摘要
前言寒武纪早期生物辐射性演化事件从某种意义来说也是底栖生物造型方式的辐射性的演化,以便适应于海床表面,以及海床表面之上和之下的不同生态空间;与此同时它们采取了形形式式的相应食性行为和采食方法。以海水中悬浮营养颗粒为生的生物不在少数,在竞争过程中,不同生物发展了不同的采食模式。本文所描述的早寒武世高足杯状海生动物Dinomischus是一种极为稀少的底表固着生物,以食悬浮性营养粒为生。这种生物以其固着柄将其采食器官(萼部)举起,以便提高采食器的高度。
To a large extent,the radiation of the Early Cambrian fauna was one of the body ground plan of benthic organisms adopting the mode of life uve on,above and below the sea floor,when ep faunal suspension-feeding,however,became a most important habit and a dominant feeding mode.Under the present study is an Early Cambrian hock glass-like animal Dinomischus,a rare component of solitary epifaunal suspension feeders tending to elevate their feeding structures through the stalk to a higher level above the sea floor. Since 1912 when Walcott made the discovery of the first Dinomischus specimen,only two additional specimens have been cumulated by Raymond in 1930,and by the Royal Ontario Museum party in 975 during the following long period,indication that these specimens are extremely rare.They are all from the Middle Cambrian Burgess Shale of British Columbia. It has been demonstrated that Dinomischus extends downward stratigraphically in Early Cambrian strata since 1987 when two specimens of the genus were obtained from the narrow strata of the lower part of the Yuanshan Member,Chiungchussu Formation at Quarry M_2,west slope of the Maotian hill,Chengjiang,Yunnan,in which a new superbly preserved Chengjiang Fauna has been encountered(Text-figs.1,2,in Chen et al.,1989).The age of the present fossils is well dated as the lower Eoredlichia Zone in the trilobite sequence. A passive suspension-feeding mechanics of Dinomischus is proposed in the present study.Dinomischus was a solitary sessile metazoan developed with an elongated stalk,which is considered to have mostly been freed from sediments,thus enabling this creature to occupy high epifaunal tiers.This genus appears to have adopted a passive suspension-feeding mode and always relied on horizontally unidirectional currents.The feeding device,calyx(compressed laterally),can be inferred to have oriented its anterior side against the currents.The calyx bract circlet opened on its anterior side where the bracts were divergent from one another allowing free entrance to currents.Formation of eddy currents within the bract circlet can be imagined to have been induced when entrance of currents was held back by the closed lateral and posterior sides of the bract circlet.This passive feeding mode seems to be such a sort of genius to process eddy feeding currents within the feeding device without any direct metabolic cost that it should therefore prove to be profitable.The rigidity of the bracts indicates that they were unable to enfold or grasp the prey.It is probable that the bracts were ciliated to sweep the food, perhaps with aid of mucous secretion to the mouth(Conway Morris,1977).The presense of a tall and slender excretory extension interpretively with an anus terminal is considered to allow an efficient dispersion of feces at a level far above the feeding device. In contrast with the passive feeding mode adopted by Dinomischus, all the living entoprocts and ectoprocts use their cillia on their tentacles to set up a feeding current.The feeding currents of entoprocts enter through intervals between the tentacles and leaves at top of the body,whereas the currents of ectoprocts enter directly through the top(Margulis and Schwartz,1982;Pearse et al.,1987).The direction of the feeding currents differs between the two groups due to their different positions of the anus that allows efficient removal of feces in each group(Text-fig.2). This passive mode of suspension-feeding without any energy cost may have been a primary contributing factor to the success of Dinomischus as epifaunal tierers,and probably also the constraint factor that caused the animal to be rare,and to be restricted within a narrow limit of environment. The biological affinity of Dinomischus has been studied by Conway Morris (1977) with the conclusion that some degree of affinity,though distant,exists between Dinomischus and the living entoprocts. Solitary epifaunal suspension feeders among broadly separated phyla,such as sponges,cnidaria,tunicates,echinoderms,ectoprocts and entoprocts have been provided with a stalk-like structure allowing the elevation of their feeding structure to a tier above the sea floor.But no provable evidence of biological affinity exists between Dinomischus and the stalked sponges, cnidaria,tunicates,echinoderms and ectoprocts,only Dinomischus is considered to ally with entoprocts by the authors. The living entoprocts may have evolved from their ancestor, Dinomischus,accompanying a reform in the feeding mode from passive to active suspension-feeding mechanics.The demineralization may have been associated with the change of the feeding mode and the bracts tend to be flexible in a manner of tentacle nature.The abandonment of excretory extension may be subsequent to the formation of a new feeding mode because such a feeding mode allows efficient removal of feces from anus within the tentacle circlet through the upward feeding current.The miniaturization in body size is interpreted as a parallel line to demineralization because the latter was associated with a declining trend of mechanic strength.Tendence of miniaturization in body size is demonstrated by the maximal length of entoprocts which becomes smaller from over 100mm in the Early Cambrian(570 Ma),to 50mm in the Middle Cambrian,and to 10mm in the present time. Internal anatomical features of Dinomischus have been previously investigated in Middle Cambrian specimens of Dinomischus isolatus by Conway Morris(1977).New fossil evidence indicates that the previous interpretation requires major revision. The preservation of one specimen in our collection seems superior to all the other ones belonging to Dinomischus and