Graptolites – survival in the palaeozoic seas

 Imagine the vast Palaeozoic oceans: graptolites (Pterobranchia, Graptolithina) appear as small and inconspicuous benthic faunal elements in the early Cambrian, later becoming one of the dominating planktic groups of macrofauna in Ordovician and Silurian marine environments. Their unusual organic housing secretions (tubaria) in the fossil record makes them one of the most important biostratigraphic markers for the Ordovician to early Devonian time intervals. The occurrence, especially of the planktic graptolites, in black shales led to the common notion of a life style that related them to anoxic or dysaerobic environments, but is merely a preservational aspect. Their demise in the early Devonian may be related to the origination and diversification of large plankton feeding organisms in the marine realm as the graptolites were unable to complete with the much faster growth and reproduction of smaller, non-colonial organisms that did not need to waste time producing a housing construction. Thus, from the later Devonian onwards, the graptolites were largely gone and the oceans were teeming with fast reproducing and growing micro-and macroplankton. Also larger predatory animals evolved as plankton feeders, that may have preyed on the graptolites. Only a few cryptic benthic species survived into the modern world.