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Paleobiology, 31(2), 2005, pp. 269–290

Evolutionary dynamics of gastropod size across the end-Permianextinction and through the Triassic recovery interval

Jonathan L. Payne

Abstract.—A global database of gastropod sizes from the Permian through the Middle Triassic doc-uments trends in gastropod shell size and permits tests of the suggestion that Early Triassic gas-tropods were everywhere unusually small. Analysis of the database shows that no specimens ofunambiguous Early Triassic age larger than 2.6 cm have been reported, in contrast to common 5–10-cm specimens of both Permian and Middle Triassic age. The loss of large gastropods is abrupteven at a fine scale of stratigraphic resolution, whereas the return of larger individuals in the Mid-dle Triassic appears gradual when finely resolved. Taphonomic and sampling biases do not ade-quately explain the absence of large Early Triassic gastropods. Examination of size trends by genusdemonstrates that the size decrease across the Permian/Triassic boundary is compatible with bothsize-selective extinction at the species level and anagenetic size change within lineages. Size in-crease in the Middle Triassic resulted from the origination of large species within genera that haveEarly Triassic fossil records and the occurrence of new genera containing large species during theMiddle Triassic. Genera recorded from the Permian and Middle Triassic but not the Early Triassic(‘‘Lazarus taxa’’) do not contribute to observed size increase in the Middle Triassic. Moreover, Laz-arus taxa lack large species and exhibit low species richness during both the Permian and the Mid-dle Triassic, suggesting that they survived as small, rare forms rather than existing at large sizesin Early Triassic refugia. The ecological opportunities and selective pressures that produced largegastropods during most intervals of the Phanerozoic evidently did not operate in Early Triassicoceans. Whether this reflects low predation or competitive pressure, r-selection facilitated by highprimary production, or physical barriers to large size remains poorly understood.

Jonathan L. Payne. Department of Earth and Planetary Sciences, Harvard University, Cambridge, Mas-sachusetts 02138. E-mail: [email protected]

Introduction

The end-Permian extinction devastated ma-rine ecosystems. Estimates of global species-level extinction range from 88–96% consider-ing the Middle and Late Permian extinctionstogether (Raup 1979) to 76–84% consideringonly the Late Permian (Stanley and Yang1994). Intense collecting efforts in the Meishanquarries of eastern China have demonstratedthat the major extinction pulse occurred at theend of the Changsingian (latest Permian), sug-gesting a global and catastrophic extinctionevent (Jin et al. 2000). The consequences of theextinction are seen in the low diversity, cos-mopolitan fauna found in the wake of theevent (e.g., Broglio Loriga et al. 1986; Schubertand Bottjer 1995; Fraiser and Bottjer 1999;Rodland and Bottjer 2001), the temporary ces-sation of metazoan reef formation (Flugel1994), and the reorganization of ecologicalsystems (Bambach et al. 2002). The preserva-tion of wrinkle structures from microbial mats

is indicative of extremely low levels of biotur-bation in Early Triassic marine sediments(Pruss et al. 2004). Stromatolites, thromboli-tes, and seafloor crystal fans in marine shelfsettings (e.g., Schubert and Bottjer 1992; Baudet al. 1997; Sano and Nakashima 1997; Ker-shaw et al. 1999; Lehrmann 1999; Woods et al.1999; Kershaw et al. 2002; Lehrmann et al.2003) and large excursions in the d13C of car-bonates not only at the Permian/Triassicboundary (e.g., Magaritz and Holser 1991) butalso through the remainder of the Lower Tri-assic and into the base of the Anisian (Baud etal. 1996; Atudorei and Baud 1997; Atudorei1999; Horacek et al. 2001; Payne et al. 2004)may reflect impoverished Early Triassic eco-systems, fluctuations in seawater chemistry, orboth. Recovery of diversity and communityecology appears to have been delayed untilthe end of the Early Triassic or beginning ofthe Middle Triassic (Batten 1973; Pan and Er-win 1994; Schubert and Bottjer 1995; Shen andShi 1996; Rong and Shen 2002). For example,

270 JONATHAN L. PAYNE

a large number of Lazarus taxa (Jablonski1986) disappear from the fossil record at theend of the Permian and return to the fossil re-cord in the Middle Triassic after an absence ofapproximately 4–5 Myr (e.g., calcareoussponges, calcifying algae, and many gastro-pods) (Batten 1973; Oliver 1980; Flugel 1985;Riedel and Senowbari-Daryan 1991; Pan andErwin 1994; Erwin 1996).

One far-reaching effect of the end-Permianextinction is the small size of Early Triassic bi-otas (e.g., Broglio Loriga et al. 1986; Schubertand Bottjer 1995; Fraiser and Bottjer 2004). Al-though much of the information on organismsize during the Early Triassic is anecdotal,Twitchett (1999) has shown that mean diam-eter of burrows in the Werfen Formation of theItalian Dolomites decreased across the Perm-ian/Triassic boundary from approximately 10mm to 2 mm. More recently, Fraiser and Bott-jer (2004) demonstrated that the assemblage ofgastropods in the Smithian Sinbad Limestoneof Utah is dominated by small taxa, in contrastto assemblages representing similar environ-ments from the Middle Permian of west Texasand the Anisian of southwest China. Further-more, they showed that gastropods largerthan 2 cm are completely absent from the Sin-bad Limestone. These regional studies high-light the need for complementary data on theglobal scale. The study presented below ad-dresses the changes in gastropod size globallyfrom the beginning of the Permian throughthe end of the Middle Triassic, using a com-prehensive database of gastropod sizes gath-ered from the literature. The data are consis-tent with the results from previous studies(Twitchett 1999; Fraiser and Bottjer 2004) andconfirm Fraiser and Bottjer’s (2004) sugges-tion that changes in the size distribution of re-gional assemblages are representative of glob-al trends in body size. Furthermore, theydemonstrate that the loss of large gastropodsoccurred between the latest Permian (Chang-singian) and earliest Triassic (Griesbachian)stages, and that substantial increase in maxi-mum size did not begin until the Anisian(Middle Triassic). They also indicate that thesmall size of Early Triassic gastropods ac-counts, at least in part, for the large numberof Early Triassic Lazarus gastropod genera.

