云南澄江早寒武世带触手的蠕形动物——Facivermis gen.nov.

EARLY CAMBRIAN TENTACLED WORM-LIKE ANIMALS(FACIVERMIS GEN.NOV.)FROM CHENGJIANG,YUNNAN

前言蠕形动物虽是非常常见的生物,由于缺乏坚硬的矿化的外骨骼,其遗骸易于腐烂,只有在特定的条件下才得以保存成为化石。因此,蠕形动物化石在地质记录中是极其罕见的。这就是科学家对于蠕形动物化石给予极大关注的重要原因。人们对各地质历史时期中蠕形动物的知识很大程度上依赖于通过对痕迹化石的研究。但同一物种由于环境的不同可以形成完全不同的痕迹。

One of the most distinctive groups of wormlike animals is known as Facivermis yunnanicus gen. et sp. nov., a possible primitive annelid which possesses 5 pairs of segmented tentacles disposed dorsally on the anterior portion of trunk. The boby is clearly segmented into rings with a spinous papillae-bearing proboscis. An intestine observed from a raised positive relief suggests the interpretation of a swallowed prey, possibly bradoriids contained therein. The fossils are normally observed to lodge slightly oblique to the bedding plane, which supports the suggestive conclusion that the animals had been burrowers before they happened to be buried in situ. The biological affinity of Facivermis has been studied in the present paper by the authors who have come to a suggestive conclusion of its annelid affinity. The most noticeable features of the present worm-like fossils are the ringing of the body,and the tentacled trunk in its anterior portion,which are believed to be not only external but involving many of the internal structures as well. Facivermis bearing an everted proboscis together with 5-pairs of segmented tentacles in its anterior tapering portion of the trunk shows a comparable affinity to living nereid polychaetes, in which, however, all the 5 pairs of tentacles are posited on the head, and their head consists of 2 main parts: the prostomium (region in front of the mouth) and the peristomium (region around the mouth). In their 5-pairs of tentacles, one pair is posited in front of the prostomium and the other 4 pairs are disposed laterally on the peristomiun. The different disposition of the tentacles between Facivermis and living nereid polychaetes is likely interpreted as resulting from an evolutionary change in the tentacled anterior trunk of primitive annelids to a fused head of living nereid polychaetes. There are confused views on the origin and carly evolution of annelids among the modern scientific community.Dickinsonia, Spriggina and Marywadea of the Ediacarian age, are interpreted as the oldest fossil records of polychaetes by a majority of the authors. But their polychacte affinity is questioned by Conway Morris (1979) and Runnegar (1982), who considered in their papers the morphological resemblances between Ediacarian polychaetes and living polychaetes as superficial. The unquestioned oldest polychaetes confirmed by the presence of bristles are known from the upper Lower Cambrian strata in Australia. The presence of parapodia in annelids is firmly evidenced by the fossil record until the Middle Cambrian while jaws were delayed in presence up to the Lower Ordovician. Conway Morris and Robison (1985) established a new class Palaeoscolecida for the worm-like metazoans with a papillate epidermis. They considered it probably as an extinct group of the Annelida, and interpreted the annular arrangement of the papillae to be reflecting metameric segmentation. The worm-like fossils in our collections dominantly possess a retractable proboscis in front of the trunk, with numerous papillae on epidermis both randomly and annularly arranged tsat renders an annelid affinity of the Palaeoscolecida strongly debatable. An alternative interpreration is also possible that the pits previously intepreted to be papillae are likely the housing of bristles; if this interpretation is correct, it would strongly support an annelid affinity of the pitted worm-like animals in the Early Cambrian. Available for the present study are five specimens of Facivermis yunnanicus gen. et sp. nov. collected from Quarries M_2 and M_3 in the western slope of the Maotian hill, about five km SE of Chengjiang. They were embedded in the laminated mudstone layers of the lower part of the Yuanshan Member. This member is the unit representing the upper part of the Chiungchussu Formation. All the fossils studied range stratigraphically in a narrow interval dated as the lower Eoredlichia Zone in the trilobite sequence. Phylum Annelida Lamarck, 1809 Class,Order and Family uncertain Genus Facivermis gen. nov. Etymology: Facis and vermis, Latin, torch and worm, with reference to the torch-like tentacles. Type species: Facivermis yunnanicus gen. et sp. nov. Diagnosis: Body bilaterally symmetrical, consisting of anterior proboscis and segmented trunk. Trunk elongated, with an anterior tapering portion on which five pairs of segmented tentacles are disposed dorsally. No evidence of setae and parapodia appearing on the trunk.Gut straight and simple. Facivermis yunnanicus gen.et sp. nov. (Pl. Ⅰ, figs. 1—4; pl. Ⅱ, figs. 1—4; text-figs. 1—4) Material: Five specimens available, with four paratypes all crowded in a small slab about 6×6cm (Pl. Ⅱ, figs. 1, 2); holotype (Cat. No. 108720, pl. Ⅰ, figs. 1, 3) represented by part and counterpart. Description: Holotype 24mm long in preserved portion, with a maximum flattened width of 1.6 mm; paratype (Cat. No. 108721, pl. Ⅱ, fig.la) larger, approximately 55mm long, with a maximum flattened width of 3 mm. Trunk elongated and cylindrical, with an anterior tapering portion disposed dorsally and bearing 5 pairs of segmented tentacles (Pl.Ⅰ, figs. 1—4; pl. Ⅱ, figs. 2a, 2b). A small and subtriangular area lying beyond the front of the trunk, which is interpreted as proboscis rather than head; proboscis clearly observed to be armed with papillae in holotype, but showing no evidence in paratypes (Cat. Nos.108721, 108722, pl. Ⅱ, figs. 2a, 2b) due to intense decay. A circular structure observed at the anterior end of the trunk in specimen Cat. No. 108721, which is interpreted as mouth in the front of proboscis (P1. Ⅱ, fig. 4; text-fig. 3a). Body clearly segmented into rings which are narrowly spaced with a density of 50—60 per cm in holotype; no evidence of setae and parapodia confirmed on the boly. Tentacles about equal in length within the same individual, disposed dorsally along the sagittal line on the anterior tapering portion of the trunk (P1. Ⅱ, fig. 4), tapering distally, and segmented into rings. Cut straight and simple, observed in holotype (Pl.Ⅰ, fig. 2) and paratypes (Cat. Nos.108721, 108722, 108724, pl. Ⅱ, figs. la, 1d, 2b), as shown in a raised positive relief or black organic film. Discussion: A dorsal view of the animal (Pl. Ⅱ, fig. 4) shows evidence of the trunk's anterior tapering portion which is clearly segmented into 5 rings, each possessing one pair of tentacles. The holotype (Pl.Ⅰ, fig. 4) shows the anterior tapering portion of the trunk in ventral view, in which the rings appear to be narrowly spaced due to possible compaction doubling. A dorsal view of the morphological aspect is expressed in text-fig. 4, but features of the animal's posterior portion are highly imaginative.