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

Abstract

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.

Keywords:

• graptolites
• Ordovician
• taxonomy
• evolution
• Scandinavia
• Sweden
• Skanegraptus janus

Acknowledgements

This research was supported by grants from the Wenner-Gren Foundations (Stockholm, Sweden), Gyllenstiernska Krapperupstiftelsen and the Royal Physiographical Society in Lund to Per Ahlberg (Lund, Sweden), who provided the access to the Krapperup drill core, kept at the Geological Department, University of Lund, Sweden. Many thanks to Kristina Lindholm (Kävlinge, Sweden) for her help with the drill core she was working on long time ago.