四川峨眉寒武纪早期的CHANCELLORIDS

EARLY CAMBRIAN CHANCELLORIIDS FROM EMEI, SICHUAN PROVINCE, SW CHINA

四川峨眉麦地坪剖面下寒武统麦地坪组上段ｃｈａｎｃｅｌｏｒｉｄｓ类骨片化石丰富，皆以磷质内核方式保存，其中首次发现具有两个中央射管的ｃｈａｎｃｅｌｏｒｉｄｓ骨片。对ＣｈａｎｃｅｌｏｒｉａｉｒｒｅｇｕｌａｒｉｕｓＱｉａｎ，Ｃｈａｎｃｅｌｏｒｉａｓｐ．，Ａｌｏｎｎｉａｓｐ．进行了详细描述。在内核标本表面，骨片的基面上首次发现了保存极好的网状微细构造。对这一微细构造解释尚有困难，但有可能为骨片原始壳体结构的内端印痕。同时，对ｃｈａｎｃｅｌｏｒｉｄｓ化石的归属及分类问题进行了讨论。

The sudden appearance, close to the Precambrian Cambrian boundary, of the e arly skeletal fossils is one of the strong evidences of the early Cambrian radiation , so called “Cambrian Explosion”(Brasier, 1979; Conway Morris, 1987). Since t hese skeletal fossils are commonly minute in size, they are usually called “the small shelly fossils”, and consist of a variety of fossil phyla, including: br achiopods, molluscs, hyoliths, proconodonts, etc., and some enigmatic fossils, s uch as tommotiids, zhijiniids, hyolithelminths, etc.(Bengtson et al .,1990). Chancelloriids, with scleritome being composed of star shaped sclerites, is one kind of the enigmatic skeletal fossils, and was previously introduced accordin g to the “spicules” typically with a central ray and 4-9 lateral rays by Walco tt in 1920 when he investigated the sponge fossils collected from the Burgess Sh ale. Since then, chancelloriid fossils have been found with a wide geographical distribution(Asia, Europe, North America, Australia, Antarctica)(Bengtson et al ., 1990)from the strata ranging from Lower Cambrian to Upper Cambrian; espec ially, they had a high density and diversity in early Cambrian. This enigmatic m etazoan with star shaped sclerites had been taken as sponges(Sdzuy, 1969)till G oryanski(1973) argued against the sponge affinity, and later many palaeontologists realized that it should be unreasonable to compare this type of sclerites with sponge spicules. In 1981,Ben gtson and Missarzhevsky precluded that the sclerites had been formed by envelopi ng sclerocytes of sponges, and put them together with wiwaxiids, siphogonuchitids and halkieriids into a new class Coeloscleritophora, based on that they all have composite exoskeleton and that the individual sclerite has a prominent internal cavity and a restricted foramen. The concept of Coeloscleritophora is stressed by Qian and Bengtson(1989), Bengtson et al .(1990), and accepted by Rigby (19 86) and some other palaeontologists. Althougth this classification scheme has been widely accepted, it does not m ean that it is flawless. Some palaeontologists were still showing doubts on the scheme (Vasil'eva, 1985;Dzik, 1996). In fact, this scheme hints some problems. F irst ,this scheme is based only on the resemblance of some characteristics of th e sclerites, but the erection of taxonomic unit of high level should base on th e internal struct ure of the soft body. Although a few soft bodied fossils so called coelosclerit ophorans have already been found (Walcott, 1920;Conway Morris, 1985; Chen et a l ., 1996), the knowledge on their internal organization is still very limited. Secondly, their ecological habitats are greatly different:chancelloriids is a k ind of sedentary animals, somewhat like sponges; while wiwaxiids and halkieriids are vagrant benthos(Conway Morris, 1985). But due to the limitation of soft bo died specimens, it is impossible to establish a better classification scheme, th erefore, the concept of the Coeloscleritophora is still adopted here. During the course of the study on the early skeletal fossils collected from th e Maidiping Formation(Lower Cambrian, Meishucun Stage) at Maidiping Section, Eme i, Sichuan Province, a lot of star shaped chancelloriid sclerites were found. T he sclerite specimens collected through decalcification by acetic acids are pres erved as phosphatic steinkerns. The described taxa herein include Chancelloria irregularius Qian, Chancelloria sp. which being characterized by its sclerites with two central rays, Allonnia sp. The reticulate microstructure is examined on the basal surfaces of the inter nal moulds, just around the foramina of horizontal and central rays. This microstructure has been observed on all chancelloriid sclerites collected from the Maidiping Formation, including articulate composite sclerites and isolated rays. The reticulation co nsists of pits and intervening ridges. The pits are circular or slightly polygon al with diameter of 4-5 μ m, the width of the interve