摘要
We present the first parsimony analysis of the agnathan subclass Galeaspida based on the analysis of 53 morph ological characters.Three most parsimonious cladograms(126 steps in length;CI=0.508;RI=0.801)were discovered.An amended classification of the Galeaspida is proposed corresponding 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 mono phyletic groups(the Eugaleaspidiformes,the Polybranchiaspidiformes and the Huananaspidiformes)are well sup-ported.It is shown that the dorsal fenestrae of the headshield evolved twice within the Galeaspida,one in the polybranchi-aspidiform 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.
基金
This work was supported by grants from the National Natural Science Foundation of China(Grant No.40332017)
the Special Funds for Major State Basic Research Projects of China(No.G2000077704)
IGCP 491.We appreciate the review of manuscript by Chang Meemann and Zhang Jiangyong.
