Body size and diversity exemplified by threetrilobite clades

TERZY TRAMMER ANd ANDRZEJ 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. Changes ofbody 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 decrease in 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 Wi gury 9 3, P L-02 -089 Wars zaw a, P oland ;Andrzej Kaim, Instytut Paleobiologii PAN, ul. Twarda 5l/53, PL-00-l14 Warszawa'Poland.

Introduction

The evolutionary history of an extinct clade consists of at least two phases: the phaseof diversification or radiation (when species number increase) and the phase of itsdecline or reduction (when the number of species drop). Cope's rule, an old (Cope1887) but recently modernized (Stanley 1973) generalization, claims that most groupshave evolved from small towards larger body size. This rule concerns only the first,diversification phase of a clade's history (Stanley 1973: figs 5-7). In effect, ourknowledge of body size evolution is biased and incomplete as it disregards theevolution of body size during the second phase, when a clade was on the wane. Theaim of the present contribution is to present an analysis of changes in body size duringthe complete evolutionary history of a clade.

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Body size and diversity: TRAMMER & KAIM

5

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Fig' 1. Body length related to cephalon length in Ptychopariina, Asaphina and Phacopida. Since body lengthand cephalon length are strongly correlated, length of cephalon can be used as a size-index. Data measuredfrom 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-earliestSilurian suborder Asaphina Salter, 1863; and by the Late Cambrian to Late Devonian.order Phacopida Salter, 1864. Body size for individual species was measured on alladult specimens of the aforementioned taxa illustrated inthe Treatise on InvertebratePaleontology (Harrington et al. 1959). Such data have been proven by our analysis tobe statistically representative. Since in most cases the illustrations intheTreatise showonly cephalons, we measured cephalons' lengths as a proxy for body size. we foundthat cephalon length is well correlated with the whole body length (Fig. 1) and thusmay be used as a size-index. Finally, we conshucted size-frequency histograms forvarious stages of the evolutionary history for each trilobite group studied.

We are aware that since 1959, when the trilobite volume of Treatise was published,a large number of new trilobite taxa has been described. we decided, however, toconduct this study on the basis of knowledge of trilobite history recorded until 1959.which, incomplete as it is, demonstrates trends in trilobite evolution. The histogramrelating nilobite species number to their body size (Fig, 8) is typical also for otheranimal taxa (compare May 1988: figs 2 and 6; Jablonsky 1996: fig.10). This validatesthe representativeness of the Treatise data.It would be interesting to see if data froma larger sample - such as the future edition of the Treatise - will confirm oulconclusions based on the limited subset.

Results

In the Ptychopariina, the phase of diversification continued from the Early to LateCambrian, and the phase of reduction from the Late Cambrian to Late Ordovician(Figs 2, 3). In the Asaphina, the phase of diversification persisted from the Middlecambrian to Early ordovician, and the phase of reduction from the EarJy to Late

ACTA PALAEONTOLOGICA POLOMCA (42) (I)

Ptychopariina

n=403

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30

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toT n=39 Ear ty

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30

20

1 0

0

20'10

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C E P H A L O N L E N G T H ( m m )

Fig. 2. Frequency distribution of cephalon length among species of Ftychopariina in various intervals of

their evolutionary history. During the Cambrian radiation phase, maximum body size and diversity

increased, while during Ordovician reduction phase the 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 cephalon size is

strongly correlated with diversity. It increased during the radiation phase, and de-creased during the reduction phase (Figs 2-5).To the contrary, we did not find strongcorrelation between maximum cephalon size and species number in the Phacopida(Figs 6,7).

Discussion

Many more trilobite species had small and intermediate cephalon sizes than large sizes(Fig. 8). Such a body size pattem is largely result of cladogenetic diffrrsion (passive

trend of McShea 1994) away from an originally small-sized ancestor as shown byGould (1938) and McKinney (1990a, b). The process of diffirsion and, in addition, thehigher total origination rate of smaller-sized species then large ones (Dial & Marzluff1988; McKinney 1990b) implies that the origin of a large trilobite species was a rareor improbable event. Thus the correlation between maximum body size and diversitymay be discussed and explained in terms of probability.

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ACTA PALAEONTOLOGICA POLONICA (42) (1)

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MiddleCambrian

LateOrdovician

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CEPHALON LENGTH (mm)

Fig. 4. Frequency distribution of cephalon length among species of 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, neededmuch more time to develop large-sized forms. This conforms with our explanation ofthe phenomenon since mammals'diversification rates were much higher than diversi-fication rates of bivalves (Stanley 1979) so that mammals achieved high diversitymuch faster.

