软体动物的早期演化——来自上扬子地台北缘的化石证据

The model hypothesis of mollusk evolution in Early Cambrian gradually formed. There are some problems to explain: the earliest microfossil bivalve fossils are found in Siberia platform area rather than Yangtze platform in which the mollusk fossils are very rich; the mollusk at the pedigree bottom of the mollusk evolution is mainly of controversial maikhanellids. The Terreneuvian mollusk fossils represent the primary stage of the evolution of the Cambrian mollusk, which is the key to solving the above problems. A large number of Terreneuvian mollusk fossils have been discovered in Nanjiang area, in the northern margin of the Yangtze Platform, which are mainly composed of monoplacophora fossils, a few of visible suspected bivalve fossils and large maikhanellids fossils. The research subject takes the fossils as the research objects. By means of scanning electron microscopy and energy spectrum analyzer, Micro-CT, polarizing microscope and other advanced instruments, much information of the fossil morphology, ornamentation, microstructure features will be obtained. The research will focus on mollusk fossil assemblages, maikhanellids and suspected fossil bivalves. The radiation of Early Cambrian mollusk in northern margin of Yangtze platform will be summarized. The Early Cambrian mollusk evolution model will be improved.

关于寒武纪早期的软体动物谱系演化模式的假说中，谱系底端存在一些难以解释的问题：目前最早的微体双壳类化石是发现于西伯利亚地台区而不是软体动物化石非常丰富的扬子地台区；作为谱系演化底端的软体动物是马哈螺类，但其归属存在争议。寒武系纽芬兰统中的软体动物化石特征代表了软体动物谱系演化的初级阶段，化石显微构造特征的发现和探究是解决上述问题的关键。位于扬子地台北缘的川北南江地区纽芬兰统中含有大量的软体动物化石，以单板类化石为主，可见疑似双壳类化石和大量的马哈螺类化石。以研究程度较低的南江地区纽芬兰统软体动物化石为研究对象，借助扫描电镜及能谱分析仪、Micro-CT、偏光显微镜等先进仪器，对化石形态、纹饰、显微构造等特征进行测试、信息读取，专注于软体动物化石组合特征和对比研究、马哈螺类和疑似双壳类化石的归属研究，总结扬子地台北缘寒武纪初的软体动物辐射方向，力求完善寒武纪早期软体动物谱系演化模式。

扬子地台北缘川北南江地区寒武系纽芬兰统以碳酸盐岩为主，含有丰富的微观软体动物化石，是研究寒武纪早期软体动物演化的理想区域。.南江地区寒武系纽芬兰统含两个小壳化石组合带：Anabarites trisulcatus - Protohertzina anabarica 带和Paragloborilus subglobosus- Purella squamulosa 带。而马边地区纽芬兰统可建立三个小壳化石组合带：Anabarites trisulcatus–Protohertzina anabarica 带Paragloborilus subglobosus–Purella squamulosa带和Watsonella crosbyi 带。结合碳同位素测试结果可知，马边地区是研究寒武系幸运阶与第二阶界线的又一理想区域。.寒武纪纽芬兰统壳类软体动物多样性显著增加。第一组合带中的壳类属皆为Maikhanella，第二组合带中多为helcionellid，第三组合带中主要是Wastonella。对helcionellid属种特征的系统总结，为helcionellid鉴定工作提供参考。而不同层位壳类形态明显转变，暗示可利用壳类化石划分生物带。.Maikhanellids壳板纹饰和显微构造与后期出现的helcionellid不同，且helcionellid具典型的胎壳。聚类分析也显示，helcionellid不是maikhanellids的直接后代。helcionellid类壳体无明显扭曲，新发现的对称性肌痕有别于腹足类干群。壳体的菱形外壳层和半珍珠显微结构反映了helcionellid的系统进化。.根据maikhanellids与siphogonuchitids骨片聚集标本的发现提出假说：maikhanellids与其他软体动物Aculifera干群一样，具有蛞蝓状形态，在背侧覆盖有帽状骨板和siphogonuchitids骨片。根据寒武纪早期单壳类软体动物的形态演化趋势，修改了软体动物系统演化树。该假说使得埃迪卡拉动物群中软体动物干群和早寒武世具有矿化骨片的软体动物之间建立了联系，有助于解释达尔文的有关生物进化悖论。.壳体保存成分与围岩性质密切相关。软体动物分类群多样性增加是生物适应环境而进化的结果。区内大量疑难化石的发现，为揭露寒武纪早期后生动物演化提供了重要证据。