Method

I constructed a global database containingsize measurements (length and width) for1612 species occurrences of gastropods fromthe beginning of the Permian through the endof the Middle Triassic, focusing especially onthe Early Triassic. Sources used to compile thedatabase are listed in the Appendix. Most datawere gathered from monographs of gastro-pods and other mollusks from single sites orregions, although data were also gatheredfrom more general papers in which gastro-pods were identified to the genus level andfigured to scale. Genus and species level tax-onomy follows the original authors except inthe case of clear consensus for an alternativeclassification in subsequent work.

Selection of Specimens. Owing to the widevariation in methods of reporting amongmonographs, only one specimen was recordedper species per monograph. Where size dataor scaled figures were available for multiplespecimens, the holotype was chosen if identi-fied. When the holotype was not figured oridentified, the specimen with the lowest mu-seum number was chosen. If museum num-bers were not available, the first specimen fig-ured was chosen, as this tended to be the best-preserved specimen (and therefore most likelyto be correctly identified). Where no museumnumbers were listed, but the specimens werelisted in strict order of size, the median spec-imen was chosen.

Of course, numerous sampling and meth-odological issues must be addressed in any at-tempt to compile a database of sizes. The sizesof individuals within a single species vary asa function of environment and age, amongother factors, and no single specimen can ac-count for all of these influences. Also, withinany higher-level classification, the number ofindividuals will vary among taxa. Therefore,determination of the average size of gastro-pods from any interval depends upon howtaxa are weighted. For example, the mean sizewill change depending on whether one spec-imen is selected per species or all measuredspecimens are used. Inclusion of all specimenspreferentially weights the more abundant spe-cies, more prolific collectors/taxonomists, and

271PERMIAN-TRIASSIC GASTROPOD SIZE

FIGURE 1. Histogram of the sizes of all gastropods inthe database (gray) and the sizes of 38 specimens of Tap-inotomaria rugosa from the Delaware Basin of Texas de-scribed by Batten (1958) (black) illustrating the fact thatthe size range of specimens within a species is muchsmaller than the size range of all species.

FIGURE 2. Timescale used in this study and abbrevia-tions used in subsequent figures. The timescale and agesfollow the 2004 IUGS stratigraphic chart for the sake ofconsistency, although the Early Triassic is subdivided atthe substage level.

FIGURE 3. Shell volumes of all gastropods in the database by epoch (A) and by age (B). Plots include only specimensthat can be assigned at the appropriate level of precision.

more fossiliferous localities. The use of a sin-gle specimen per species per monograph hasbeen chosen as a reasonable compromise be-tween taking one datum per species and in-cluding all abundance data. By using thismethod, geographically widespread taxa arecounted in each locality, rather than only once.Variation of size within species is small incomparison with the variation across the en-tire data set (Fig. 1), supporting the approxi-mation of species size by the size of a singlespecimen (see ‘‘Results’’ for further investi-gation of this approach).

Measurement. Specimens were selectedonly when both maximum shell height (par-allel to coiling axis) and width (perpendicular

to coiling axis) could be determined. Mea-surements were taken from texts wheneveravailable, or measured with digital calipers tothe nearest 0.1 mm from scaled figures. Themeasurement of size used here is the shell vol-ume, approximated as a cone with the maxi-mum height and width of the measured spec-imen. The conical approximation compareswell with actual shell volume determined bydisplacement and can be used to estimate bio-mass (Powell and Stanton 1985).

Results

The timescale used is shown in Figure 2 andthe sizes of all gastropods in the database areplotted in Figure 3. The results are shown atepoch- (Fig. 3A) and age-level (Fig. 3B) strati-graphic resolution. Only those specimens thatcan be confidently assigned at the chosen level


TABLE 1. Summary statistics of gastropod size (in log mm3), number of specimens, and number of source studiespresented at three levels of stratigraphic resolution. For reference in the size units: 0 5 1 mm3; 3 5 1 cm3; 5 5 100cm3 (about the size of an orange).

Interval Code Maximum Mean Median Minimum nNo. of

sources

Middle Triassic T2 5.73 2.15 2.17 21.20 732 17Early Triassic T1 3.32 1.46 1.62 21.30 71 12Permian P 5.72 2.12 2.17 22.48 809 41

Middle Triassic T2 5.73 2.15 2.17 21.20 732 17Early Triassic T1 3.32 1.46 1.62 21.30 71 12Late Permian P3 4.66 1.33 1.50 22.48 215 8Middle Permian P2 5.72 2.27 2.17 0.09 277 20Early Permian P1 5.35 2.40 2.35 20.40 285 22

Ladinian T2-2 5.73 1.87 1.97 21.20 408 9Anisian T2-1 4.57 2.48 2.52 0.23 324 10Spathian T1-4 3.32 1.19 0.71 20.56 9 2Smithian T1-3 2.59 1.05 1.22 21.07 27 1Dienerian T1-2 2.56 1.98 1.89 1.42 5 1Griesbachian T1-1 2.40 1.15 1.62 21.30 15 6Changsingian P3-2 4.48 1.24 1.45 22.48 137 3Wuchiapingian P3-1 4.66 1.41 1.49 20.72 74 6Capitanian P2-3 2.26 1.48 1.22 0.87 8 1Wordian P2-2 5.08 2.27 2.23 0.31 179 12Roadian P2-1 4.62 2.17 1.96 0.09 70 12Kungurian P1-3 4.82 2.29 2.32 20.40 138 8Artinskian P1-2 3.85 2.76 2.90 1.28 9 5Assel./Sakmar. P1-1 4.40 3.27 3.54 1.46 9 5

of stratigraphic resolution are included in eachplot. The logarithmic scale was chosen be-cause it is more useful in statistical compari-son of right-skewed distributions typical ofbody sizes among species within a clade, andbecause evolutionary rates of change are typ-ically measured on logarithmic scales (e.g.,darwins, haldanes; Gingerich 1983). It also hasthe advantage of clearly representing changesat the small end of the size spectrum that areextremely compressed on a linear scale.