The reduction phase. - To explain why maximum body size decreases alongwith decreasing diversity during the reduction phase, two hypotheses can be suggested:a deterministic one and a stochastic null hypothesis. The deterministic hypothesisassumes elevated extinction and/or reduced origination rates of large-bodied trilobitespecies in comparison with small-sized ones, as postulated by Stanley (1979) and Vrba

Asaphina

Body size and diversitv: TRAMMER & KAIM

vo1o090

80

70

60

50

40

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a m€uimumbody size

r = 0.840

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GEOLOGIC T IME (Ma)

Fig. 5. Changes of the maximum body size compared to changes of the diversity in Asaphina, plotted as apercentage of maximum. Note the strong correlation between size and diversity (r = 0.840). Data fromFig. 4. Time scale after Harland et al. (1990).

(1983) for all large species. But the assumption mentioned above is not necessary toexplain the pattern visible in Figs 2-5. The null hypothesis, on the other hand, assumesthat the taxonomic attrition of species during the reduction phase is nonselective withrespect to species'body size. For successive time periods, it assumes equal extinctionprobability q for all species present, no matter if small, intermediate or large (for thepositive test see Appendix). To make the problem simpler, the hypothesis omits theinfluence of species origination because during the reduction phase the rate of extinc-tion greatly exceeds the rate of speciation (compare Harrington et al. 1959; Foote1988). An equal extinction probability for all the species does not mean an equal totalprobability ofextinction for all body sizes present, because individual body size classesare not equally numerous at the beginning of the reduction phase (see e.g., size-fre-quency histogram for the Late Cambrian in Fig. 2).Large-bodied trilobite species arerare; in general, the smaller are trilobite species, the more frequent they are (Figs 2,4and 8). For example, only one Late Cambrian Ptychopariina species falls into thecephalon size class of 118-120 mm, but over 40 species fall into the size class of24 mm (Fig. 2). If one species from the size class 118-120 mm became extinct, the

ACTA PALAEONTOLOGTCA POLONTCA (42) (t)

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8

4

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10 20

CEPHALON LENGTH (mm)

Fig. 6. Frequency distribution of cephalon length of Phacopida species in various intervals of theirevolutionary history. Data (including stratigraphic range of the clade) from Harringlon et. al. (1959).

Body size and diversitv: TRAMMER & KAIM

o/o1oo

363

G E O L O G I C T I M E ( M a )

Fig. 7. Changes of maximum body size compared to changes of diversity in Phacopida. Note the weakcorrelation between size and diversity (r = 0.242).DatafromFig. 6. Time scale after Harland et al. (1990).

whole size class would vanish. But even if 35 species from the size class 24 mmbecame extinct, this class would continue.

In accordance with the laws of probability concerning compound events, whenextinction probability of each species is 4, then the probability of simultaneousextinction of two species is f , the probability of simultaneous extinction of threespecies equals q3 and the probability of extinction of ru species equals { . The morespecies-rich a group of species, the smaller is the probability of its complete extinction.Since classes of small body size are species-rich and classes oflarge body size are verypoor in species, classes of large-bodied species are characterized by a higher totalprobability of extinction in comparison with the classes of small-bodied species,despite the fact that the loss of species diversity is nonselective with respect to bodysize. During the waning of a clade, large species become extinct faster than small ones(Figs 2, 4), not because they are larger but because they are less numerous (compareMcKinney 1990b:p.107,frg.4.I0;Jablonsky 1996:p.277). Attrrtionof acladeduringits decline will tend to cause gradual extinction of size classes in order of theirincreasing species richness (that means classes including smaller and smaller species).In effect, the maximum body size decreases along with decreasing diversity during thereduction phase of a clade's history Gigs 2*5).

517

90

80

70

60

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40

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1 00

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r =0.242

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ACTA PALAEONTOLOGICA POLONICA (42) (I)

C E P H A L O N L E N G T H ( m m )

Fig. 8. Distribution of body size in species of Trilobita. Length of the cephalon is used as the size-index.The peak on the side of small sizes results from much higher speciation rate of the smaller species. Datameasured from Harrington et al. (1959).

We prefer this stochastic hypothesis, instead of the deterministic one, as it is thesimpleE null hypothesis. Another reason is that it seems to be supported by investiga-tions concerning cranidial shape outline which suggest nonselective species extinctionduring the decline of the Ptychopariina and Asaphina (Foote 1993).

