossils in our study range in a narrow interval dated as the lower Eoredlichia Zone in the trilobite sequence. Sponges have been widely accepted as an independent group of animals which are separated from Eumetazoans belonging to the Subkingdom Parazoan. They lack organized tissues and possess the potential of regeneration of the entire organism from the smallest fnagments; for these reasons their origination has been considered to be an early event in the history of animals. Their mineralized spicules would be expectedly rich in the Late Precambrian rocks in case of their early origination. However, the confirmation of this expectation has failed in fossil record. Only rare spicules have been reported from the strara of the Pre-Meishucun Stage, and part of them are still questionable.The lowest occurrence of less questionable spicules in China were described by Zhong Hua 1977 and Tang Tian-fu, Zhang Jun-ming and Jiang Xian-jian 1978 from the Late Proterozoic Doushantou Formation of lower Vendian equivalent in the Yangtze Gorges, western Hubei. Their recorded monaxons occur in the thin sections, measuring 0.1--1 mm long, and 0.01--0.02 mm across. Therefore, the sponges might have appeared in the Vendian Stage before their rapid dispersion and diversification in the Early Cambrian. These sponges are likely of later origin than eumetazoans. The fossil hecord confirms that the eumetazoans have flourished in he Vendian Stage, and possessed a long preceding period for their existence (Chen, 1988). The sponges were the largest group of suspension-feeding sessile epifauna among the Chengjiang metazoans. The competitive interaction between the sessile organisms may have been very intense due to their dependence on the same or similar nutrient resources. Evidence of niche segregation among the sponges is visualized from their tiering complexity, which is interpreted by Bottjer and Ausich (1986) as the vertical distribution of organisms within the benthic boundary layer. The Leptomitid sponges mostly occupied an epifaunal tier of 10cm, and the sponges of Choaia a low epifaunal tier (+5).The sponges with a short Conical body in shape such as Choaia may have used their large coronal spicules and thus have adapted themselves to keeping their central discsefly anchored on muddy substrate with the water circulation passing through the dense spicular meshes. The highest tier (+30cm) is represented by a new undescribed taxon of Demospongea which was globularly shaped, possessing a relatively thick and four-layered skeleton. Family Leptomitidae de Laubenfels, 1955 Genus Leptomitus Walcott, 1886 Type species: Leptomitus zitteli Walcott, 1886 Leptomitus teretiusculus sp. nov. (Pl.Ⅰ, fig. 4; Pl.Ⅴ, fig. 1—8; Pl. Ⅵ, fig. 2; text-fig. 1) Material: 14 specimens available for the present study, all compressed. Diagnosis: Tubular, thin-walled sponges with double-layered skeleton composed of monaxons, ranging up to ll0mm in length and approximately 12mm in flattened width. Outer layer of skeleton consisting of a vertical thatch of fine monaxons and inserted vertical rods composed of large oxeas in echelon structure; inner layer consisting of unbounded monaxons approximately 0.25mm apart. Description: Holotype (Cat. No.108480) being a nearly complete specimen, 80mm long, with a narrow base 1mm wide, which expands gradually upwards to a maximum flattened width of approximately 6mm at height of 63 mm; remaining part of the specimen tubular. Oscular margin unpreserved. Skeletal net composed of three size series of monaxons. Coarse oxeas making up the distinct rods which extend vertically to the full length of the sponge. Oxeas 3—4mm long, and 0.1mm across. Rods 0.16mm apart in the upper part where 27—28 of them appear, and 0.08 mm apart near the base of the specimen where they are reduced to 20. Interspaces of rods filled with the smallest monaxons which are generally poorly preserved, approximately 0.2—0.3mm long, and 0.01mm across in size as observed from Cat. No. 108482. Horizontal monaxons of inner layer slightly coarser than the fine monaxons of the outer layer, approximately 0.5mm long, 0.02mm across, and 0.25mm apart. Discussion: This new taxon bears a general resemblance to Leptomitus lineatus (Walcott, 1920). But in the former the sizes of sponges and spicules