最早的栉水母动物化石——华南伊迪卡拉纪的“八臂仙母虫”

The Ediacaran ctenophore(Eoandromeda octobrachiata) from South China

华南伊迪卡拉系陡山沱组保存的磷酸盐化球状化石中有一部分被认为是最早的后生动物胚胎化石,但由于缺乏成年或幼年过渡期的化石证据,对这些最早微化石的动物属性还一直存在争议。最近在贵州江口县翁会地区伊迪卡拉(震旦)系陡山沱组的页岩中发现一个与"庙河生物群"相似的宏体碳质压膜化石群,其中包含一个盘状的宏体化石新类型——八臂仙母虫(Eoandromeda octobrachiata,Tang et al.2008),具有八条螺旋状辐射的旋臂,封闭在一个包膜内。这种八辐射结构在八射珊瑚和栉水母动物中都可见到,但同时具有的螺旋特性与现生的所有类群都无法对比。在贵州陡山沱组的"瓮安生物群"中发现的球状胚胎化石同样具有螺旋性状,尽管还没有直接的个体发育证据,但我们认为新属种可能代表这些螺旋状胚胎化石的成年实体化石。最新报道的南澳大利亚Rawnsley石英岩层中也发现相似的印痕化石,被归为同一类别,表明华南的"庙河生物群"与澳大利亚的"伊迪卡拉生物群"时代相当。翁会化石库中少量出露的三辐射化石与俄罗斯"白海生物群"的典型分子—Albumares和Anfesta可以对比。新发现的倾斜保存的八臂仙母虫化石和碳质条带呈子午线状排列的核桃状化石,可能是该类化石归属栉水母类的有力证据,这一发现推测可将栉水母类的最早化石记录从"澄江生物群"的早寒武世推前大约3千万年。

Abstract： Some phosphatized spheroidal remains in the Ediacaran Doushantuo Formation, South China, have been interpreted as the embryos of early animals. Because of the lack of adult or juvenile transitional fossil evidence,however, the animal affinities of these microfossils are open to debate. Macroscopic carbonaceous compressions in the Ediacaran （Sinian） Doushantuo shales of South China （Wenghui, Jiangkou County, Guizhou Province）, which provide a new-Miaohe-Biota-type-taphonomic window on the Ediacaran biological diversity, include the discoidal macrofossil Eoandromeda octobrachiata （Tang et aI., 2008）, with octoradiaIly arranged spiral arms that were probably housed within an outer membrane. Such an octoradial structure can be seen among the cnidarian octocorals and the ctenophores. The spiral feature is not seen in the extant groups, but it occurs in the Ediacaran ＂trilobozoan＂ Tribrachidium. Some globular embryos in the Doushantuo Formation at Weng＇an, Guizhou Province, have a similar helicospiral feature. Although there is no direct evidence from ontogeny, the authors regard E. octobrachiata as being the adult stage of these Doushantuo spiral embryos. The octoradial Ediacaran macrofossil recently reported to be preserved as casts and molds in the Rawnsley Quartzite in South Australia was classified as belonging to the same taxon. The Miaohe biota in South China and the Ediacaran biota in Australia are doubtlessly contemporaneous. A couple of triradial fossils from the Wenghui lagerstatte also suggest a possible comparison with the Ediacara trilobozoan fossils-Albumares and Anfesta from the shores of the White Sea in Russia. The recent discovery of a few skewed E. octobrachiata and walnut-shaped fossils with several carbonaceous meridian bands believed to represent lateral compressions of the same organism strongly suggests that this octoradial fossil is a ctenophoran.