The lack of evident dependence of maximum body size on diversity in thereduction phase. - As mentioned earlier, the maximum body size and diversity areweakly correlated in Phacopida, so it may happen that although diversity decreases, themaximum body size increases (Figs 6, 7). This strongly suggests a taxonomic attritionthat is selective with regard to body size during the reduction phase. In general,elevated origination and/or reduced extinction in regions ofmorphospace occupied bylarge-bodied species had to exist, otherwise large-bodied species, which belong tospecies-poor size classes, should have vanished first when diversity decreased. Inparticular, the origin of new large-size species in the face of decrease of total diversityresulting from the extinction of smaller species in the Late Ordovician (Fig. 6) hascaused the lack of evident dependence of maximum body size on diversity. The dataof Foote ( 1 993) also suggest selective species reduction during the decline of Phacopi-da and seem to support our conclusions in this respect.

Test for presence or absence of selectivity. - As shown above, there are twobasic patterns of body size evolution during the reduction phase of a clade. The firstone with a high correlation between the diversity and the maximum body size (Figs 3,5), and the second one that is characterizedby weak correlation between these twovariables (Fig. 7). The fust pattern may serve as an indicator that the clade decline isnonselective with regard to body size, while the second one may indicate a cladereduction selective with respect to size.

Size-diversity relationship ofrecenttaxa. - The analysis ofthe interdependenceof body size and diversity in the fossil record may add a historical perspective to thestudy of body size patterns of species diversity of Recent taxa. Size-frequency plot forspecies of a Recent taxon may be compared to a single film frame while time-series

10 Body size and diversity: TRAMMER & KAIM

plots for fossil species to a whole film sequence. Rosenzweig (1995: pp.73-:77)presented a sufilmary of knowledge of the body size patterns of the species diversityamong Recent groups. Most extant organisms display more numerous intermediate-size species than either very large or very small ones. But there are also groups wheresmall body sizes are most common. Rosenzweig (1995) admits that ecologists cannotexplain the size-diversity patterns of Recent taxa. The message from the fossil record(Stanley 1973 and this paper) is that the unimodal size-diversity relationship mayappear at the beginning of the diversification phase, and at the termination of thereduction phase of the history of a clade when diversity is relatively low. The positivelyskewed body size histograms are typical of periods of the acme of a clade whendiversity is relatively high. Thus the shape of the body size histogram of a particularRecent taxon may depend on the cunentphase of the evolutionary history of the clade.

Conclusions

1. Cope's rule did not concern a complete evolutionary history of a clade but onlyits radiation phase.

2. Cope's rule may be redefined as follows: 'During the radiation phase of a clade,the maximum body size is diversity dependent in such a way that increase in diversityis associated with increasing maximum body size'.

3. The radiation phase is selective with respect to body size because small-bodiedspecies have much higher total speciation rates than large-bodied ones.

4. During the decline of a clade, the maximum body size is in principle diversitydependent such that decrease in diversity is associated with a decreasing maximumbody size.

5. There are two basic patterns of maximum body size modifications during thedecline of a clade. The first pattern occurs when strong correlation between thediversity and the maximum body size exists, and is indicative of nonselective speciesattrition. The second pattern occurs when the correlation between diversity and themaximum body size is wealg and is indicative of the attrition which is selective inrespect to size.

Acknowledgments

We thank an aronymous reviewer, M.J. Benton, M. Foote, K. Malkowski, and M.L. McKinney forhelp and discussion during the preparation ofthis paper.

References

Cope, E.D. 1887. The Origin of the Fittest, 1426. D. Appleton & Co, New York.Dial, K.P. & Marzluff, J. 1988. Are the smallest organisms the most diyerse? - Ecology 69,1620-1624.Foote, M. 1988. Survivorship analysis of Cambrian and Ordoviciantrilobites. - Paleobiologyl4,258J1l.Foote, M. 1993. Discordance and concordance between morphological and taxonomic diversity. -

P aleobiolo gy 19, 185204.

ACTA PALAEONTOLOGICA POLONICA (42) (I)

Gould, S.J. 1988. Trends as changes in variance: a new slant on progress and directionality in evolution. -

Journal of Paleontolo gy 62, 319-329.Harlan4 w.B., Armstrong, R.L., Cox, A.V., Craig, L.8., Smith, A.G., & Smith, D.G. 1990. A Geologic

Time Scale 1989, 1J63. Cambridge University Press, New York.

Harrington, H.J., Henningsmoen, G., Howell, B.F., Jaanusson, V', Lochman-Balk, Ch', Moore R'C',

Poulsen Ch., Rasetti, F., Richter, E., Richter, R., Schmidt, H., Sdzuy, K., Struve, W', Stormer, L''

Stubblefield, C.J., Tripp, R., Weller, I.M., and Whittington, H.B. 1959. Systematic descriptions. 1n.'

R.C. Moore (ed.), Treatise on Invertebra.te Paleontology, Part O, (Arthropoda 1),01704539. Geo-

logical Society of America and University of Kansas Press, Lawrance, Kansas.

