Article: Ordovician graptolites from the Great Paxton borehole, Cambridgeshire

The borehole penetrated beds with a lower Llanvirn fauna containing Glyptograptus cf. denatus (Brongniart), some other small and unidentified diplograptids, a Paraulograptus? sp., Didymograptus pluto sp. nov. (pendent), and a Didymograptus sp. (extensiform). The assemblage is quite different from others in Britain, but matches well with several from the Baltic and east European regions. D. pluto is described, with details of its proximal development, astogeny, and intrapopulation variation. Comparisons with D. bifidus (J. Hall) and D. murchisoni (Beck), which are species of quite different ages, show that convergent evolution strongly affected these pendent didymograptids. The different styles of proximal development (sicular shape, budding sequence) are the best means of unravelling these homeomorphies.