Traditional analyses of Early Phanerozoic marine diversity at the genus level show an
     explosive radiation of marine life until the Late Ordovician, followed by a phase of erratic
      decline continuing until the end of the Palaeozoic, whereas a more recent analysis
       extends the duration of this early radiation into the Devonian, 'Ihis catch-au approach
   hides an evolutionary and ecological key event long after the Ordovician radiation: the
   rapid occupation of the free water column by anisnals during the Devonian.  Here, we
   explore the timing of the occupation of the water column in the Palaeozoic and test the
   hypothesis that ecological escalation led to fundamental evolutionary changes in the
    mid-Palaeozoic marine water colunin.  According to our analyses demersal and nektonic
    modes of life were probably initiaffy driven by competition in the diversity-satlirated
     benthic habitats together with the availability of abundant planktonic food.  Escalatory
     feedback then promoted the rapid rise of nekton in the Devonian as suggested by the
     sequence and tempo of water-column occupation.