ECOLOGICAL ASSESSMENTS REVEAL COMPLEX HISTORY OF PALEOZOIC NEKTONIZATION

Owing to its excellent fossil record, the marine benthos dominates models of faunal succession and macroevolution. The pelagic realm has received considerably less attention, and the limited data available tend to emphasize intra-clade interactions over global patterns. This combination may obscure major biospheric transformations behind events in the benthos. Recently, it was suggested that the water column was underexploited by active swimming organisms, or nekton, during the Great Ordovician Biodiversification Event (GOBE), when most higher taxa originated. Rather, distinct clades developed nektic morphologies simultaneously during a Devonian phase of escalating competition in the crowded nektobenthos. This Devonian Nekton Revolution (DNR) purportedly transformed the structure of aquatic ecosystems, establishing a configuration that has been roughly maintained to the Recent. The DNR hypothesis relies on the diversity metrics of several predominantly nektic, planktic, or nektobenthic higher taxa. This approach masks significant differences within sub-taxa and overlooks pelagic organisms nested within benthic clades. Here we test the DNR hypothesis using all Paleozoic entries within the Paleobiology Database and Sepkoski’s Compendium. Each non-benthic clade was assessed at the finest taxonomic resolution possible in order to describe its ecology. Genera were assigned to one of three megaguilds representing varying degrees of swimming proficiency: plankton, nektobenthos, or nekton. The resultant dataset reveals a complex history of nektonization that was initiated earlier and progressed more gradually than postulated by the DNR narrative. There does not appear to have been any pronounced, discrete nektonization event after the GOBE. Early Paleozoic oceans may have been significantly more complex and “modern” than previously envisioned, with successive biotic events altering faunal composition through time, but not substantially revolutionizing ecosystem fundamentals.