摘要
前言早寒武世海绵在文献中虽屡有报道(Rigby, 1976, 1986),但多为分散式保存的骨针,关于海绵骨骼整体化石的报道则极鲜见(Walcott,1886, 1889; Rigby, 1987)。云南澄江是世界上迄今所发现的保存大量完整海绵骨骼化石和层位最低的唯一产地。澄江海绵化石群的研究为重建海绵类早期历史提供了新的较可靠的科学依据。长期以来人们根据早寒武世地层中所发现的大量海绵骨针均为四射类型的事实,推测这一时期的海绵分异度低,主要由六射海绵少数分子所组成,普通海绵极稀少,还未得到发展(Rigby,1976,1986)。澄江海绵化石群的研究所得出的结论与以上的推测恰恰相反,实际上,早寒武世筇竹寺期的海绵在演化上已达到了一个新的高度,普通海绵已高度分异并成为海绵类中最主要的优势生物类型。在分异度、物种结构、骨骼结构方式和复杂程度等方面均表现了筇竹寺期海绵动物群与中寒武世布尔吉斯海绵动物群具有惊人的相似性;这些相似性表明,海绵动物在早寒武世早期(筇竹寺期)已达到演化上的第一高峰,在其后筇竹寺期到布尔吉斯期将近3千万年的地质历程中处于缓慢的渐进式进化过程。
Although isolated spicules of sponges are known, virtually in all continents with greatly abundant Lower Cambrian rocks, the preservation of verified articulate skeletons is extremely rare in the past records. The Chengjiang Lagerst(?)tten is of special importance in fossil sponge studies for the record of its older age and the excellent preservation. Becaause of the biased preservation, the knowledge of Lower Cambrian sponges has been heavily dependent on the isolated spicules which are largely known as isolated stauractines, and which have led to assumption by various authors that sponges in Early Cambrian had a limited diversity, consisting largely of Hexactinellid protosponges with a few rare demosponges. On the contrary, a supported evidence from the Chengjiang Lagerst(?)tten suggests that the sponges, particularly Demospongea in the Early Cambrian Chiungchussu stage underwent very rapid diversification which led to many indefinite morphologic types, with an evolutionary complexity which might stand comparison with those of the Middle Cambrian Burgess Shale. The striking similarity of the sponge fauna between the Chengjiang Lagerst(?)tten and the Burgess Shale suggests that sponges have retarded their evolutionary pace from the Chiungchussu stage for at least a period of 30 Ma.
Among the demosponges, Leptomitus has a simple and thin double-layered skeleton and stratigraphically lowest occurrence which have led to assumption as the central stock of the Demospongea. Fossil evidences indicate that many elements of demosponges have adopted from Leptomitus a virtually unchanged pattern of double-layered skeleton. None of multiple-layered skeletons have been known among the Cambrian sponges apart from the present new genus Quadrolaminiella. This new form is characterized by a unique four-layered and double-netted skeleton. The outer net consists of the 1st (outermost) and 2nd layers of oxeas, which extend parallel and vertical to the length of the sponges respectively; inner net formed by the 3rd and 4th layers, extending upward diagonally clockwise and counter clockwise respectively. Quadrolaminiella might have derived from Leptomitus as if it had developed a thicker skeleton with an additional inner net.
All the specimens available for the present studies were collected from Quarries M3 and CF5 on the W and NW slopes of the Maotian hill, about five kin, SE of Chengjiang. They were embedded in the mudstone layers in the lower part of the Yuanshau Member, a unit representing the upper part of the Chiungchussu Formation. Biostratigraphically the specimens under study range within a narrow interval dated as the lower Eoredlichia zone.
Phylum Porifera Grant, 1872 Class Demospongea Sollas, 1875 Order Monaxonida Sollas, 1983 Family QuadroIaminiellidae fam. nov.
Tubular to conical sponges, with four-layered walls composed of monaxons arranged in four la yers and two nets; outer net consisting of the 1st(i.e., the outermost) layer and the 2nd layer, with spicules arranged vertically in the 1st layer and horizontally in the 2nd layer; inner net diagonally oriented, with spicules extending clockwise in the 3rd layer and counterclockwise in the 4th (i.e., the innermost) layer.
Genus Quadrolaminiella gen. nov.
Type species: Quadrolaminiella diagonalis gen. et sp. nov.
Diagnosis: Large, tubular to conical sponges with four-layered and double-netted skeleton composed of monaxons. 1st (i.e., the outermost) layer consisting of coarse, widely spaced oxeas, extending as parallel rods virtually to the entire length of the sponge; space between the rods absent, with finer oxeas. 2nd layer produced by finer oxeas which are horizontally arranged and narrowly spaced, combining with the 1st layer to produce the outer net. Moderately coarse oxeas of the 3rd layer clockwise oriented, occurring in bundles or unbundled; 4th layer produced by counterclockwise oriented oxeas combining with those of the 3rd layer to form the inner net. Spicules of the inner net diagonally oriented with reference to the principal axis of the sponge.
Discussion: This new taxon is characterized by its complex four-layered skeleton arranged in double nets.
Quadrolaminiella diagonalis gen. et sp. nov. (P1s.Ⅰ—Ⅳ; P1. Ⅴ, fig. 2; text-figs. 1, 3)
Material: Seven specimens available, all compressed and represented by the part and couterpart.
Diagnosis: Large, tubular or conical sponges with a maximum length of 30cm and 12cm across. 1st layer composed of coarse oxeas forming uniformly parallel, vertically continuous, rodlike structures; 2nd layer formed by the finer oxeas; 3rd layer with monaxons at mid-size between those of the 1st and 4th layers, and clockwise oriented; 4th layer with rods counterclockwise oriented.