are much smaller. Genus Leptomitella Rigby, 1986 Type species: Leptomitus metta Rigby, 1983 Leptomitella conica sp. nov. (Pl. Ⅵ, figs. 1,3,4; text-fig. 2) Material: Only two specimens available for the present study. Diagnosis: Conical, thin-walled sponges with skeletons composed of monaxons arranged in two layers.Outer layer consisting of a vertical thatch of finest monaxons and inserted vertical rods; inner layer with bundled monaxons larger than the fine monaxons of outer layer in size. Description: Holotype (Cat. No. 108485, pl. 6, figs. 3, 4) 20mm long, expanding upward to a maximum flattened width of 6mm near the top. Paratype (Cat. No. 108486) 14mm long, with a maximum flattened width of 5mm. Outer layer of holotype skeleton consisting of coarse oxeas 5—7mm long, 0.03mm across; vertical rods 0.32mm apart in upper part of specimen.Coarse oxeas in paratype 6—7mm long, and 0.03 across; rods approximately 0.08—0.11mm apart. Inner layer consisting of both bundled and unbundled monaxons which are 1—1.5mm long, 0.015—0.03mm across; spicule bundles consisting of 5—10 spicules each, spaced 1mm part in upper part of the sponge. Discussion: The present new taxon is closely related to Leptomitella in having spicules of the inner layer occurring in bundles. Leptomitella was previously known only from the Bolaspidella polymerid trilobite zone in the Middle Cambrian Marjum Formation of the House Range in western Utah, and was considered as a more advanced form than Leptomitus. The new fossil evidence from Chengjiang convinces us that the bundled inner layer of skeletons need not be more advanced than the unbundled inner layer. In fact, Leptomitella appeared contemporaneously with Leptomitus in Early Cambrian. Besides, the fossil evidence shows that the leptomitids with spicules occurring in bundles in their inner layer of skeletons predominated over the unbundled leptomitids both in number of individuals and in species in the Chengjiang sponge fauna. Leptomitella conica sp. nov. differs from L. metta (Rigby, 1983) in its smaller size of sponge body and in its inner layer of monaxons both bundled and unbundled in arrangement. The bundles of L. conica are spaced more closely with a smaller number of spicules. Leptomitella confusa sp. nov. (Pl. Ⅲ, figs. 1—4; text-fig. 3) Material: Two specimens available for the present study. Diagnosis: Relatively large, cylindrical, thin-walled sponges with double-layered skeletons composed of three series of monaxons. Outer layer consisting of both finest monaxons and coarse oxeas which are arranged vertically in a pin stripelike fabric; inner layer of monaxons arranged horizontally in both bundled and unbundled forms. Description: Holotype (Cat. No.108487 A, B) represented by part and counterpart specimens, incomplete, 91mm long, with a maximum flattened width of 50 mm. Coarse oxeas up to 10mm long, 0.04mm across; rods approximately 0.4mm apart. Fine monaxons in outer layer 1.5—2mm long and 0.01mm across; interspace between the coarse oxeas with approximately 5—6 fine spicules. Monaxons in inner layer intermediate-sized between coarse and fine spicules in outer layer;monaxons approximately 5mm long, and 0.02mm wide, arranged in loosely bundled and unbundled patterns with bundles about 3—4mm apart. Bundled fabric distinct in major part of the fossil, but less clear at one end (interpreted to be the lower terminal). Specimen Cat. No. 108488 represented by part (possibly upper part) of a sponge, 78mm long, with a maximum flattened width of 37mm. Spicule bundles in inner layer rather broadly spaced, 4—6mm apart. Discussion: So far this new taxon is known of from only two incompletely preserved specimens which show no evidence of constriction, indicating that they possess one of the basal and oscular terminals of the sponges. Therefore, the original sponges must be very long in size. This new taxon is closely related to Leptomitella metta (Rigby, 1983), but the latter has spicules occurring both in bundled and in unbundled fabric. Genus Paraleptomitella gen. nov. Type species: Paraleptomitella dictyodroma gen. et sp. nov. Diagnosis: Tubular to balloon-shaped, thin-walled sponges with double-layered Skeletons consisting of monaxons; coarse oxeas slightly curved, and interlocked with one another to form elongated nets filled with vertically arranged fine monaxons in outer layer of skeletons; inner layer of skeletons consisting of bundled monaxons. Discussion: Paraleptomitella appears most similar to Leptomitella (Rigby, 1986) in general structure and shape. However, in the new genus the coarse oxeas are slightly curved and interlocked with one another to form an elongated fabric in the outer layer of the skeleton. Paraleptomitella dictyodroma gen. et sp. nov. (Pl.