Astogenetic aspects of the dendroid graptolite Airograptus (Late Cambrian-Early Ordovician) and their phylogenetic implications

Based on a Backscatter Electron (BSE) examination of partially 3-dimensional graptolite specimens of Airograptus from the Tremadocian (Early Ordovician) Yehli Formation at Dayangcha, Baishan of Jilin Province, NE China, the thecal structure and astogenetic aspects of this genus (except for the missing proximal part) are described and illustrated in detail. In the present collection, no furcating but only peltate thecal apertural processes are confirmed. A comparison between the presumably benthic dendroid Airograptus and Rhabdinopora, the earliest planktonic graptolite, indicates that there are remarkable similarities in astogenetic aspects between them, although there are also differences in the development of dissepiments and thecal apertural processes. A possible ancestor-descendent relationship between the two genera is herein proposed.