盔甲鱼亚纲(无颌类)的系统发育关系

基于对53个形态学特征的讨论,首次开展了无颌类儒甲鱼亚纲的简约性分析,获得了3 个最简约的分支图,其步长为126,一致性指数为0．508,保留指数为0．801。在此基础上,修订了盔甲鱼亚纲的分类。研究结果表明,志留纪兰多维列世至温洛克世的汉阳鱼类、修水鱼类和大庸鱼类属于盔甲鱼亚纲的基干类群。在其余盔甲鱼类中,识别出3个大的单系类群, 分别是真盔甲鱼目、多鳃鱼目和华南鱼目。在盔甲鱼亚纲中,头甲背窗的起源至少发生了两次,一次在多鳃鱼目支系中,另一次在华南鱼目支系中。基于背窗特征而建立的大窗鱼类并不是一个单系类群。盔甲鱼类的年代分布指示该类群的两次辐射演化,分别是兰多维列世特列奇期基干盔甲鱼类和真盔甲鱼目的辐射演化,和早泥盆世洛霍考夫期多鳃鱼目和华南鱼目的辐射演化。

PHYLOGENETIC RELATIONSHIPS OF GALEASPIDS (AGNATHA)

We present the first parsimony analysis of the agnatban subclass Galeaspida based on the analysis of 53 morphological characters.Three most parsimonious cladograms(126 steps in length;Cl =0.508;RI=0.801)were discovered.An amended classification of the Galeaspida is proposed cor- responding to the present analysis.Our results suggest that hanyangaspids,xiushuiaspids and dayon- gaspids from the Llandovery-Wenlock of Silurian are basal galeaspids.Within the remaining galeaspids, three major monophyletic groups(the Eugaleaspidiformes,the Polybranchiaspidiformes and the Hua- nanaspidiformes)are well supported.It is shown that the dorsal fenestrae of headshield evolved twice within the Galeaspida,one in the polybranchiaspidiform lineage,and the other in the huananaspidiform lineage(nested within the Huananaspidae).The chronological distribution of galeaspids highlights two radiations of the group,one for basal galeaspids and eugaleaspids in the Telychian(Llandovery)of Silurian,and the other for polybranchiaspidiforms and huananaspidiforms in the Lochkovian of Early Devonian.