Ⅰ, figs. 1—3; pl.Ⅱ, figs. 1—5; text-figs, 4, 5) Material: Up to 20 specimens available for the present study. Diagnosis: Tubular, thin-walled sponges attaining to 95 mm long, with a maximum flattened width of 12 mm. Double-layered skeleton composed of monaxons. Outer layer consisting of coarse oxeas which are slightly curved and interlocked with one another to form elongated nets filled with vertically arranged fine monaxons. Inner layer consisting of horizontal monaxons arranged in bundles. Base of sponges narrow; oscular margin rounded. Description: Holotype (Cat. No. 108489 A, B) associated with a paratype (Cat. No. 1084900 A. B) both appearing nearly complete, represented by part and counterpart of the specimens. Holotype 92 mm long, cone-shaped in its basal 4.4 mm, expanding upward to a maximum flattened width of approximately 11.3 mm at midlength of sponge; upper part of sponge subcylindrical, gradually becoming constricted upward, with a rounded oscular margin. Skeletons double-layered, consisting of three series of monaxons in size. Coarse oxeas slightly curved, 5mm long, 0.05 mm across, and interlocked with one another to form elongated nets. Inner layer composed of small horizontal monaxons, about 1 mm long and 0.02 mm across; spicule bundles 2 mm apart in upper part, and narrowly spaced downward to a minimal limit of 0.5 mm apart. Spicules of inner layer unbundled in upper 15 mm of sponge. Specimen Cat. No. 108490 90 mm long, with its base unexposed; maximum flattened width 9.5 mm; spicule bundles 1.5 mm apart. Specimen Cat. No. 108491 (Pl. Ⅰ, figs. 1—3) relatively small, incomplete, 34 mm long, with a maximum flattened width of 9.5 mm near the oscular margin, which is broadly rounded. Coarse oxeas 1.5 mm long; elongated nets 0.3 mm wide and 1 mm long. Horizontal monaxons occurring in bundles approximately 1 mm apart in most part of the specimen but becoming unbundled in top 5 mm. Spicule nets of outer layer in Cat. No. 108492 0.3mm wide, and 2mm long; spicule bundles 1.1—5mm apart, with approximately 8 spicules in each bundle. Discussion: This new taxon differs from Leptomitella metta (Rigby, 1983) and Leptomitella confusa in its characteristic coarse oxeas which are curved and interlocked with one another to form elongated nets. Paraleptomitella globula gen. et sp. nov. (Pl.Ⅱ, fig 6; Pl. Ⅳ, figs.1—3; text-fig. 6) Material: Only two specimens available for the present study. Diagnosis: Balloon-shaped, thin-walled sponge with double-layered skeleton composed of monaxons; coarse oxeas in outer layer slightly curved, interlocked with one another to form elongated nets. Inner layer composed of horizontally arranged monaxons occurring in bundles in most part of sponges but in unbundled fabric in the top part. Description: Holotype (P1. Ⅳ, figs. 2, 3; Cat. No. 108493) 67 mm long, slender in lower 45 mm, and egg-shaped in upper 22 mm, with a maximum flattened width of 14 mm. Oscular margin broadly rounded. Coarse oxeas 3—4mm long, 0.05 mm across, and interlocked with one another to form vertically elongated nets which are 1.1—1.3 mm long and 0.32—0.48 mm wide. Inner layer composed of horizontally arranged monaxons (probably oxeas) which are approximately 1 mm long and 0.02 mm across; spicule bundles 1.5 mm apart, with 8—12 monaxons each. Paratype (Cat. No. 108494; pl. Ⅳ, fig. 1) 37 mm long, slender in lower part, and egg-shaped in upper 20 mm, with a maximum flattened width of 10 mm. Coarse oxeas up to 7 mm long, and 0.1 mm wide; spicule nets averagely 0.6 mm wide, and 10 mm long. Oscular margin broadly rounded. Spicule bundles 1.1 mm to 1.8mm apart, composed of monaxons approximately 1 mm long and 0.025 mm wide. Discussion: The balloon shape of the skeletons is characteristic of the present new taxon.
出处
《古生物学报》
CAS
CSCD
北大核心
1989年第1期17-31,123-128,共15页
Acta Palaeontologica Sinica