Maximum. The most obvious changeacross the Permian/Triassic boundary is thedecrease in maximum size among all gastro-pods, and the subsequent increase in maxi-mum size during the Middle Triassic (Table 1,Fig. 3). Figure 4 shows conical sketches, drawnto scale, of the largest gastropod specimensfrom each interval. There is a striking absenceof large gastropods during the Early Triassic.The largest Permian gastropods have volumesup to 500,000 mm3 (500 cm3), correspondingto a length and diameter of approximately 12cm, with common 5–6-cm specimens. In con-trast, the largest Early Triassic gastropod hasa volume of 2000 mm3 (2 cm3), 250 timessmaller than the largest Permian gastropod.

The largest known gastropod from the Gries-bachian is 250 mm3 (0.25 cm3). Remarkably,not a single Griesbachian specimen is greaterthan 1.5 cm long. The first Triassic gastropodsto exceed a volume of 1000 mm3 (1 cm3) or alength of 2 cm occur in the Spathian, at the endof the Early Triassic.

The decrease in maximum size across thePermian/Triassic boundary appears abrupt atall levels of resolution, whereas the increasethrough the Triassic appears increasinglygradual with greater stratigraphic precision.The largest Anisian gastropod in the databaseis 20,000 mm3 (3.7 cm high and 4.4 cm wide),and the largest Ladinian gastropod slightlyexceeds 500,000 mm3, corresponding to alength of 24 cm and a width of 9 cm. Numer-ous Anisian and Ladinian specimens are larg-er than the largest Early Triassic gastropodsby nearly an order of magnitude. FollowingPowell and Stanton’s (1985) empirical deter-mination of a correlation between shell vol-ume and biomass with a slope of 0.77 (on a logscale) for modern gastropods, the largest gas-tropods in the Early Triassic likely had only1/100th the biomass of the largest Permian andMiddle Triassic gastropods, and the largest


FIGURE 4. Conical sketches of the largest known gastropod specimens from the Permian and Triassic drawn toscale in height and width.

Griesbachian gastropod was 300 times smallerthan the largest Permian and Middle Triassicspecimens.

One specimen of the bellerophontid gastro-pod Retispira bittneri with a diameter of 5 cmwas reported as Early Triassic in age (Yoch-elson et al. 1985). This specimen is twice thelength and nearly seven times the volume ofall other Early Triassic specimens, and ap-proximately 50 times the volume of the nextlargest Griesbachian specimen. According toYochelson et al. (1985: p. 100) this specimenfrom Wyoming was found in float ‘‘as a moldon a piece of dolomitic limestone . . . .’’ At thisparticular locality, dolomitic Permian stratacrop out upslope of Triassic rocks, thus pro-viding the possibility that the float might de-

rive from Permian outcrops. Yochelson et al.(1985: p.100) went on to state, ‘‘Thus, from itslithology there was the possibility that therock bearing the holotype of this species wasPermian float. It is perfectly possible for ablock of Permian limestone to be moved downthe slope and onto the Triassic and then bemasked by other debris so as to simulate trueoutcrop.’’ Although they located a possibleTriassic source bed for the specimen (also theholotype and only type material), it couldonly be determined by lithological similarityand the presence of bellerophontid gastro-pods in the source bed. However, the largestbellerophontids on the figured slab from thepresumed Early Triassic source bed (Yochel-son et al. 1985: Figs. 1–4) are only about 1 cm


FIGURE 5. Shell volumes of silicified Late Permian gas-tropods described by Pan and Erwin (2002) comparedwith the shell volumes of all other Late Permian speci-mens in the database.

in diameter. The fact that the holotype wasdiscovered in float from an area with fossilif-erous Permian beds upslope and is an extremeoutlier in size locally and globally supportsthe interpretation that it derives from Permianstrata. Therefore, this specimen was excludedfrom the database on the basis of its uncertain(and likely incorrect) age assignment.

Minimum. Changes in minimum size aremore difficult to assess. Previous workers(e.g., Jablonski 1996) have noted that thesmallest molluscan species are often the mostpoorly sampled despite constituting a signif-icant fraction of the diversity (see, e.g., Bouch-et et al. 2002). Because they are sufficientlysmall (, about 2 mm) to make collection,preparation, and identification difficult (see,e.g., Kidwell 2001), it is likely that the smallestgastropods are the most poorly sampledthrough this entire interval. A recent mono-graph by Pan and Erwin (2002) of silicifiedgastropods from the latest Permian of south-ern China includes many species much small-er than any found elsewhere in my data set.The size distribution of the silicified speciesdescribed by Pan and Erwin (2002) contrastssharply with the size distribution of speciesfound in all other studies of the Late Permian(Fig. 5), clearly illustrating the importance ofpreservation and preparation for sampling thesmall end of the size spectrum. Although theabundance of small gastropod species in theLate Permian could reflect a real evolutionaryphenomenon, it seems more likely that smaller

gastropods are missing from other time inter-vals because they are less well preserved. Si-licification is capable of preserving in exqui-site detail much smaller specimens than mightotherwise be preserved; therefore, the mini-mum size recorded for a given time intervalmay depend strongly on the abundance andenvironmental settings of silicified faunas aswell as the subsequent preparation of thespecimens in the laboratory. It also appearsthat through time the largest species havebeen described earlier than the smallest, andso small species may be missed in some tax-onomic work if there is a great abundance oflarger forms (see discussion of monographicbias below). Because large forms are absent inthe Early Triassic, the sampling of smallerforms may be more complete than would beexpected for a comparable amount of work onanother time interval. Thus, given that bothtaphonomic and sampling biases most strong-ly affect the small end of the size spectrum, itseems best to say that there is no discernibletrend in minimum size through the Permian–Triassic interval.

