THE DEVONIAN NEKTON REVOLUTION

Impressive discoveries of Neoproterozoic and Early Paleozoic Fossillagerstaetten drew attention to evolutionary and macroecological processes within these timespans. It almost seemed that nothing essential happened after the Ordovician. Contrariwise, some major ecological fluctuations have been recorded from Devonian times, several having less prominent Silurian precursors. Classic examples are the radiation of land plants, jawed fish and land animals.

Marine invertebrates show significant ecological and morphological changes: major cephalopod groups such as bactritoids as well as ammonoids evolved and reef growth increased until the Late Devonian crises. Both the rise and fall of dacryoconarids occurred, graptolites became extinct, and various mollusc clades modified early ontogenetic strategies during the Devonian, documenting a planktonic turnover. In addition, there is a drastic macroecological change in marine faunas from a demersal and planktonic habit towards a nektonic habit.

Various interpretations may explain this Devonian Nekton Revolution: (1) demersal and nektonic modes of life were initially driven by competition in the diversity-saturated benthic habitats or (2) the availability of rich planktonic food resources. Escalatory feedbacks probably promoted the rapid evolution of nekton in the Devonian. Both these radiations and Late Devonian mass-extinctions were linked to the increasing nutrient input to sea waters during eutrophication episodes.