Devonian features in Ludlovian graptolites — no guide to ancestry

摘要

A diverse group of LudlovianMonograptusspecies is characterized by the retarded growth of thecal hoods, either proximally M. colonus(Barrande),M. chimaera(Barrande) andM. hauptiKühne or throughout the rhabdosome M.micropoma(Jaekel). In all these species the hoods are secondary structures, i.e. later growth additions to the thecae. By contrast, in the DevonianMonograptusspecies the thecal hoods are primary structures, i.e. the outward extension of the dorsal thecal wall. and are formed by undelayed growth that typically is completed before growth of the succeeding theca begins. It is among Ludlovian (M. uncinatusTullberg) and Pridolian (M. simihPřibyl) forms with homologous hoods that the roots of the majority of the DevonianMonograptusspecies might be found. The LudlovianM. micropomaandM. hauptishould not be affiliated to the group ofMonograptus vomerinus(Nicholson), which is highly characteristic of, and restricted to, the late Llandovery and Wenlock graptolite fauna