湖北三峡地区埃迪卡拉系灯影组“蝌蚪状”遗迹化石*

遗迹化石是埃迪卡拉纪存在两侧对称动物最有力的证据。但多数埃迪卡拉纪遗迹化石为简单、水平的表面爬迹或潜穴。在湖北三峡地区灯影组石板滩段含典型埃迪卡拉软躯体化石的地层中新发现一类形态特别的化石,呈蝌蚪状,一端膨大,一端细管状。通过对化石形态、同位素分析以及沉积学特征的研究,说明该化石为遗迹化石,而不是实体化石。蝌蚪状化石为一种复合迹,垂向活动形成球状的膨大端,平行藻席层活动形成近于水平的潜穴,反映了造迹生物垂向切穿藻席层并沿藻席层进行觅食的行为。该发现说明了在埃迪卡拉纪晚期已有两侧对称动物开始形成较为复杂的潜穴。

EDIACARAN TADPOLE-LIKE TRACE FOSSILS FROM THE DENGYING FORMATION IN THE YANGTZE GORGES AREA,HUBEI PROVINCE

The definitive evidence for the presence of bilaterian animals in the Ediacaran comes from trace fossils.However,most of them are limited to simple surface trails and relatively shallow burrows,representing simple behaviors of wormlike animals that lived close to the sediment surface.Here a new tadpole-like trace fossil was described from the Shibantan dark grey thinbedded laminated micritic limestone of the Dengying Formation in the Yangtze Gorges area,Hubei Province,and co-occurred with remarkably soft-bodied Ediacara-type fossils and simple trace fossils.The tadpole-like fossil consists of a narrow burrow and expanding bulb structure.Based on morphological characters, isotope analysis and sedimentary observations,the tadpole-like fossil is confirmed to be a kind of trace fossil rather than a body fossil.The tadpole-like trace fossils,closely associatedwith microbial mats,represent a probing behavior of new feeding style.The tadpole-like fossils can be interpreted as compound traces that are made up of two types of traces in contact.Respectively,the expanding bulbous represents vertical probing and the narrow burrow represents horizontal behaviors of trace maker.Regardless of the precise taxonomic affinity of the producers,the tadpole-like trace fossil is made by a bilaterian.They can penetrate the microbial mat to exploit into deeper sediment,and form modest complex burrows during the latest Ediacaran Period.