ACLADISTIC STUDY ON THE ORIGINATION OF THE EARLY DICRANOGRAPTIDS DURING THE MIDDLE-LATE ORDOVICIAN TRANSITION
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Abstract:
Based on the graptolite materials from the Hulo Formation，western Zhejiang Province and the Miaopo Formation，western Hubei Province，a phylogenetic analysis of the early dicranograptids and their related taxa is conducted in this paper. The result of the analysis based on 13 taxa and their 25 characters. The 13 taxa include the early dicranograptids Jiangxigraptus mui，J. wuningensis，J. sextans，J. vagus，J. intortus and Dicranograptus irregularis，reletated biserial taxa Undulograptus austrodentatus，U. sinicus，Archiclimacograptus caelatus，Dicaulograptus hystrix and the disputable taxa Undulograptus ?praesextans，Undulograptus dicellograptoides，and outgroup Exigraptus uniformis. The result showed that the early dicranograptids are grouped as a monophyly, and the “uni-biserial” character state of Dicranograptus and Jiangxigraptus are homologous instead of independent origin，and Dicranograptus is likely to derive from Jiangxigraptus. This sequence basically matches the stratigraphic occurrence，indicating that the cladogram and derived tree seem to represent the true phylogeny. An alternative analysis including the biostratigraphic attribute as a character produced identical result，implying a possible correlation between age and the distance of phylogenetic relations without ghost line. The character of the “two uniserial and reclined stipes” developed in Undulograptus dicellograptoides and Jiangxigraptus mui may have independent，respectively origins in Darriwilian Stage，showing a homoplasy or an iterative evolution instead of phylogenetic evolution.
