早寒武世带网状鳞片的蠕形海生动物

EARLY CAMBRIAN NETTED SCALE-BEARING WORM-LIKE SEA ANIMAL

前言寒武纪早期动物爆发性演化事件是地质历史中的一个重大不解之谜,达尔文对此曾给予极大的注意但深感费解(Mayr,1982;Conway Morris,1987)。近三十余年来,世界各地前寒武纪晚期地层内陆续发现了许多软躯干后生动物化石。这些化石资料表明后生动物在寒武纪之前早已出现了(Chen,1988;Glaessner,1984)。由于早期动物由软躯干所组成,极难成为化石。到了寒武纪时。

Among all the sudden origins of faunas, none bothered Darwin more than the sudden appearance of most of the major animal phyla in the Lower Cambrian rocks, while nothing was found in the Precambrian strata during Darwin's time. Although the present fossil record of metazoans has been extended backward as far as 740 Ma (Chen, 1988),virtually all these older fossils are soft-bodied. The marvelous radiation of invertebrates was indeed a sudden event in the Early Cambrian time, but due to its combination with the biomineral event, a wide range of skeletal materials appeared across the whole spectrum of life. All the hard parts are small, known as the small shelly fossils, which usually include molluscs, hyolithids, protoconodonts, worm tubes, and a great array of scales, and spicules. Much of the small shelly fossils' fascination lies in scale and spicule fossils because of their enigmatic nature combined with an extraordinary diversity in forms. One of the most distinctive groups known as Microdictyon bore phosphatic, netted scales which occur mostly in isolation after being disjoined from each other following the death of the animal. The fossils reported in this paper are wormlike animals armored laterally with 10 pairs of etted scales in Microdictyon identity, mainly collected from the Maotian hill, about five km SE of Chengjiang, located at 102°57＇30'E, 24°39＇30'N at an elevation of 1920 m Text-fig.1). Several quarries have been opened for fossil collection since 1984. The present fossils were from Quarry M_2 at the western slope, and Quarry CF_3 at the northwestern slope respectively. Only one piece of isolatedly preserved fossil scale was collected from near the Dapotou village, 3km SW of the Maotian hill (DJ_1) (Text-fig.1). All the fossils were embedded in the mudstone layers of the lower part of the Yuanshan Member which is the unit with its name given to the upper part of the chiungchussu Formation in which a superbly preserved new fauna is encountered (Text-fig. 2). This member comprises predominantly fine grained mudstones which may represent a deeper outer shelf environment laterally intercalated with extensive blanket-type /deposits of very tranquil water conditions. Periodic fine sand tongues (1—50 cm thick) are interpreted to mark the distal storm layers which were deposited offshore under storm-induced current flow or storm surge ebb flow. The age of the Yuanshan Member is well dated by the occurrence of the earliest trilobites such as the eodiscoid Mianxiandiscus, the redlichiids Eoredlichia and Wutingaspis, and the polymerid Parabadiella. The fossils under study range stratigraphically in a narrow interval dated as the lower Eoredlichia Zone in the trilobite sequence. The generic name of Microdictyon was previously introduced by Bengtson, Matthews and Missarzhevsky 1981 for separately preserved netted scales which were broadly documented in Asia, Europe, North and Central America, Australia in strata ranging from the Tommotian Stage to Middle Cambrian. The taxonomy of Microdictyon at species level has been established upon the isolated single-element scale-like specimens including such species in the form of single elements as Microdictyon effusum Bengtson, Matthews and Missarzhevsky 1981; M.rhomboidale B. M. M. 1986; M.cf.rhomboidale in B. M. M. 1986; M. robisoni B. M. M. 1986; Microdictyon n. sp. 1 B. M. M. 1986; and M. ? tenuiporatum B. M. M. 1986. A new classification of natural taxonomic units which are based on the recognition of assemblages as the remains of individual organisms will be consequently promoted by the present study. This animal is confirmed to carry ten pairs of netted scales with an outline varying from irregularly angular, oval, vertically oblong, regularly rounded, and horizontally oblong (Text-fig. 3). The netted scales have been proved to cover vertically much of the trunk; they are considered to be responsible for the trunk's vertical, supporter. But much of the animal's body was flexible, and not covered with scale-like sclerites. The body of the animal is believed to be internally divided into eleven segments, with the anterior first one making up the head, and the most posterior one bearing the anus; all the other nine segments are essentially identical. The segmentation is externally indicated with a series of aligned scales and soft legs. The scale-like sclerites are concavo-convex plates comprising a reticular framework layer and an overlying capping layer, both possibly having rendered much improvement of mechanical strength against the tension force made by musclepulling. The Microdictyon was a bilaterally symmetrical worm-like creature presumably adapted itself to the free living model. The presence of ten pairs of unsegmented and