View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002and Early to mid Devonian kept total arthropodpredator diversity up until the later part of theMiddle Devonian (Fig. 10c). Adrain et al., 2000demonstrate that Silurian alpha (i. e., within habitat)diversity of trilobites recovered to Ordovician levels.This illustrates the important point that global andlocal patterns may be decoupled, as Adrain et al.also argued. In this case, beta (between region)diversity may have decreased, possibly because theglobal trilobite fauna was re-established from a smallnumber of surviving taxa, but those surviving taxawere able to recapture the full range of life habits(niches) that trilobites had occupied prior to theOrdovician extinction. Those surviving taxa mayhave had some properties that contributed to otherchanges observed in the evolutionary dynamics ofthe group. The trilobites show a very high rate ofturnover in the Cambrian and Early Ordovician,attesting to their well-known high rate of evolutionat that time (which gives them great biostratigraphicutility). Their turnover (compared to their continuingstanding diversity) dropped off noticeably in the laterOrdovician and remained at values proportionallyless than in the Cambrian for the rest of thePaleozoic.Figure 10c illustrates that there was aprogressive shift in diversity dominance ofpredatory arthropods throughout the Paleozoic.Trilobites dominated the Cambrian and Ordovician;the eurypterids and malacostracans became asignificant proportion of total predatory arthropoddiversity in the Silurian and Devonian, althoughtrilobites continued in the majority. Then, as boththe trilobites and eurypterids declined in the laterDevonian, the malacostracans retained theirdiversity, which even increased in the Carboniferousbefore also declining to low end-Paleozoic levels.Malacostracans were the only major arthropodmarine predator taxon to survive the end-Permianextinction, and so they are solely responsible forthe post-Paleozoic increase in diversity of predatorymarine arthropods (Fig. 9b). Although notdocumented in detail in Figure 9b, much of thisdiversification from the Cretaceous to the laterCenozoic has been the radiation of the brachyurancrabs with their varied pincer-type claws. In fact,the highly varied limb differentiation ofmalacostracans clearly contrasts with the nearlyuniform and undifferentiated limbs of trilobites.These features attest to the increase in specializationthat characterizes at least some of the Cenozoicincrease in predator diversification.Echinoderms.—The predatory echinodermsalso have a pattern of higher early to mid-Paleozoicdiversity, lower diversity from the LateCarboniferous through the Triassic, and then a riseto a new peak of diversity in the later Mesozoic(Fig. 11a). Unlike the arthropods and more like theanthozoa, they only maintain, but do not increase,Late Cretaceous diversity levels in the Cenozoic.The infrequent but quite large turnover peaks alsoindicate that the fossil record of predatoryechinoderms is rather poor, but the total diversitypeaks track the same general diversity pattern asthe standing diversity plot. Figure 11b illustratesthat, although some ophiuroids and echinaceanechinoids are also predators, the vast majority ofpredatory echinoderms have always been starfish.Echinoids did not become effective predators untilthe modified and stronger lantern of the Echinaceaevolved (Parker, 1982).Vertebrates.—Predatory vertebrates have adiversity history (Fig. 12a) similar to that ofgastropods or of all predators combined, but notlike that of the Anthozoa, Cephalopoda,Arthropoda, or Echinodermata. Their genusdiversity fluctuated up and down from theOrdovician through the Early Cretaceous butremained relatively low, with boundary-crossingstanding diversity at or below 50 genera throughoutthe 400-million-year interval. It more than doubledin the Late Cretaceous and from there has increasedfive-fold during the Cenozoic.Figure 12b shows that conodontophorids werethe diversity-dominant marine vertebrate predatorsin the Ordovician and Silurian. They were replacedas diversity dominants by the combination of thefish classes as a whole in the Devonian. Thecombined fish groups have remained the diversitydominantvertebrate marine predators, with tetrapodsmaking up only a small proportion of Mesozoic andCenozoic vertebrate marine predator taxa.334