Mean and Median. At the coarsest strati-graphic resolution, the mean and median siz-es in the Early Triassic are smaller than in thePermian and the Middle Triassic (Table 1).With finer stratigraphic subdivision, both thePermian decrease and Triassic increase ofmean and median size appear to be moregradual. The Middle to Late Permian decreasemay result from the extinction at the end ofthe Guadalupian (Stanley and Yang 1994;Shen and Shi 1996), a more gradual trend to-ward small size through the Permian, or both.Owing to the small number of studies of LatePermian gastropods and the numerous smallspecies described by Pan and Erwin (2002), itis difficult to distinguish biological and sam-pling factors in the apparent size change. Forthis reason, I bin the Permian data together formost comparisons of the Permian with theEarly and Middle Triassic. Binning reducesthe effect of relatively poor and biased sam-pling of the Late Permian faunas but also ob-scures patterns within the Permian. Sizetrends within the Permian are addressed sep-arately whenever possible.


FIGURE 6. Histograms of shell volume. A, All described Early Triassic specimens. B, Early Triassic specimens in-cluded in the database.

FIGURE 7. The size of each of the five largest specimenspresented here is the median of five iterations of simplerandom sampling of 71 specimens without replacement.This analysis demonstrates that the lack of large speci-mens in the Early Triassic is not due merely to the small-er number of specimens described from that interval.

Is the Size Decrease a Sampling Artifact?

In addition to evolutionary change, thequality of the fossil record, the way in whichfossils were collected and described, and theway in which my database was compiledcould account for an apparent drop in maxi-mum size, and it is necessary first to explorethe possibility that one or more of these fac-tors might account for the observed pattern.

Use of Single Specimens. Because the data-base consists of only a single specimen perspecies per monograph, one might worry thatthe compilation strategy excluded much largerEarly Triassic gastropods from the study. Theinclusion of all Early Triassic gastropods illus-trated or reported to scale in the literature,however, changes the minimum but not themaximum size and does not affect the shapeof the Early Triassic size distribution (Fig. 6).

The largest specimens reported from LowerTriassic strata are much smaller than the se-lected specimens taken from the Permian andMiddle Triassic literature. Inclusion of all re-ported specimens from all time intervalscould only enhance the contrast between theEarly Triassic and the surrounding intervals.

Sample Size. Many fewer gastropods havebeen described from the Early Triassic thanfrom the Permian or Middle Triassic. Becausethe maximum size of a given time interval candepend strongly on the number of specimensin that interval, equivalent sample sizes are re-quired for meaningful comparison of the max-imum size. Comparability is achieved by sub-sampling the data to the same number of spec-imens from each time interval. Subsamplingof the Permian and Middle Triassic data to thenumber of Early Triassic specimens (71) dem-onstrates that the decrease in maximum sizeis not merely a consequence of the small num-ber of specimens (Fig. 7). All Early Triassicspecimens fall below the top five Permianspecimens even if only 71 randomly selectedPermian specimens are considered. The larg-est Early Triassic gastropod would be morethan an order of magnitude smaller than thelargest specimen from the Permian, LatePermian, or Middle Triassic even if the sam-ples consisted of equal numbers of specimens.In fact, on average, the largest Early Triassicspecimen would have ranked thirteenth out of71 in the subsampled Permian data set, andthe next four largest Early Triassic specimenswould have ranked seventeenth, eighteenth,


nineteenth, or twenty-fourth, respectively, ifthey had been in the subsampled Permiandata. If only Late Permian data were includedfor subsampling, the largest five Early Triassicspecimens would have ranked seventh, tenth,eleventh, twelfth, or fourteenth. The largestEarly Triassic specimen would be the fifthlargest Middle Triassic specimen and the nextfour Early Triassic specimens would rankninth, tenth, eleventh, or sixteenth. Even atcomparable levels of sampling, the largestPermian, Late Permian, and Middle Triassicgastropods are significantly larger than thelargest Early Triassic specimens, demonstrat-ing that relatively sparse sampling of the Ear-ly Triassic cannot explain the absence of largeEarly Triassic gastropods.

Taphonomy. Changes in quality and styleof preservation could, in principle, shape thesize changes observed in the Permian and Tri-assic. It is difficult to explain the absence oflarge Early Triassic forms in this way, how-ever, because few taphonomic factors are bi-ased against the preservation of larger speci-mens. Even taphonomic processes known tobe biased toward preserving small specimens,such as silicification, preserve numerous spec-imens in the Permian that are larger than allreported Early Triassic gastropods (e.g., Yoch-elson 1956; Erwin 1988; Pan and Erwin 2002).Silicification of Early Triassic fossils is ex-tremely rare in any case (Schubert et al. 1997;Kidder and Erwin 2001), and therefore is in-capable of accounting for the observed pat-tern. If anything, the taphonomic bias of silic-ification explains the apparently small size ofthe Late Permian gastropods in aggregate. Thesmall Early Triassic gastropods are knownprimarily from cast and mold preservation.Therefore, taphonomy cannot explain the ab-sence of large specimens during the Early Tri-assic.

Monographic Bias. Might there be manylarge gastropods from known Early Triassiclocalities that have yet to be collected or de-scribed? A plot of the size of gastropod spe-cies versus the year in which they were de-scribed indicates that larger species tend to bedescribed earlier, and smaller species later(Fig. 8). Not only do the largest specimens de-scribed tend to be smaller in more recent

monographs, but so, too, do the smallest gas-tropods. This is true whether the data are con-sidered only within time intervals (Fig. 8B–D)or by geographic region (Fig. 8E). To quantifythis effect and determine whether it is statis-tically significant, one can test trends in par-ticular percentiles of the data by using quan-tile regression. This method is similar to stan-dard linear regression but may be applied toany quantile within a data set. Quantileregression of size against year of descriptionreveals statistically significant decreasingtrends in the ninetieth, fiftieth, and tenth per-centiles of the sizes of gastropods describedsince 1950 (Table 2). The trend is most pro-nounced at the tenth percentile, providing ev-idence that the smallest gastropods are likelythe most poorly sampled through the entirestudy interval. Although gastropods continueto be described over a great range of sizes, thelarge end of the size spectrum was delimitedvery early, whereas species smaller than thosedescribed previously continue to enter the lit-erature. The tendency to collect and describelarger species earlier is especially apparent forthe Permian and Middle Triassic, where manystudies have been completed through theyears (Fig. 8B,D). The trend is less apparentfor the Early Triassic (Fig. 8C), owing in partto the paucity of data. Therefore, althoughlarger specimens may still be collected frompreviously sampled Early Triassic localities, ifsignificantly larger Early Triassic gastropodsexist, they will likely be found in new locali-ties. Indeed, if significantly larger specimensare found, it will be informative to know whatlocal and regional environments harboredthem.

