PHOSPHATIZED ANIMAL EMBRYOS FROM THE NEOPROTEROZOIC DOUSHANTUO FORMATION AT WENG'AN, GUIZHOU, SOUTH CHINA
PHOSPHATIZED ANIMAL EMBRYOS FROM THE NEOPROTEROZOIC DOUSHANTUO FORMATION AT WENG'AN, GUIZHOU, SOUTH CHINA
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DOI：
10.1666/0022-3360(2000)074
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发表时间：
2000-09
期刊：
影响因子：
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通讯作者：
S. Xiao;A. Knoll
中科院分区：
文献类型：
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作者：
S. Xiao;A. Knoll
摘要新元古代陡山沱组最上层的磷块岩。（570 ± 20 Ma）中发现了球形微体化石，其特征是外部覆盖物的雕刻和细胞分裂的精确模式。Megaspaera和Parapandorina最初分别被解释为problematica和volvocacean绿色藻类，但新发现的种群支持它们被重新解释为早期动物的卵和胚胎。化石的大尺寸（直径约半毫米），持久的包膜，精确的卵裂模式和推断的生理学与团藻类或其他藻类有着根本的不同，但这些都是在卵裂早期保存的动物胚胎的预期特征。因此，Megaspaera被解释为包裹在其卵壳内的卵，而Parapandorina则代表了同样和完全分裂的Stereoblastula。尽管它们保存得很好，但Megaspaera和Parapandorina的系统发育位置不能自信地解决，主要是因为缺乏可识别的成年形态，它们可能与之相关。Megaspaera和Parapandorina的个体特征可以在现存海绵、刺胞动物和两侧动物的卵和胚胎中找到。其他三个独特的球形形式，Megaclonophycus onustus，Caveasphaera costata n。gen.和sp.，螺旋藻呢bulbifera？发生在同一个存款中。对这些化石的解释更有问题，尽管它们也可能是动物胚胎。
Abstract Phosphorites of the uppermost Neoproterozoic Doushantuo Formation (ca. 570 ± 20 Ma) exposed at Weng'an, South China, contain globular microfossils characterized by distinctively sculpted outer coverings and precise patterns of cell division. Forms assigned to the genera Megasphaera and Parapandorina were originally interpreted as problematica and volvocacean green algae, respectively, but newly discovered populations support their reinterpretation as the eggs and embryos of early animals. The fossils' large size (about half a millimeter in diameter), persistent enveloping membrane, precise cleavage pattern, and inferred physiology are fundamentally different from those of volvocaceans or other algae, but are expected features of animal embryos preserved in early stages of cleavage. In this light, Megasphaera is interpreted as an egg enveloped within its egg case, and Parapandorina represents equally and totally cleaving stereoblastulas. Despite their exquisite preservation, the phylogenetic placement of Megasphaera and Parapandorina cannot be resolved with confidence, due largely to the absence of recognizable adult morphologies to which they might be linked. Individual characters of Megasphaera and Parapandorina can be found in eggs and embryos of extant sponges, cnidarians, and bilaterians. Three other distinctive globular forms, Megaclonophycus onustus, Caveasphaera costata n. gen. and sp., and Spiralicellula? bulbifera? occur in the same deposit. Interpretations of these fossils are more problematic, although they may also be animal embryos.