fortunately its internal anatomy shows clear.The calyx cavity is demonstrated by the saccate nature of two separate masses which are interpreted as parallel to the visceral mass and the gut sac of living entoprocts respectively(Textfigs.3,4).It is reasonable to suggest that the gut sac in the low calyx bears two vertical extensions which are regarded as oesophagus and intestine.The intestine may be connected with the excretory extension,and the anus would thus have been situated at the distal terminal of the excretory extension far beyond top of the bract circlet. The space between visceral mass(considered as gut sac in Conway Morris,1977)and calyx was regarded by Conway Morris(1977)to have been occupied by radiated strands which helped support the gut sac(here interpreted as visceral sac).However,no evidence of such strands has been observed in our specimen,instead,the visceral mass is documentarily fixed on the calyx through the bracts. Phylum Entoprocta Family Dinomischidae Conway Morris,1977 emend.Chen,Hou et Lu herein Stalked calyx with distal rigid bract circlet.Mouth and excretory extension lying within the bract circlet.Anus opening at distal terminal of excretory extension. Genus Dinomischus Conway Morris,1977 emend.Chen,Hou et Lu herein Type species: Dinomischus isolatus Conway Morris,1977 Emended diagnosis:Sessil solitary metazoan with body consisting of stalked calyx which is symmetrical bilaterally, and compressed laterally.Bract circlet possessing about 18 rigid bracts which are mostly upright but obliquely convergent on its anterior side.Calyx cavity containing visceral mass in upper part and gut mass in lower part.Excretory extension interpreted to be posited on posterior side of body within the bract circlet.Anus opening at distal terminal of the excretory extension. Dinomischus venustus sp.nov.(P1.I,figs.1—5; text-figs.3,5—7) Diagnosis: Body large,hock glass-like,with a maximum vertical length over 100mm.Calyx laterally compressed,with a conic outline in anterior(or posterior)view,and a domed outline in lateral view;calyx cavity containing visceral mass in upper part,and gut mass in lower part;excretory extension slender,vertically extending from posterior portion of the calyx.Calyx encircled distally with about 18 bracts which were mostly upright in a close arrangement to one another but in a convergent arrangement in the anterior ones.Bract angular and smooth on outer side,but wrinkly on lateral and inner sides. Description: Holotype(Cat.No.108478)represented by part and counterpart,lying parallel to the bedding plane after being compacted posteroanteriorly,showing that the matrix layers separate the body into portions,and thus the fossil lies at slightly different levels.Discrimination of the part from the counterpart appearing to be subjective.Counterpart showing an inner view of calyx cavity.Excretory extension lying parallel to the bedding plane at a higher level. Stalk incomplete;its preserved upper portion buried at an oblique angle with bedding.Part embedded in an overlying position of counterpart,in which the internal anatomy of visceral and gut masses is expresed by positive relief.Preserved length measuring 66mm,represented by incomplete stalk,calyx,calyx bracts and enlongated excretory extension.Calyx conical in outline,with a maximum width of 9.5mm as measured from its distal terminal.Visceral mass bowl-shaped in posterior(or anterior)view,with a maximum width of about 8mm as measured from near its top.Visceral mass fixed through rigid bracts with upper calyx.Gut mass conformably lying on the low calyx in posterior(or anterior)view,with upper 1.8mm of it contracted upward.Nine bracts noticed from this specimen,which are plate-like,with wrinkly inner surface,upright in posterior(or anterior)view and clustered with one another at pointed distal terminal.Bracts measuring about 12mm in length,with a 10mm free portion.Excretory extension measuring 0.8mm across,with an uncompacted diameter of 0.5mm.Preserved distal end extending to 44mm beyond the bract circlet.Two wedge-like projects noticed on the surface of the gut mass. Paratype(Cat.No.108479)preserved in a lateral compacted manner,with a length of 41mm represented by calyx,bract circlet and excretory extension,demonstrating a broadly domed outline of calyx in lateral view.Maximum lateral width measured from top of calyx to be 12mm.Excretory extension lying near the posterior margin of calyx.Nine bracts noticed,with the posterior six upright and clustered and the anterior three divergent from one another.Outer view of the bract observed to be smooth and angular. Discussion: General morphological and internal anatomical features and interpretive feed- ing mode are expressed in Text-fig.3.The length of stalk and presence of holdfast are reconstructed based on the evidence reveaded by the Canadian Middle Cambrian species Dinomischus isolatus Conway Morris,1977. Variation of the calyx profile among the specimens of Dinomischus venustus sp.nov.,and D.isolatus Conway Morris,is interpreted to be a reflection of the calyx after being compacted in different directions,so that the cross sections of the two species are elliptical with lateral compression.Nine bracts have been noticed in the USNM specimen of the Canadian species and in the two specimens of the Chinese species,providing the evidence that the complete number of bracts is about 18.The posterior five are(provably)observed to be divergent from one another,but the others clustered and upright. The present taxon bears a strong resemblance to the Middle Cambrian Canadian taxon,Dinomischus isolatus Conway Morris.The difference between the two taxa lies.in their body sizes.In addition,the visceral mass(interpreted as gut by Conway Morris,1977)of the Canadian species seems to be comparatively narrower and elongated.
出处
《古生物学报》
CAS
CSCD
北大核心
1989年第1期58-71,137,共14页
Acta Palaeontologica Sinica
