介形类系统分类与起源演化的形态、分子和化石证据

介形类(Ostracoda)因其丰富的化石记录和广布的海陆现生代表类群，而被认为是进化生物学中研究生物多样性产生机制和演变历程的颇具潜力的重要模式生物。介形类在甲壳亚门中的谱系发生位置、起源及其内部各类群间的系统关系还存在诸多争议。基于其体制构造的形态学特征，介形类被归入甲壳亚门下的颚足纲(Maxillopoda)，但来自18S rDNA序列数据分析却显示Maxillopoda不是单系群。基于化石记录和壳体形态特征，高肌虫(Bradoriida)长期以来被认为是介形类的一个祖先类群，但保存有软躯体的早寒武世化石的研究表明，Bradoriida不是介形类甚至可能也不属于甲壳类。不同的研究者所强调的壳体和肢体形态特征各异，导致介形类最大的现生类群速足目(Podocopida)的四个超科之间的关系也存在诸多推测。壳体和肢体特征在系统演化意义上的不兼容，需要分子生物学等证据的介入。分子、形态和化石证据的积累及各种信息整合是系统演化研究的必然趋势。

THE ORIGIN AND PHYLOGENETIC RELATIONSHIPS OF OSTRACODA BASED ON MOLECULAR, MORPHOLOGIC AND FOSSIL DATA

Ostracods are potentially very good model organisms for evolutionary studies because they combine excellent fossil records with a wide extant distribution, and therefore are suitable for studies on both patterns and processes leading to extant diversity. There are still controversies concerning the phylogenetic position and systematic relationships among different major lineages of Ostracoda, and their origins are hardly resolved. Ostracoda are considered to be a member of Maxillopoda based on the body plan and other morphologic characters, which were confirmed by Podocopina and Platycopina segment studies; but 18S rDNA sequence analysis does not support Maxillopoda as a valid monophyletic taxon. Bradoriids have long been regarded as ostracods, because they share some similarities in shell characters with palaeocopids; but newly found bradoriid fossils with soft parts preserved in the Lower Cambrian indicate that they do not belong to Ostracoda. A number of hypotheses on the phylogenetic relationships among major groups of Podocopida had been proposed, with no general consensus reached so far. It is suggested that future studies emphasize on combining ostracods morphological characters, fossils and molecular phylogeny in order to better resolve phylogenetic disputes of Ostracoda.