elongated soft legs provides a convincing argument that this animal was not a benthos because its kind of legs is greatly unfavorable for the benthos to burrow into the sediments or crawl across the surface of sea floor. Such a kind of soft legs would perform a favorable function of adhesive mechanics for attachment on objects floating near the surface of the water. An argumentation about the animal lodging on jellyfish has arisen from the evidence of fossil record that part of the Microdictyon specimens were found in association with jelly-fish remains (Pl. Ⅲ, fig. 6). The Microdictyon with a bilaterally symmetrical body plane, paired elongated legs and partially armored body with high mechanical strength of scales, indicates that this animal did not live exclusively on floating subjects. Instead, it might take much time for the animal to adapt itself to a nektonic life and use its soft legs for sliding or swimming. Although the phyletic position of Microdictyon remains enigmatic, the authors believe that the closest affinities of this creature are possibly related to the annelids but with most important differences between them. One of the great differences is that the animal possessed a much smaller number of segments than annelids, and its body was partially armored with phosphatic scales. Based on observation of morphology and structure, Bengtson, Matthews and Missarzhevsky (1986) believed that the scales of Microdictyon could not grow by accretion of the mineralized tissue and the increase in their size would be achieved by moulting growth of the animal, which provides further evidence tending to confirm the animal as a minor distinct phylum of worm-like high invertebrates. Phylum uncertain Worm-like high invertebrates, possessing a series of netted scales in pairs on lateral sides and the same number of unsegmented soft legs. Family Microdictyonidae fam. nov. Worm-like animals bilaterally symmetrical, with netted scales in pairs and the same number of unsegmented soft legs laterally. This new family include Microdictyon Bengtson, Matthews and Missarzhevsky 1981 from Lower to Middle Cambrian and Milaculum Muller 1973 from Upper Cambrian to Middle Ordovician. Genus Microdictyon Bengtson, Matthews et Missarzhevsky 1981, emend. Chen, Hou et Lu Type species: Microdictyon effusum Bengtson, Matthews and Missarzhevsky 1981. Emended diagnosis: Worm-like ani- mal slender and bilaterally symmetrical, armoured with 10 pairs of netted scales on lateral sides, and 10 pairs of elongated and unsegmented legs. Scale phosphatic, constructed by a framework layer with hexagonal meshes, and an overlying capping layer making up the nodes and crests on top of hexagonal mesh walls. Holes rounded in the meshwork. Distribution: Worldwide in the Lower and Middle Cambrian. Microdictyon sinicum sp. nov. (Pls.Ⅰ, Ⅱ, Ⅳ, figs. 1—5; Pl. Ⅲ, figs. 1—6; text-figs. 3—6) Material: Two relatively, complete specimens with soft body, gut, completely or near completely paired scales and incomplete soft legs; eight incomplete specimens with partially preserved soft body and incomplete assemblage of scales; two pieces of separately preserved scales. Diagnosis: Animal small, worm-like, slender, up to 8 cm long and 0.3 cm across, with 10 pairs of netted scale and 10 pairs of unsegmented legs. Scales thin, concavo-convex, posited on lateral sides of trunk, covering vertically much of body; two-fold internal structure, with inner framework layer consisting of hexagonal meshes and outer capping layer to form crests on top of the mesh walls and nodes on top of the walls' junctions. Holes rounded in meshwork. Scale assemblage recognized by the natural occurrence of components in an individual animal with their alignment angular (anterior first and second pairs), oval (third to sixth pairs), vertically elongated-oblong (seventh pair), circular (eighth pair) or horizontally elongated-oblong (ninth and tenth pairs) in outline. Alimentary canal simply slender, straightly extending anteroposteriorly, with anus near the posterior end. Description: Holotype represented by part (Cat. No. 108286 A, pl.Ⅰ, fig. 1; text-fig. 5) and counterpart (Cat. No. 108286B, pl.Ⅰ, figs. 2—5) of a complete mature individual, with an unfolded length of about 79 mm. Trunk roughly preserved in lateral compaction; its exact orientation usually detectable from dislocation of the paired scales. Right side of trunk likely dislocated against left side antero-ventrally in its anterior 15 mm; ventrally in following 10mm; ventro-posteriorly in the interval between the sixth and seventh pairs of scales; posteriorly near the eighth pair of scales; and dorsoposteriorly in the following posterior portion. Body slender, 4mm in compressed width, with original width accounted as in the order of 2.8 mm; a domed dorsal profile on sites of scales in anterior first to seventh pairs dorsally, but an even profile on sites of eighth up to tenth pairs of scales. Faint annulations preserved in interval between the seventh and eighth pairs of