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SINCE it was first suggested by Lapworth (1897) it has been generally accepted that the Graptoloids evolved from a form of Dictyonema, with a long, thread-like nema, in which ‘the pendent mode of attachment is the only one possible’. I disputed this (Kirk 1969) suggesting that if ciliary feeding currents had been drawn towards the thecae, a Dictyonema of this kind could have been maintained in an upright position on a nema attached distally to the sea floor, and that it could have become detached and independently moving as an adult, so accounting for its ubiquitous distribution. Starting from such an upright rhabdosome I attempted to show how the graptoloids might have evolved into a great variety of independently moving forms in which the sicula always opened upwards, thus obviating the necessity for explaining the unlikely reversion from pendent, through reclined, back to upright or scandent, and also the difficulty of attaching many of the forms to floating seaweed of which the existence was highly questionable.
In the discussion of this thesis, Bulman (Kirk 1969, p. 280) disputed my basic premise of an upright Dictyonema flabeUiforme as the starting point of graptoloid evolution, and cited the occurrence of ‘bulb-like’ floats at the proximal end of some D. flabelliforme rhabdosomes, which could ‘only be interpreted as a proximal buoyancy device’ so that ‘whether suspended or floating the proximal end would be uppermost’.
In the same discussion (Kirk 1969, p. 283 et seq) Rickards also challenged me on many points, among which
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