Biostratigraphy, palaeobiogeography and morphology of the Llandovery (Silurian) graptolitesCampograptus lobiferus(M’Coy) andCampograptus harpago(Törnquist)

Abstract

Synopsis The surviving type material of Campograptus lobiferus (M’Coy, 1850) from the convolutus Biozone (mid-Llandovery) of the Birkhill Shales Formation is redescribed. The material is from the Moffat and Beattock Bridge district of the Southern Uplands, Scotland, but the exact location is unknown. Comparison of Campograptus lobiferus with surviving type material of the closely related Campograptus harpago (Tornquist, 1899) suggests that they may be discriminated by thecal morphology: in harpago the uniformly tubular metatheca grows approximately at right angles to the rhabdosome before bending back through 180° so that the aperture faces dorsally; whereas in lobiferus the metatheca is initially broader and more triangulate and the aperture faces proximally or proximo-dorsally. Campograptus lobiferus occurs in the leptotheca, convolutus and lower sedgwickii biozones of the Llandovery, whereas the rarer harpago is most common in the upper part of the convolutus Biozone, perhaps ranging into the lower sedgwickii Biozone. Welsh populations of C. lobiferus (although not those from Anglesey) show consistently more slender morphologies than the Scottish type material, possibly as a result of geographical or ecophenotypic variation. C. lobiferus had global distribution, whereas C harpago seems limited to the Avalonia, Baltica and possibly Perunica palaeocontinents of the Silurian. The morphological similarity of the metatheca of C harpago and that of the younger Llandovery ( crenulata-spiralis graptolite biozones) Lapworthograptus grayae (Lapworth) demonstrates convergent evolution.