Environmental Bias. Nearly all monographsfrom all time intervals appear to represent ei-ther carbonate or mixed carbonate and silici-clastic shelf environments from relatively lowpaleolatitudes. There is no obvious change inthe environments represented across the var-ious time intervals. For example, the smallgastropods from the Sinbad Limestone occurin deposits indicative of normal marine, openshelf habitats (Fraiser and Bottjer 2004). How-ever, gastropod size may respond to local eco-logical and environmental factors not record-ed in this study (e.g., proximity to reefs, co-


FIGURE 8. Shell volume graphed as a function of the year in which the description was published. A, All data. B,Permian data. C, Early Triassic data. D, Middle Triassic data. E, All specimens from the U.S.A. These graphs dem-onstrate that the largest gastropods tend to be described earlier in any given time interval or area, whereas thesmallest gastropods tend to be described later.

occurring organisms, minor fluctuations inwater depth). The data available do not pro-vide sufficient paleoecological information torule out completely local environmental bias-es that could account for the pattern. Thebroad similarity of environments studiedthrough all of the intervals, on the other hand,indicates that the pattern does not result sim-ply from the sampling of different environ-ments through time. Sampling and methodo-

logical biases are unlikely to account for thepattern of size change observed.

How Was the Decrease inSize Accomplished?

A variety of biological scenarios could ac-count for the generally small size of Early Tri-assic gastropods and the absence of large in-dividuals: (1) the extinction of larger speciesor higher-level taxonomic groups (size-biased


TABLE 2. Quantile regression results for regressions ofvolume in log(mm3) on year of publication for gastro-pods described since 1950. The relevant test in these cas-es is against a slope of zero, in which case there is norelationship between size and year of description. Re-gression and significance tests were performed usingBLOSSOM, a statistical package available through theU.S. Geological Survey (www.fort.usgs.gov/products/software/blossom/blossom.asp).

Quantile

Slopelog(mm3)/

yearProbability

true slope 5 0Signi-ficant?

90th 20.028 ,0.0002 Yes50th (median) 20.033 ,0.0002 Yes10th 20.055 ,0.0002 Yes

FIGURE 9. Gastropod sizes sorted by the status of the genus in relation to the end-Permian extinction. A, Permian.B, Late Permian. C, Early Triassic. D, Middle Triassic. Categories: E, extinct by the end of the Permian; S, survivingand occurring in the Early Triassic; L, surviving from the Permian to the Middle Triassic as a Lazarus genus; N1,new in the Early Triassic; N2, new in the Middle Triassic.

extinction), (2) the occurrence of new smalltaxa (size-biased origination), (3) the increasedrelative abundance of small taxa (changing rel-ative abundances), (4) the evolution of many lin-eages toward smaller sizes (size change withinlineages), or some combination of these factors.Constraining the processes governing gastro-pod size change from the Permian through the

Middle Triassic may shed light on the mech-anisms responsible for the generally slow re-covery from the end-Permian extinction.

Size-biased Extinction. Might the decreasein maximum size reflect the extinction of gas-tropod groups containing large species? InFigure 9 all genera have been categorized onthe basis of their record through the Permian–Triassic interval. The categories are: generathat go extinct in the Permian (E), survivorgenera that are found in the Early Triassic (S),Lazarus genera that disappear during the Ear-ly Triassic but reappear in the Middle Triassic(L), genera new in the Early Triassic (N1), andgenera new in the Middle Triassic (N2). Whenall of the Permian data are pooled, membersof extinct genera cover the same basic sizerange as members of genera that survive intothe Triassic (Fig. 9A) and are, on average,smaller than survivor genera (Table 3). Thesame is true when only Late Permian data areconsidered (Table 3, Fig. 9B). Among the sur-vivors of the extinction, the Lazarus genera


TABLE 3. Maximum and mean size (in log mm3) and number of specimens presented by the genus survival statusacross the Permian/Triassic boundary. For reference in the size units: 0 5 1 mm3, 3 5 1 cm3, 5 5 100 cm3 (aboutthe size of an orange).

Generatime

Survivor

Max Mean n

Lazarus

Max Mean n

Extinct

Max Mean n

New in EarlyTriassic

Max Mean n

New in MiddleTriassic

Max Mean n

T2 4.78 2.46 164 3.55 2.13 133 3.90 2.41 44 5.73 1.98 344T1 3.07 1.60 39 3.11 1.15 27P3 4.66 1.69 60 3.93 1.13 28 4.48 1.18 117P2 5.72 2.62 65 3.89 2.31 35 5.08 2.14 173P1 4.88 2.62 52 4.35 2.43 54 5.35 2.34 172

are less common than those present in the Ear-ly Triassic, and the Lazarus genera containvery few large specimens, especially in theLate Permian when only a single species ex-ceeds 1 cm3. The significance of the small sizeof Lazarus genera will be discussed below(see Lazarus taxa or Waldo taxa?). While size-selective extinction may have occurred at thespecies level (at least within survivor genera),at present it is not possible to test this scenariobecause within the current taxonomy no in-dividual species cross the Permian/Triassicboundary while retaining the same speciesname. The end-Permian extinction was notsize-biased for gastropods at or above the ge-nus level, but could have been size-biased atthe species level.

