The Caledonides of the British Isles--reviewed. 1979. Geological Society of London. Extinction of the graptolites

dendroids in the late Carboniferous. There is insufficient information for the other six graptolite orders to suggest when they might have become xtinct, so that in these cases it s impossible as yet to speculate how extinction occurred. The dendroids survived apparently unchanged from their first appearance in the Mid-Cambrian until the late Carboniferous, whereas the graptoloids enjoyed four major and spectacular evolutionary bursts in a planktonic, largely tropical environment: those of the Arenig/Llanvirn; the gracilis Zone; the Llandovery; and the Ludlow. The post-Ludlow strata record three further smaller diversifica-tions, of successively decreasing speciation, one in the Silurian (Ptidoli) and two in the Devonian (Lochkov & Pragian), the last marking the extinction f the graptoloids. Between the evolutionary bursts are evolutionary lows in terms of species diversity. After examining these major revolutionary events (for which speculative reasons are given) as preludes to the extinction of the graptoloids, it is concluded that this final act was result of a gradual restriction of the particular ecological niche occupied by the morphologically less varied Devonian monograptids. The nature of this and other planktonic niches is as yet unknown. A general reduction, following the Ordovician, of the planktonic biomass, and the phytoplank