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Body Size and Diversity Exemplified by Three Trilobite Clades

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Body Size and Diversity Exemplified by Three Trilobite Clades Body size and diversity exemplified by three trilobite clades TERZY TRAMMER ANdANDRZEJ KAIM Trammer, J. & Kaim, A. 1997. Body size and diversity exemplified by three trilobite clades.- Acta Palaeontologica Polonica 42, l, l-12. Cope's rule concerns only the radiation phase of a clade, overlooking the phase of the clade decline; thus it is incomplete. Changesofbody size during the entire evolutionary history of a clade are exemplified here by three trilobite groups - ftychopariina, Asaphina and Phacopida. Increasing diversity ofthe clade is associated with increase in maximum body size during the radiation phase, and decreasing diversity is generally associated with a decreasein maximum body size. Two basic patterns of the maximum body size changes are observed dwing the decline of the clade. The first one is characterized by a high correlation between diversity and the maximum body size, and indicative of species atfrition that is nonselective with respect to the body size. The second one is characterized by a weak correlation between diversity and maximum body size, and typical of selective species attrition in relation to size. Jercy Trammer, Instytut Geologii Podstawowej, Ilniwersytet Warszawski, al. Zwirki i Wigury 93, PL-02 -089 Warszaw a, P oland; Andrzej Kaim, Instytut Paleobiologii PAN, ul. Twarda 5l/53, PL-00-l14 Warszawa' Poland. Introduction The evolutionaryhistory of an extinct cladeconsists of at leasttwo phases:the phase of diversificationor radiation (when speciesnumber increase)and the phaseof its decline or reduction (when the number of speciesdrop). Cope's rule, an old (Cope 1887)but recentlymodernized (Stanley 1973) generalization, claims that most groups have evolved from small towards larger body size. This rule concernsonly the first, diversification phase of a clade's history (Stanley 1973: figs 5-7). In effect, our knowledge of body size evolution is biased and incomplete as it disregardsthe evolution of body size during the secondphase, when a clade was on the wane.The aim of the presentcontribution is to presentan analysisof changesin body sizeduring the complete evolutionary history of a clade. Body size and diversity: TRAMMER & KAIM ^45 E I -s z^^ ILI JU J 225 4. zo T u lc o 10 5 0 40 60 80 loo 12o J:"" ,.J:tt" (';T Fig' 1. Body length related to cephalonlength in Ptychopariina,Asaphina and Phacopida.Since body length and cephalonlength are strongly correlated,length of cephaloncan be used as a size-index.Data measured from Harrington et al. (1959). The problem will be exemplified by three trilobite groups:the Early Cambrian- Late Ordovician suborder Ptychopariina Richter, 1933; the Early Cambrian-earliest Silurian suborderAsaphina Salter, 1863; and by the Late Cambrianto Late Devonian .orderPhacopida Salter, 1864. Body size for individual specieswas measuredon all adult specimensof the aforementionedtaxa illustratedinthe Treatiseon Invertebrate Paleontology(Harrington et al. 1959).Such data have been proven by our analysisto be statisticallyrepresentative. Since in mostcases the illustrationsintheTreatise show only cephalons,we measuredcephalons' lengths as a proxy for body size.we found that cephalonlength is well correlatedwith the whole body length (Fig. 1) and thus may be used as a size-index.Finally, we conshuctedsize-frequency histograms for various stagesof the evolutionary history for eachtrilobite group studied. We areaware that since1959, when the trilobite volumeof Treatisewas published, a large number of new trilobite taxa has been described.we decided,however, to conductthis study on the basisof knowledgeof trilobite history recordeduntil 1959. which, incompleteas it is, demonstratestrends in trilobite evolution.The histogram relating nilobite speciesnumber to their body size (Fig, 8) is typical also for other animaltaxa (compareMay 1988:figs 2 and6; Jablonsky1996: fig.10). This validates the representativenessof the Treatisedata.It would be interestingto seeif datafrom a larger sample - such as the future edition of the Treatise - will confirm oul conclusionsbased on the limited subset. Results In the Ptychopariina, the phase of diversification continued from the Early to Late Cambrian, and the phase of reduction from the Late Cambrian to Late Ordovician (Figs 2, 3). In the Asaphina,the phaseof diversificationpersisted from the Middle cambrian to Early ordovician, and the phase of reduction from the EarJy to Late ACTA PALAEONTOLOGICAPOLOMCA (42) (I) 1oT n=2 rara 0 l----.--- -+ OiOovician Ptychopariina 10T n=B Middre n=403 O+-___.@-&-q Ordovician t8 toT n=39 Earty (/) 101& Ordovician tlJ oF6@G*@al--- - ul [L 50 @^ IL o30 t ,2o u rn dl l z 30 20 Middle Cambrian 10 0 30 20 Early '10 Cambrian 0 CEPHALON LENGTH (mm) Fig. 2. Frequency distribution of cephalon length among speciesof Ftychopariina in various intervals of their evolutionary history. During the Cambrian radiation phase, maximum body size and diversity increased,while during Ordovician reduction phasethe marimum body size and diversity decreased'Data (including stratigraphic range of the clade) from Harrington et aI. (1959). Ordovician (Figs 4, 5). We found that in both groups, maximum cephalonsize is strongly correlatedwith diversity. It increasedduring the radiation phase,and de- creasedduring the reductionphase (Figs 2-5).To the contrary,we did not find strong correlationbetween maximum cephalonsize and speciesnumber in the Phacopida (Figs6,7). Discussion Many moretrilobite specieshad smalland intermediate cephalon sizes than large sizes (Fig. 8). Such a body size pattem is largely result of cladogeneticdiffrrsion (passive trend of McShea 1994) awayfrom an originally small-sizedancestor as shown by Gould (1938)and McKinney (1990a,b). The processof diffirsion and,in addition,the highertotal originationrate of smaller-sizedspecies then large ones(Dial & Marzluff 1988;McKinney 1990b)implies that the origin of a large trilobite specieswas a rare or improbableevent. Thus the correlationbetween maximum body size and diversity may be discussedand explainedin termsof probability. 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Frequency distribution of cephalon length among speciesof Asaphina in vmious intervals of their evolutionary history. During the Cambrian-Early Ordovician diversification phase,the maximum body size and diversity increased, while during the Ordovician phase of the reduction, maximum body size and diversity decreased.Data (including stratigraphic range ofthe clade) from Harrington et aI. (1959). rapidly during only in about 10 to 15 Ma of radiation.Bivalves, in contrast,needed much more time to develop large-sizedforms. This conforms with our explanationof the phenomenonsince mammals'diversification rates were much higherthan diversi- fication rates of bivalves (Stanley 1979) so that mammalsachieved high diversity much faster. The reduction phase.- To explain why maximum body size decreasesalong with decreasingdiversity during the reductionphase, two hypothesescan be suggested: a deterministicone and a stochasticnull hypothesis.The deterministichypothesis assumeselevated extinction and/or reduced origination rates of large-bodiedtrilobite speciesin comparisonwith small-sizedones, as postulated by Stanley(1979) and Vrba Body size and diversitv: TRAMMER & KAIM vo1o0 Asaphina 90 80 m diversity 70 a m€uimum bodysize 60 r = 0.840 50 40 \\ 30 20 10 \ c c (s (sc o.S >,'6 :c)nt.S 9'F 9'< e.9 =F ,:Y O .=o qB 9E
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