1979/01/01 by T. N. Koren’, R. B. Rickards
paper · doi:10.1144/gsl.sp.1979.008.01.54
Summary Graptoloids became extinct in latest Early Devonian (Pragian) times, and dendroids in the late Carboniferous. There is insufficient information for the other six graptolite orders to suggest when they might have become extinct, so that in these cases it is 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 diversifications, of successively decreasing speciation, one in the Silurian (Přídolí) and two in the Devonian (Lochkov & Pragian), the last marking the extinction of 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 phytoplankton component on which the graptolites probably fed, was probably largely responsible for the extinction of this order of graptolites. Further, the changes in the nature of the sea margins during the Caledonian orogeny, and the rise of the land plants, are speculatively related to both the extinction of the graptoloids and, later, the dendroids. Competition with other active nekton feeders, except possibly the dacrionarids, is considered unlikely.