Jabloniky, D. 1996. Body size and macroevolution. 1n.' D. Jablonsky, D.H. Erwin & J.H. Lipps (eds)'

Evolutionary Paleobiology, 256-289. University of Chicago Press, Chicago'

MacFadden, B.J. 1986. Fossil horses from 'Eohippus'(Hyracotherium) to Eqaus: scaling, Cope's Law, and

the evolution of body size. - P al e obiolo gy 12, 35 5-3 69.

May, R.M. 1988. How many species are there on Earth'! - Science24l'1441-1449'McKinney, M.L. 1990a. Classifying and analysing evolutionary trends. 12.' K.J. McNamara (ed.), EvoI-

utionary Trends, 23-58. The University of Arizona Press, Tucson.

McKinney, M.L. 1990b. Trends in body-size evolution. In: K.I. McNamara (ed.), Evolutionary Trends,

75-l 18. The University ofArizona Press, Tucson.McShea,D.W. lgg4.Mechanismoflarge-scaleevolutionarytrends.-Evolution48,l747-1763.Miiller, A.H. 1961. Grossabliiufe der Stammesgeschichte,l-116. G' Fischer, Jena.

Miiller, A.H. 1963. Lehrbuch der Paleiozoologie. Allgemeine Grundlagen,l-387. G. Fischer, Jena.

Rosenzweig, M.L. 1995. Species Diversity in Space and Time, 1-436. Cambridge University Press,

Cambridge.Stanley, S.M. 1973. An explanation for Cope's ntle. - Evolution21,l-26.Stanley, S.M.lg7g.Macroevolution: PattemandProcess,l-332. W.H. Freeman, San Francisco.

Vrba, E.S. 1983. Macroevolutionarytrends: new perspectives ontheroles ofadaptationandincidentaleffect.- Science22l,387-389.

11

ZaleLnofi0 migdzy maksymalnq wielko6ciq cialaa zt62nicowaiieil gatunkowym na przykladzie trzechgrup trylobit6w

JERZY TRAMMER i ANDRZEJ KAIM

Streszczenie

Ewolucyjna historia wymarlej grupy sklada sig przynajmniej z dwtfaz: zfazy radiacji- gdy liczba gatunk6w sig zwigksza , orazfazy redukcji - kiedy liczba gatunk6w spada.

W fazie radiacji, zgodnie zrcgul4Cope'a, wrazzrosn4c4 liczb4 gatunk6w, roSnie tez

na drodze kladogenerycznej dyfu4i (Gould 1988; McKinney 1990a, b) maksymalna

wielko6i ciala w grupie (Fig. 2--7), tzn. pojawiaj4 sig gatunki obejmuj4ce osobniki

o corazto wigkszych rozmiarachciala. Natomiast w fazie redukcji zachodzi odwrotnyproces: zmntejszq4cej sig liczbie gatunk6w howatzyszy r6wnoczesny spadek maksy-malnej wielkoSci ciala w grupie @ig. 2a). Silna korelacj a migdzy liczb4 gatunk6w

a maksymalna wielkoscia ciala w czasie redukcji grupy (Fig. 3, 5) wskazuje, zepowoduj4ce redukcjg wymieranie odbywalo sig bez selekcji na wielko6i. Slaba kore-

lacjamigdzy obiema wielkoSciami (Fig. 7) jest natomiast przejawem istnienia doboru

zaleinego od rozmiar6w ciala w czasie wymierania'

12

Appendix

Body size and diversity: TRAMMER & KAIM

expected

Table 1 and Fig. 1. Expected and found species number of Early Ordovician Ptychopariina to test the nullhypothesis.

size class (mm) o2 1 / +o G8 8-10 10,t2 I2-14 14-16 r6-18

speclesnumber

Late Cambrian(found) 12 43 30 3 I 24 l 3 l l 1 l 5

EarlyOrdovician

found 5 i0 9 3 2 I 1

expected zXl 813 naL 6!2 5jl2 2X1 zXl 2+l l+ l

size class (mm) t8t0 a 1 a ^ 24-26 2628 28-i0 30 32 J Z--)+

speclesnumber

Late Cambrian(found) l 2 4 2 l 4 0 I

EarlyOrdovician

found 0 0 0 0 0 0

expected l+t 0d-1 l+t 0t1 1+t 1+l 0 0

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24 26 28 30 32 34 36

L E N G T HC E P H A L O N

Expected species number for each size class was calculated according to the equation:

N z = N r p t { t t r ' p . t t - p )

where: Nz - expected Early Ordovician species number in a given size class, Nr - Late Cambrianspecies number found in a given size class, p - probability of species survival (p = Ntz/Nrr), Ntr -total found Late Cambrian species number, Nz - total found Early Ordovician species number.