Proximal structure and development in the Ordovician graptolite Parisograptus Chen and Zhang, 1996

The proximal development of Parisograptus Chen and Zhang is described from three–dimensionally preserved specimens. The unique development features an origin of proximal thecae like a string of pearls vertically upon each other on the reverse side of the rhabdosome with the dorsal sides of the initial stipes placed side by side. The development differs strongly from that found in the superficially similar Arienigraptus in which the first thecal pairs grow downwards side by side, even though the rhabdosome shapes are quite similar. It represents a first step towards the development of a completely biserial rhabdosome and eventually leads to the biserial, monopleural glossograptids.