View - Kowalewski, M. - Virginia Tech

BAMBACH—SUPPORTING PREDATORSIn an earlier study, I discussed possibleinfluences on energetics in the marine ecosystem,emphasizing the changes that may have taken placewhen terrestrial biomass initially increased on landduring the Devonian and when angiospermsbecame dominant on land in the Cretaceous andEarly Cenozoic (Bambach, 1999). In that study, Inoted major changes in diversity dominance of bothpredator and prey taxa in the Devonian, whenevolutionary volatility in predators began toincrease, although overall diversity did not. Theincrease in proportion of predator taxa in theCenozoic was also noted, although the evaluationof the number of predator taxa in that paper shouldbe regarded as preliminary to the more complete,but still predominantly estimated, analysispresented here. I argued that both intervals ofchange reflected change in the energetics of theecosystem. These intervals of change map withSignor and Brett’s (1984) “precursor to theMesozoic marine revolution” and Vermeij’s (1977)classic analysis of the Mesozoic marine revolution.However, the two major changes in predators as aproportion of the diversity of the marine faunadocumented in the current study, which occur inthe recovery intervals following the devastation ofthe two era-bounding mass extinctions, do notcoincide with the suggestion of Vermeij thatrevolutions in the history of life are not linked tocrises (Vermeij, 1995, p. 145).When we consider the proportion of diversityof several different groups of taxa within thepredators as a whole, a pattern of more continuousturnover in predator diversity dominance emerges.The three groupings illustrated in Figure 20 wereselected because of some general similarities in thediversity histories of the individual taxa. Trilobites,eurypterids, arthropod incertae sedis, conodonts,placoderms, and acanthodians all reached theirmaximum diversity in the Paleozoic, weregenerally confined to the Paleozoic, and were allextinct by the end of the Triassic. When groupedtogether they comprise over 90% of the diversityof Cambrian predators and decline continuouslyin their share of predator diversity after that(Fig.20a). Predatory polychaetes and cephalopodseach display a fairly even diversity path throughthe bulk of the Phanerozoic; and anthozoans,echinoderms, and chondrichthyans have peaks ofdiversity in the Paleozoic and Cenozoic withmarkedly lower diversity in between, but their post-Paleozoic diversity increase is not dramatic andthey fit with the polychaetes and cephalopods inhaving high Paleozoic diversity with Cenozoicdiversity still in the same general range. Summedtogether (Fig. 20b), these groups, which eachreached a Paleozoic peak of diversity and alsopersisted through the Mesozoic, became thediversity dominant predators in the OrdovicianRadiation (rising from less than 10% to 70% ofpredator diversity during that time). Their share ofpredator diversity averaged about 70% through therest of the Paleozoic, fluctuating from 58% to 75%.In the Mesozoic it decreased some, but remainedover 50%, and averaged slightly below 60%, ratherthan over 65% as in the bulk of the Paleozoic. Notethat this group of taxa maintains a relatively stableproportion of predator diversity through the laterPaleozoic and early Mesozoic, despite many oftheir constituent taxa having lower absolutediversity during that interval, because total predatordiversity was lower during that time. Late in theEarly Cretaceous the share of predator diversityheld by these taxa began to drop continuously, andit is now at a low point of 10%. A third group oftaxa, the gastropods, malacostracans, Osteichthyes,and tetrapods, all have diversity maxima in theCenozoic and little or no Paleozoic diversity.Grouped together (Fig. 20c), these taxacontinuously increased their share of predatordiversity through the Phanerozoic. They comprisedless than 10% of predator diversity until the mid-Devonian, but passed 70% of total predatordiversity in the Paleocene.Many of the taxa in the groups that reachedhigh diversity in the Paleozoic are relatively lowenergy-expendingpredators (starfish, anthozoa).In my earlier paper (Bambach, 1999) I alsodocumented that the turnover in diversitydominance in predators in the Devonian may havebeen a replacement of taxa with lower energeticsby those with higher energetics (lower and higher347