Description: Holotype (P1.Ⅰ; P1. Ⅴ, fig.2; Cat. No. 108497 A,B) separated into two parts through the plane in reference to spongocoel of the sponge, thus the spicules appearing in the form of casts both in part and counterpart. Innermost layer of skeleton occurring on uppermost surface on both parts; each of the next outer layer appearing in proper order from the uppermost.
Distinction of the part from the couterpart of holotype appearing subjective; the part represented by a fragment 172mm long, with a maximum flattened width of 84mm at a height of 68mm from base. Base short-conical, with an expanding angle of 80°. Innermost layer of skeleton occurring at the uppermost surface of rock, composed of oxeas up to 12—15mm long, 0.04mm across; rods approximately 0.8mm apart, narrowing downwards. Unlerlying 3rd layer consisting of oxeas 7—10mm long, 0.03—0.04mm across; rods 0.4—0.7mm apart, with finer spicules filled in the interspace of the rods. Following underlying 2nd layer consisting of horizontal fine monaxons 5—8mm long, 0.025mm across, and 0.02mm to 0.04mm apart. Oxeas of the outermost layer coarse, 10—15mm long and 0.04mm across; rods 1.6mm apart at a height of 10mm from base of the sponge; 40 rods observed from the specimen, suggesting that about 100 rods with reference to the entire sponge. Counterpart (P1.Ⅴ, fig. 2) 182mm in length, and 84mm in maximum flattened width, with its skeleton occurring in the same order of relative position in the matrix.
Figured paratype (Cat. No. 108498A, B) represented by a fragmentary sponge 156mm long, witha maximum flattened width of 115mm, separated into part and counterpart through one side of skeleton. 4th layer in the part preserved in the form of cast; rods counterclockwise oriented, about 0.05 mm across and 0.8—1.0mm apart near the base, and 1.2—1.4mm apart in the major part of the sponge. Overlying 3rd layer consisting of spicules appearing as casts; rods clockwise oriented, 0.04mm across, and 0.15—0.5mm apart near the base and 0.8—1.2mm apart in the major part of the sponge. Spicules in the 2nd layer preserved as impressions, arranged in bundles in its lower 20mm, and unbundled in the remain portion of the sponge; each bundle consisting of 8—10 spicules, and 0.5—0.7mm apart. Spicules in the 1st layer preserved in the form of impressions, about 0.06mm across, 0.23—0.30mm apart in the lower part, and 0.4—0.8mm in the middle and upper parts of the sponge. Counterpart (P1.Ⅳ; Cat. No. 108498B) represented by a fragment 139 mm long, with a maximum flattened width of 75mm. Spicules preserved as casts in the 1st and 2nd layers, and impressions in the 3rd and 4th layers.
Paratype (Cat. No. 108499A, B) represented by a fragmentary sponge 139mm long, with a maximum flattened width of 51mm, separated into part and counterpart through one side of the skeleton. Rooted base 3.2mm long, with a flattened width of 2.2mm, expanding rapidly to 51mm in flattened width and 13mm in length, then becoming tubular in the upper part. Spicules of the 4th layer inthe part (P1. Ⅱ, figs. 1, 2) preserved as casts, 10—15 mm long, 0.01—0.02mm across, and 0.5—1 mmapart in the middle part of the sponge, with finer sspicules 0.5—1mm long and 0.0015mm across, filled in interspace of rods. Spicules in the overlying 3rd layer preserved as casts; rods clockwise oriented, with spicules 3—5mm long, 0.003mm across,and 0.15—0.21mm apart in the middle and upper parts of the sponge. Monaxons preserved as impressions in the following 2nd layer, 10—15mm long, 0.003mm across, 0.3—0.4mm apart in the middle part, and 0.15—0.2mm apart near the base of sponge. Rods preserved as impressions in the 1st layer, 0.4—0.6mm apart in the middle and upper parts, and 0.8mm near top of the sponge; spicules 12—15mm long 0.03—0.04mm across. Spicules appearing as impressions in the 3rd and 4th layers and as casts in the 1st and 2nd layers of the counterpart (Cat. No. 108499B).
Discussion: The restoration of the four-layered and double-netted structure of the skeleton(text-fig. 1) is mainly based on the observation of the part and counterpart belonging to the holotype (P1. Ⅰ; P1. Ⅴ, fig. 2). The holotype was fortunately split on the surface through spongocoel of the sponge into two specimens; each of them preserved with skeleton possessing duplicate four-layered structure in the same manner of casts. The relative position of the four spicule-layers shows a mirror symmetry, with the innermost layer occurring at the uppermost surface of rock. However, the majority of specimens were separated through one side of skeleton.
The morphological features of the sponge are basically confirmed by the holotype and one of the paratypes (P1. Ⅲ, fig. 1); the restoration of the oscular pattern is highly speculative because of lacking supporting evidence. A tendency toward narrowing upward near osculum is supported by observation on one of the paratypes (Cat. 108499A, B; P1. Ⅱ, fig. 1). The thickness of the skeleton is uncertain; its innermost layer is observed to extend onto an extreme margin of the skeleton from one of the paratypes (P1. Ⅱ), suggesting that the skeleton is thin, possibly only four spicules thick.
Quadrolaminiella
出处
《古生物学报》
CAS
CSCD
北大核心
1990年第4期402-414,507-512,共13页
Acta Palaeontologica Sinica
