Synopsis

The Rhuddanian (Llandovery, Silurian) monograptid taxa praematurus Toghill and cyphus Lapworth are reassessed by review of existing and new collections. The eponymous species of the late Rhuddanian cyphus Biozone is now widely regarded as a typical Coronograptus, with essentially simple tubular thecae. Nevertheless, Lapworth, in drawing up his original description, was clearly referring mainly to biform graptolites of the Monograptus revolutus group, prominent on the type rock slabs. Nomenclatural stability is best served by referring to cyphus in its common usage, i.e. in the interpretation of Elles & Wood, largely based upon Lapworth’s material from Dob’s Linn, Scotland. The monograptid praematurus, a mid-Rhuddanian taxon, was originally described as a variety, and evolutionary precursor, of Coronograptus cyphus. However, it possesses more complex, geniculate, thecae, with well-developed thecal hoods, and probably represents one of the earliest Huttagraptus species. Huttagraptus? praematurus has a range restricted to the lower atavus Biozone and may be a subzonal marker for that interval. The roots of Coronograptus are obscure, but, in possessing essentially simple tubular thecae, it might be a separate offshoot from an Atavograptus stock.

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