Devonian features in Ludlovian graptolites — no guide to ancestry

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Abstract
A diverse group of Ludlovian Monograptus spp. is characterized by the retarded growth of thecal hoods, either proximally [M. colonus (Barrande), M. chimaera (Barrande) and M. haupti Kunne] or throughout the rhabdosome [M. colonus (Barrande), M. chimaera (Barrande) and M. haupti Kunne] 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 Devonian Monograptus spp. the thecal hoods are primary structures, i.e., the outward extension of the dorsal thecal wall, and are formed by undelayed growth that is completed before growth of the succeeding theca begins. It is among Ludlovian (M. uncinatus Tullberg) and Pridolian (M. similis Pribyl) forms with homologous hoods that the roots of the majority of the Devonian Monograptus species are found. The Ludlovian M. micropoma and M. haupti should not be affiliated to the group of M. vomerinus (Nicholson), which is highly characteristic of, and restricted to, the late Llandovery and Wenlock graptolite faunas.

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