The proximal development of the Middle Ordovician graptolite Skanegraptus janus from the Krappegrup drill core of Scania, Sweden

The new axonophoran genus and species Skanegraptus janus from the Middle Ordovician Holmograptus lentus Biozone of the Krapperup drill core (Scania, SW Sweden) bears a proximal development pattern with an apparent paired origin and metathecal upward growth of th12 and th21 on the reverse side, a pattern that is identical to the externally visible structures of Mitchell’s patterns G and K and can be compared closely to the pattern E, F, H and I astogenies. It sheds new light on the evolution of the proximal development types and their complex changes and cautions to identify these patterns if internal details, such as the development of the crossing canal(s) and the precise origin of primordial thecae, are unknown from growth line evidence.