Size-biased Origination. Genera new in theEarly Triassic are not significantly different insize from the survivors present during theEarly Triassic (Table 3, Fig. 9C). There is no in-dication that the occurrence of new small gen-era during the Early Triassic can account forthe observed pattern (in large part becausethere are so few genera new in the Early Tri-assic). On the other hand, Middle Triassic sizeincrease occurred within survivor genera aswell as new Middle Triassic genera (N2) (Ta-ble 3, Fig. 9D). Those new genera all belong tofamilies with Early Triassic representatives,suggesting a close relationship to the survivorgenera. Lazarus genera and genera withinfamilies that originated in the Middle Triassicdo not contribute any of the largest specimensin the Middle Triassic. Thus, the Middle Tri-assic increase in maximum size results fromsize increase and diversification within thesurvivor genera and related genera within the

same families, to the exclusion of Lazarus gen-era and genera within new families.

Changing Abundance within Database. Exist-ing data are insufficient to study the abun-dance of individuals within species throughthe Permian–Triassic interval. Many studiesdo not report abundance data and those thatdo usually do not adequately explain the sam-pling strategies used. Within my data set,however, changes in the number of species oc-currences within genera of different sizescould influence the global size trend. Becausea single specimen was selected to represent itsspecies for each monograph, the number ofspecies occurrences in my database is a metricthat combines geographic range and speciesrichness. The change in the number of speciesoccurrences within genera is not correlatedwith the mean size of the family in the initialinterval for either the Permian–Triassic (Fig.10A) or the Early–Middle Triassic transition(Fig. 10B). In other words, the average size ofa genus during one time interval does not pre-dict its success across either of these transi-tions. There is no evidence that the observedsize trends were caused by genera with small-er average size becoming more widespreador species rich across the Permian/Triassicboundary, nor is there evidence that generawith larger average size became more wide-spread or species rich from the Early to Mid-dle Triassic.

Size Change within Lineages. The Early Tri-assic representatives of nearly all genera weresmaller than their Permian and Middle Tri-assic relatives. Table 4 shows maximum,mean, and minimum size, and number of spe-cies occurrences within the database by time


FIGURE 10. A, Change in abundance from the Permian to the Early Triassic plotted against mean size in the Perm-ian. B, Change in abundance from the Early to Middle Triassic plotted against mean size in the Early Triassic. Thelack of correlation between these variables demonstrates that size did not predict success (i.e., abundance in thedatabase) in the subsequent interval for either the Permian or the Early Triassic.

interval for each genus that crosses the Perm-ian/Triassic boundary.

Figures 11–14 compare changes in mean,maximum, and minimum sizes within generafrom the Permian through the Middle Trias-sic. Change in mean is plotted on the horizon-tal axis and change in minimum (Figs. 11, 12)or maximum (Figs. 13, 14) is plotted on thevertical axis. The advantage of these plots isthat the distribution of trends among generacan be easily assessed. Points in the upperright quadrant represent increases in bothvariables, and points in the lower left repre-sent decreases.

Change in mean and change in minimumare not strongly correlated from the Permianto the Early Triassic (Fig. 11A), whereas bothmean and minimum increase in most groupsfrom the Early to the Middle Triassic (Fig.11B). Comparison of the Permian and theMiddle Triassic shows a predominance of in-crease in mean and minimum size among sur-vivor genera (Fig. 12A), and more scatteredchanges among Lazarus genera (Fig. 12B). Asdiscussed above, heterogeneous sampling ofthe smallest gastropods may shape apparentchanges in minimum size through time.

Decrease in both maximum and mean sizeoccurs in most genera across the Permian/Tri-assic boundary (Fig. 13A), and size increaseoccurs from the Early to Middle Triassic (Fig.13B). It is quite clear that most genera showpronounced size decreases, often by morethan an order of magnitude, from the Permianinto the Early Triassic. The subsequent in-

crease from the Early to Middle Triassic iseven more pronounced. Similarly consistentdifferences in mean and maximum size arenot seen in comparing the Permian with theMiddle Triassic among either the survivorgenera present in the Early Triassic (Fig. 14A)or Lazarus genera, with only Permian andMiddle Triassic records (Fig. 14B). However,most survivor genera experienced a net in-crease in size from the Permian to the MiddleTriassic, whereas net size increase and de-crease were equally common among Lazarusgenera. The balanced numbers of genera ex-periencing net increases and decreases inmaximum and mean size from the Permian tothe Middle Triassic demonstrate that system-atic increase or decrease in mean and maxi-mum size among nearly all genera is not typ-ical of any comparison between two relativelyclose time intervals, but rather reflects the un-usually small size of Early Triassic gastro-pods.

Summary. Two scenarios are compatiblewith the decrease in size across the Permian/Triassic boundary. Size-selective extinction atthe species level and within-lineage size de-crease (of at least the largest species) can eachaccount for the data. Because the identificationof likely ancestor-descendant pairs across thePermian/Triassic boundary is extremely dif-ficult, sufficient data do not exist to determinewithin-lineage size trends through this inter-val, as would be needed to distinguish be-tween these two possibilities. Evaluating sizetrends within lineages at the species level


would require a well-defined phylogeneticframework and large numbers of specimens ineach species to produce robust results, a pros-pect beyond the scope of the current data. Thesize increase through the Early and MiddleTriassic, on the other hand, must have includ-ed size increases within lineages, because noEarly Triassic genera or Late Permian Lazarusgenera include species as large as the largestspecies found in the Middle Triassic. Indeed,many of the largest Middle Triassic speciesare members of survivor genera and thereforelikely represent size increases within somesurvivor lineages (Fig. 9B). Of course, size-se-lective origination and within-lineage sizechanges are not strictly distinct phenomena,as all new species must descend from otherspecies. New species that are either larger orsmaller than all possible ancestral speciesmust represent instances of within-lineagesize changes. The small number of mono-graphs and unusual preservation (silicifica-tion) of many Late Permian gastropods com-promise the analysis of size changes withingenera from the Late Permian to the Early Tri-assic. Determining whether the pattern of sizechanges from the Permian to the Early Triassicresults from processes acting only at theboundary, or whether it was also (or even pri-marily) driven by processes operating at theend of the Guadalupian (or more gradualchanges within the Permian) will dependupon more complete sampling of the LatePermian from multiple localities. Such dataare necessary if we hope to disentangle the ef-fects of the Guadalupian and end-Permian ex-tinctions on gastropod size.

