A preliminary study on the relationship of the early dicranograptids based on cladistic analysis

Abstract

A cladistic analysis on the early dicranograptids and their presumably related taxa was conducted by using PAUP 4.0. For the analysis, 12 taxa were selected including six species of the early dicranograptids (Jiangxigraptus mui, Jiangxigraptus wuningensis, Dicellograptus sextans, Dicellograptus vagus, Dicellograptus intortus and Dicranograptus irregularis), some biserials (Levisograptus dicellograptoides, Levisograptus austrodentatus, Levisograptus sinicus, Archiclimacograptus caelatus and Dicaulograptus hystrix) and an outgroup (Exigraptus uniformis). Twenty-four characters, including a biostratigraphic character, were adopted to describe their morphological changes from Darriwilian (Middle Ordovician) to early Sandbian (Late Ordovician). For a comparison, cladistic analyses of 12 species with 23 characters (excluding the biostratigraphic character) and 13 species (including Levisograptus? praesextans) with 24 characters were conducted. The results indicate that all the included species of Dicellograptus, Dicranograptus and Jiangxigraptus consistently form a group that is well supported at an over 50% bootstrap level; J. mui and J. wuningensis consistently form a sister group in all the cladograms. An alternative analysis excluding the biostratigraphic character produced an identical result to that with the character included, which implies a possible correlation between age and distance of phylogenetic relations. In the results, D. vagus is a primitive sister group to the rest of the dicranograptids, suggesting a likely secondary derivation of the proximal biserial scandency in Dicranograptus. The inclusion of the proximally scandent L.? praesextans in the analysis turns out that the species is more distantly related to the dicranograptids than A.caelatus.

Keywords::

• relationship
• Darriwilian–Sandbian
• early dicranograptids
• cladistic analysis

Acknowledgements

We thank Chuck Mitchell, Jörg Maletz, Dan Goldman and Chen Xu for helpful discussions and comments. Financial supports from the Ministry of Science and Technology of China (2011ZX05008-001-10) and the Natural Science Foundation of China (41172034 and 41221001) are acknowledged. This is a contribution to the IGCP Project 591 – The Early to Middle Paleozoic Revolution.