八臂仙母虫(Eoandromeda octobrachiata)新材料及其意义

New Data of Eoandromeda Octobrachiata and Their Indications

在中国贵州江口地区陡山沱组新发现的化石材料表明,伊迪卡拉/震旦纪(Ediacaran/Sinian)的宏体化石——八臂仙母虫Eoandromeda octobrachiata的原始生活状态可能为一个较高的圆锥形生命体,具有沿着侧面锥状螺旋排列的八条旋臂。旋臂表面保存有与栉水母动物的栉板可能同源的横向有机质条带,规则排列的旋臂可能与栉板带同源,具有右旋八辐射对称的特征;但是缺乏诸如触手、平衡石、极区构造和两辐射对称等的冠群栉水母类的典型性状,由此,Eoandromeda被解释归属为一类早期的干群栉水母动物。若干精美化石还保存了明显的中央环带,与八条栉板带状的旋臂相联接,和中寒武世栉水母化石的反口面圆环极为相似,推测与神经系统和运动摄食能力的发育有关,表明它有可能具备在近海或远洋浮游掠食的生活方式。在伊迪卡拉/震旦纪早期陡山沱组化石记录中出现八辐射的栉水母类化石,与后生动物种系发生的Planulozoa假设(栉水母动物Ctenophora是刺胞动物Cnidaria+两侧对称动物Bilateria的姐妹群)最为一致。

Recent discoveries in South China show that the Ediacaran Doushantuo macrofossil Eoandromeda octobrachiata had a high conical body with eight arms in helicospiral arrangement along the flanks.The arms carried transverse bands supposed to be homologous to ctenophore ctenes（comb plates）.Eoandromeda is interpreted as an early stem-group ctenophore,characterized by the synapomorphies ctenes,comb rows,and octoradial symmetry,but without crown-group synapomorphies such as tentacles,statoliths,polar fields,and biradial symmetry.Some exceptionally preserved fossils still have distinct central ring concatenated with eight comb-row-type arms.The authors infer that this derived apicalorgan-like or mouthlip-like structure as in crown coelenterata might be related to the development of neural or sensory system and the ability of locomotion or prey, and that the octoradial taxon probably had a pelagic mode of life.The early appearance in the fossil record of octoradial ctenophores is most consistent with the Planulozoa hypothesis（Ctenophora is the sister group of Cnidaria＋Bilateria）of metazoan phylogeny.