Discussion

Lazarus Taxa or Waldo Taxa?

In addition to the small size of the speci-mens, Early Triassic gastropod faunas are un-usual for the number of genera known fromPermian and Middle and Late Triassic stratathat have never been found in Early Triassicdeposits (Batten 1973; Erwin 1996). Thesemissing taxa, known as Lazarus taxa, disap-pear from the fossil record for a significantlength of time before reappearing relativelyunchanged (Jablonski 1986). Lazarus genera

(those missing from the Early Triassic) rep-resent more than half of the known Early Tri-assic gastropod diversity, according to therange-through method (Erwin 1996). Severalexplanations have been proposed to accountfor the extended absence of so many generafrom the fossil record. Prominent among theseis the ‘‘refugium’’ scenario (Jablonski andFlessa 1986; Kauffman and Erwin 1995), inwhich Lazarus taxa survived the end-Permianextinction relatively unchanged in isolatedand to-date unsampled locations, migratingback into sampled localities in the Middle Tri-assic. Alternative explanations include the‘‘impersonator’’ (or Elvis) scenario (Erwinand Droser 1993), in which Middle Triassicspecimens represent taxa superficially similarbut unrelated to Late Permian forms. In athird alternative, rarity of fossils due to lowpopulation sizes has been proposed to explainthe Early Triassic Lazarus taxa (Wignall andBenton 1999). Finally, Erwin (1996) noted thattaphonomic factors can help to explain the re-cords of at least those Lazarus genera (Luciella,Gosseletina, Borestus, Dictyotomaria, Glyptoto-maria, Helicospira, and Brochidium) that haverelatively thin and fragile shells, are preservedonly via silicification, or are difficult to iden-tify without extremely good preservation.

Although the proposed scenarios are notmutually exclusive, determining the role ofany one of these processes in the overall pat-tern has proven difficult. The data presentedabove, however, provide direct evidence for acommon factor shared by all of the Lazarusgenera. A comparison of the survivor (S) andLazarus (L) genera in the Permian shows thatLazarus genera do not contain any of the larg-est Permian specimens (Table 3, Fig. 9A,B). Allof the largest Permian (and Late Permian)specimens fall into either extinct (E) or survi-vor (S) genera, suggesting that sampling ofthe largest forms is quite good. Taxa contain-ing large species that survived the end-Perm-ian extinction have all been observed withinthe Early Triassic, whereas those that have notbeen found in the Early Triassic do not reap-pear in the Middle Triassic. Thus, small sizewas a necessary (though not sufficient) con-dition for gastropod Lazarus genera. Mostmembers of the Lazarus genera, when seen in


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the Permian, were less than 2 cm long, andonly a single Late Permian species exceeds 1cm3. A comparison of mean sizes (Table 3)demonstrates that Lazarus genera were con-sistently smaller than the survivor genera. Thefact that only small-sized genera became Laz-arus genera is consistent with the abundantevidence that the small end of the size spec-trum is the most poorly sampled. If Lazarusgenera experienced the decrease in size typi-cal of survivor genera it would have decreasedfurther their likelihood of being collected andidentified.

Numerous survivor genera, however, hadsmall sizes in both the Permian and the Tri-assic. The majority of survivor genera presentin the Early Triassic have robust shells (Erwin1996), whereas many Lazarus genera are thinshelled and preserved predominantly by silic-ification (Erwin and Pan 1996). In addition tothese taphonomic differences, survivor generahad broader geographic distribution andhigher species richness in the Permian (Erwinand Pan 1996). My database reveals that thegap in species richness widened from thePermian to the Middle Triassic. In the Perm-ian, Lazarus genera averaged 3.8 species pergenus, whereas survivor genera averaged 7.7species. By the Middle Triassic Lazarus gen-era had increased slightly to 4.3 species pergenus whereas survivor genera more thandoubled in species richness to 16.2 species pergenus. The enhanced gap in species richnessbetween survivor and Lazarus genera sug-gests that differences in response to the end-Permian event and/or subsequent recoverydynamics added to this pattern. The distri-bution of long stratigraphic gaps among taxareflects not only differences that existed dur-ing the Permian, but also differences in diver-sity enhanced by the environmental and eco-logical conditions established in the aftermathof the end-Permian extinction. For example,several survivor genera (especially Wortheniaand Coelostylina) radiated in the Middle Tri-assic while diversity within Lazarus generaremained essentially static.

Lazarus genera were small, contained fewspecies, and were preserved primarily via si-licification prior to the end-Permian extinc-tion. This recipe for a poor fossil record was


FIGURE 11. Change in minimum size compared with change in mean size for genera from the Permian to EarlyTriassic (A) and the Early to Middle Triassic (B).

FIGURE 12. Change in minimum size compared with change in mean size from the Permian to the Middle Triassic.A, Survivor genera (with Early Triassic fossil record). B, Lazarus genera (without Early Triassic fossil record).

enhanced following the extinction by contin-ued low species richness in the Triassic andcomplete failure to produce any species oflarger size. Rather than persisting unchangedin isolated refugia, many Lazarus genera maybe in the same deposits that are known to con-tain Early Triassic gastropods, but are simplysmall, rare, and/or poorly preserved enoughto preclude collection and identification (todate). These fossils may be hiding from us inthe very deposits we have been investigating.Rather than Lazarus, they may have more incommon with Waldo (see, if you can, Hand-ford 1987), a cartoon character from a series ofchildren’s books who wears a red and whitestriped hat and is (sometimes) found in col-orful drawings filled with so many other peo-ple, animals, and objects that he is often quitedifficult to locate despite being in plain view.The challenge of the books is to locate Waldoamidst all of the other distracting elements of

the picture. Like Waldo, many Lazarus gastro-pods may be hiding in plain sight.