scales. Legs slender and unsegmented, tapering from their bases to terminals; right leg of fifth pair measuring 11 mm long, 1.7 mm and 0.7 mm across at its base and terminal end respectively. Alimentary Canal simply straight, extending for much part of body in a subventral position, with terminal anus near the posterior end. Specimen Cat. No. 108287 (Pl. Ⅱ, fig. 1; text-fig. 6) nearly complete, only lacking posterior trunk. Preserved portion measuring 39 mm along the curvature of the trunk, with original length probably in the order of 45 mm, represented by much part of trunk with scales from the 1st to 9th pairs. Trunk measuring 2.5 mm across, with original diameter probably in the order of 1.6 mm; its dorsal profile appearing domed at location of the 1st to 7th pairs of scales. Surface showing faint annulations.Alimentary canal simply straight. Soft legs incompletely preserved. Specimens Cat. Nos. 108289—108296 (Pl. Ⅲ) consisting of all rather small individuals with original length probably in the order of 15 mm; Cat. Nos. 108290 and 108291 being dorsoventrally compressed, with trunk exposed in dorsal view; Cat. No. 108289 being ventrolaterally compacted; while Cat. Nos. 108294—108296 having been buried in immediate association with a fossil jelly-fish. EDAX analysis indicating that all the scales have experienced diagnostic changes of silicification. Capping layer of the scale (Cat. No. 108297, pl. Ⅳ) consisting of such chemical compositions as SiO_2 79.77%; Al_2O_3 11.93%; FeO 6.3%, MgO 1.4%; and TiO_2 0.56%. Scale thin, weakly convex; framework layer consisting of hexagonal meshes (Pl. Ⅰ, figs. 2—5; text-fig. 4); capping layer made up of crests and nodes, with the nodes situated at three wall junctions (Pl.Ⅳ, fig. 4). Pipes circular in cross section, extending across the hexagonal network with their inner portion exposed (Pl. Ⅱ, figs. 3—5; text-fig. 4), about 0.15 mm in maximum diameter, which tends to decrease toward the periphery. Holes about 134 in number. Outline size and orientation varying among the scales of an individual. Anterior Ist pair of scales rounded anteriorly, and bluntly angular posteriorly; 2nd pair broadly rounded anteriorly, slantingly straight posteriorly, and bluntly angular postero-ventrally; 3rd to 6th pairs of scales all egg-shaped, with ventral end blunter than the dorsal end; 7th pair vertically elongated and oblong in shape; 8th pair regularly rounded, both 9th and 10th, pairs horizontally elongated and oblong in shape with the 10th pair much smaller than the 9th pair. The length and width dimensions and spacing of scales within an individual measured systematically from Cat, Nos. 108286A. 108287 and 108289, are shown in Tables Ⅰ and Ⅱ as follows: Remarks: The general aspect of the Microdictyon animal has been outlined in the reconstruction map (Text-fig.3). Since comparatively little information is available on the morphology of the anterior terminal, the presence of an elongated proboscis is highly tentative. The stratigraphical and geological distribution of the Microdictyon animal is extended by evidences of separately preserved scales which are well-known in acid residues of Middle to Lower Cambrian rocks. A number of taxa based on the single scale-element have been accumulated, such as Microdictyon effusum Bengtson, Matthews and Missarzhevsky 1981; M. rhomboidale B. M. M., 1986; M. cf. rhomboidale in B. M. M., 1986; and M. robisoni B. M. M. 1986; Microdictyon n. sp. 1 in B. M. M., 1986; and M. tenuiporatum B. M. M., 1986. The types species of Microdictyon, M. effusum was established for the egg-shaped scale element which came from Malyi Karatau, South Bothnian area, Kazakhstan near the base of the Lower Cambrian Shabakty Formation. It can be distinguished from the present species by its elongated and strongly convex scales with mushroomshaped nodes. The dorso-ventral length to anteroposterior width ratio of the egg-shaped scale element in the Yunnan species ranges from 1.3:1 to 1.7:1, whereas that of M. effusum varies from 1.8:1 to 2.3:1. Microdictyon rhomboidale was erected for the regularly outlined scales probably of the first paired element. It is characterized by its rhombic scale which is angular posteriorly and anteriorly, and concave ventrally. Microdictyon robisoni was based on twenty incomplete scales belonging to circular and angular scale elements respectively, which were found from the Middle Cambrian Swasey Formation, Utah. The strongly convex scale and tall, spike-shaped nodes are characteristic of this taxon. Microdictyon n.sp.1 in Bengtson et al., 1986, is represented by the four incomplete scales probably belonging to the circular scale element, collected from the Lower Cambrian Campito Formation, Nevada. These scales differ from their Yunnan equivalent in their rather strong convexity, and mushroom-shaped nodes. Microdictyon tenuiporatum is based on the circular scale element collected from the Tommotian stage, Siberia, with the scale characterized by its irregular or tetragonal netted framework.