Selective Pressures on Size

As discussed above, the loss of all large gas-tropods across the Permian/Triassic bound-ary is compatible with two scenarios: the ex-tinction of all larger gastropod species andrapid size decrease in those large lineages thatsurvived the extinction. Assessing the relativeimportance of these two processes will re-quire more highly resolved phylogenetic in-formation about Permian and Triassic gastro-pods.

Regardless of which scenario(s) occurredacross the boundary, the absence of large gas-tropods throughout the Early Triassic sug-gests that selective pressures inhibited size in-crease until the Middle Triassic. This impres-sion is reinforced by the more rapid size in-crease observed in land mammals (Alroy


FIGURE 13. Change in maximum size compared with change in mean size for genera from the Permian to EarlyTriassic (A) and the Early to Middle Triassic (B).

FIGURE 14. Change in maximum size compared with change in mean size from the Permian to the Middle Triassic.A, Survivor genera (with Early Triassic fossil record). B, Lazarus genera (without Early Triassic fossil record).

1998) and planktonic foraminifera (Arnold etal. 1995; Schmidt et al. 2004) following theend-Cretaceous mass extinction. Low primaryproductivity (Twitchett 2001) and the pre-dominance of opportunistic (r-selected) spe-cies (Fraiser and Bottjer 2004) have been sug-gested as ecological explanations for the smallsize of Early Triassic gastropods in aggregate.In the absence of convincing evidence for lowproductivity, however, the small size and lo-cally high abundance of gastropods in EarlyTriassic strata are perhaps more likely to re-flect an overabundance of food resources in arelatively empty ecosystem. The bivalve faunaof Tristan de Cunha (the most isolated islandin the South Atlantic) consists exclusively ofsmall taxa (,5 mm) and provides an excellentmodern analogue for this scenario. Gould(1977: p. 327) pointed out that these bivalvesevolved toward small size after colonizing theislands and suggested that this occurred ‘‘as acommon response to superabundant resourc-

es in one of the world’s emptiest ecospaces.’’Reduced predation pressure and competitionin the empty ecosystems of the Early Triassicoceans may also have favored small size. Val-entine (1973) was the first to recognize that thereduced defensive ornamentation of Early Tri-assic invertebrates in comparison to theirPermian predecessors may reflect decreasedpredation pressure. Indeed, the intensity ofshell crushing and shell drilling appears tohave decreased from the Permian into the Tri-assic, reaching very low levels in the Early Tri-assic (Boyd and Newell 1972; Kowalewski etal. 1998, 2000; Walker and Brett 2002; Oji et al.2003). The phenomenon of competitive dis-placement of size among similar species with-in communities was first suggested by Brownand Wilson (1956) and Hutchinson (1959) andmost widely recognized in Darwin’s finches(Grant 1986). The very low diversity of EarlyTriassic communities (e.g., Schubert and Bott-jer 1995) would have minimized such com-


petitive interactions, limiting the role of thispotentially important factor in increasing thesize range of gastropods. Physical stressesmay also have constrained the body size ofEarly Triassic gastropods. Large perturba-tions of the global carbon cycle throughout theEarly Triassic suggest that a series of distur-bances began at the end of the Permian andcontinued until the Middle Triassic (Payne etal. 2004). Size increase may have been inhib-ited by low oxygen content in the water col-umn (e.g., Wignall and Hallam 1992), the ef-fects of decreased carbonate saturation in thesurface ocean on biomineralization related toepisodic increases in carbon dioxide (e.g.,Knoll et al. 1996), or other as yet unknown en-vironmental factors. Given the global natureof the pattern, whatever the driving mecha-nism(s) it must have operated, perhaps epi-sodically, for several million years.

Conclusions

The absence of large gastropods from theEarly Triassic fossil record is not solely the re-sult of poor or biased sampling of Early Tri-assic strata. Rather, it reflects the extreme rar-ity or total absence of large gastropods duringthe Early Triassic. The aggregate changes ingastropod size on the global scale mirrorthose observed within regional assemblagesby Fraiser and Bottjer (2004). The decrease insize at the family and genus levels suggeststhat at least some lineages experienced signif-icant size reduction across the Permian/Tri-assic boundary, although data sufficient tostudy size changes within individual lineagesthrough the Permian-Triassic interval are notcurrently available. The small size of all spe-cies within Lazarus genera prior to the end-Permian extinction suggests that the surviv-ing species in these genera were likewisesmall, and thus difficult to collect and identi-fy. Lack of silicification and low species rich-ness of Lazarus genera were also contributingfactors. Increase in maximum size may havebegun in the Spathian but occurred primarilyin the Middle Triassic. Middle Triassic size in-crease was not universal; it was largely re-stricted to those families and genera that sur-vived the extinction and appeared in the EarlyTriassic fossil record. Whether the small size

of Early Triassic gastropods reflects primarilyecological or environmental selective pres-sures on body size remains poorly under-stood. More-detailed stratigraphic resolutionof the Spathian-Anisian transition, both forgastropods specifically and for the marine bi-ota generally, is needed to define the taxonom-ic, temporal, and biogeographic pattern ofsize increase and, perhaps, illuminate the pri-mary mechanisms underlying the absence oflarge gastropods during the Early Triassic.

Acknowledgments

I owe many thanks to R. Bambach, D. Bott-jer, D. Erwin, A. Knoll, C. Marshall, M. Rex, S.Pruss, and an anonymous reviewer for helpfuldiscussions and comments; D. Erwin and A.Nutzel for providing taxonomic information;and E. Chang for introducing me to SAS.Thanks to M. Markey, J. Brocks, and Y. Shenfor translating articles into English from Rus-sian, German, and Chinese, respectively. Fi-nancial support for this work was provided bythe Department of Defense (National DefenseScience and Engineering Graduate Fellowshipto the author) and the NASA Astrobiology In-stitute. The complete gastropod size databaseis available from the author upon request.

Literature CitedAlroy, J. 1998. Cope’s rule and the dynamics of body mass evo-

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