COMBINING MORPHOMETRIC AND PALEOECOLOGICAL … · Appropriate samples for rigorous, quantitative...

Palaeontologia Electronica http://palaeo-electronica.org

PE Article Number: 8.2.33ACopyright: Paleontological Society October 2005Submission: 18 February 2004. Acceptance: 18 April 2005.

Tang, Carol and Pantel, Jelena H. 2005. Combining Morphometric and Paleoecological Analyses: Examining Small-Scale Dynamics in Species-Level and Community-Level Evolution. Palaeontologia Electronica Vol. 8, Issue 2; 33A:10p, 370KB; http://palaeo-electronica.org/paleo/2005_2/combine/issue2_05.htm

COMBINING MORPHOMETRIC AND PALEOECOLOGICAL ANALYSES: EXAMINING SMALL-SCALE DYNAMICS IN SPECIES-LEVEL AND

COMMUNITY-LEVEL EVOLUTION

Carol Tang and Jelena H. Pantel

ABSTRACT

By combining two powerful types of quantitative techniques—landmark morpho-metric and paleocommunity analyses—we can examine the role of ecology in evolu-tionary processes by determining whether major morphological changes co-occur withsignificant changes in paleocommunity structure. Whereas morphological changesusing landmark morphometric analysis indicate evolutionary changes within a lineage,changes in paleocommunity metrics such as abundance and dominance can be usedto indicate ecological changes and identify species of special interest.

This preliminary study documents both major ecological and morphologicalchanges within the scallop “Pecten” thompsoni from two closely spaced stratigraphichorizons of the Neogene marine sequence of the Dominican Republic. Ecologically,the abundance and dominance of this species differs dramatically between the twosamples. Morphometrically, while size does not change significantly, the two popula-tions exhibit statistically different shapes. In addition, there appears to be a shift inontogenetic patterns where initially an allometric pattern exists but then changes to anisometric growth relationship. These different morphological patterns are consistentwith changes in paleoenvironmental conditions. We suggest that a coupled approachof morphometric and paleocommunity analyses may focus future studies on intervalsand taxa where evolutionary change may be directly correlated to concomitant ecolog-ical changes.

Carol Tang. Department of Invertebrate Zoology and Geology, California Academy of Sciences, 875 Howard Street, San Francisco, California, 94103, USA [email protected] Jelena H. Pantel. Department of Invertebrate Zoology and Geology, California Academy of Sciences, 875 Howard Street, San Francisco, California, 94103, USA currently: Section of Integrative Biology, University of Texas at Austin, 1 University Station C0930. Austin, TX 78712 USA [email protected]

Key Words: Dominican Republic, bivalves, evolutionary paleoecology, Neogene

TANG AND PANTEL: SMALL-SCALE DYNAMICS IN SPECIES AND COMMUNITY EVOLUTION

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INTRODUCTION

The fossil record clearly documents bothchanges in taxonomic lineages and communitycomposition. However, it is unclear how closelyrelated the processes and patterns of species evo-lution and community changes may be. The evi-dence to suggest that the role of biotic interactionsshape the evolutionary paths of lineages remainscontroversial (e.g., Valentine 1973; Kitchell 1985;Gould 2001; Vermeij 2004).

Among neontologists, however, local ecologi-cal interactions between species have been citedas a major influence on speciation and evolution.The geographic mosaic theory of co-evolution hasprovided a new way of considering the role of localecology and biotic interactions in allopatric specia-tion processes and evolutionary patterns. Forexample, in examining the interactions between aspecies of moth and a plant in western US states,Thompson (1997, 1999) documented that thereexisted mutualistic, commensalistic, or even antag-onistic relationships between the two speciesdepending on the presence or absence of otherinsect species. These differences in biotic interac-tions can be correlated with different genetic popu-lations (Thompson 1999) and thus, can drivespeciation through time (Forde et al. 2004).Another implication of these observations andinterpretations is that within small geographicareas, “[I]nteractions may show selection mosaics,such that different traits and outcomes are favoredby natural selection in different communities”(Thompson 1999).

In a paleontological context, the geographicmosaic theory of co-evolution suggests that paleo-community composition and biotic interactions mayvary geographically and temporally even at smallscales, and that these differences may result inevolutionary changes. This theoretical frameworkmay, therefore, bridge the fields of evolutionarybiology and paleoecology as well as neoecologyand population genetics. In this paper, we explorethe potential of using quantitative morphometricand paleocommunity composition data to examinewhether correlations exist between community-level and species-level patterns. Quantitative mor-phometrics allows for the evaluation of species-level microevolutionary changes whereas the anal-ysis of paleocommunity composition provides afirst-level proxy for evaluating biotic interactionsand ecological fluctuations. If evolutionarychanges within lineages as evidenced by morpho-logical change are concurrent with major changesin community structure, it would allow us to seri-ously reconsider the role of biotic interactions in

determining patterns of species evolution in thefossil record.

In order to investigate the relationshipbetween ecological and evolutionary processes,one must have appropriate depositional systemsand fossil samples which will allow for robust quan-titative analyses. One such depositional system isin the Neogene marine sequences of the CibaoValley of the northern Dominican Republic. Thissystem has become a major focal point of manyimportant paleobiological studies for many goodreasons. The fossils have been studied since themid 1800s with much modern study based on col-lections made by E. and H. Vokes and theNaturhistorisches Museum of Basel, Switzerland,since the 1970s (see Jung 1986 for review of mol-luscan studies and NMITA web site, http://porites.geology.uiowa.edu/). The marinesequences exposed in river gorges in the northcentral highlands are very fossiliferous and arebelieved to represent fairly continuous depositionin a gradually deepening basin (Saunders et al.1986).

Many paleontologists around the world havebeen involved in examining the Dominican Repub-lic Neogene fauna. Experts in taxonomic groupshave systematically surveyed the samples col-lected by the Naturhistorisches Museum of Basel(many monographs have been published in Bulle-tins of American Paleontology). Given the long his-tory of taxonomic study, the alpha taxonomy andbasic identification of fossils found in these rocks iscomparatively good even if higher level taxonomicreorganizations have not yet been completed formost of the groups. In addition, paleobiologistshave long used this system to examine patterns ofevolution, including documenting rates of changeand investigating the role of heterochrony in speci-ation processes (e.g., Anderson 1994; Cheethamet al. 1994; Nehm and Geary 1994).

MATERIALS

Appropriate samples for rigorous, quantitativeanalyses of both morphological and paleoecologi-cal change were collected by one of the authors(CT) during an expedition to the Rio Gurabo andRio Cana of the Dominican Republic in the summerof 2000. Bulk fossil sampling was employed wherematrix and fossil materials were collected togetherwithout discrimination along restricted vertical andlateral intervals. Five sub-samples were taken ateach locality; collectively these are referred to asone bulk fossil sample. These sampling localitiesare closely aligned to sampling localities of the


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Naturhistorisches Museum of Basel collection(Saunders et al. 1986).

By restricting the lateral extent from which thesamples were collected, we minimize as much aspossible the effect biological and taphonomicpatchiness may have on assemblage composition.The average lateral extent of the samples was 1.02m (median = 0.87 m); 26% of the samplesextended less than 0.50 m horizontally. Similarly,by restricting vertical thicknesses, the time averag-ing represented within samples was minimized asmuch as possible. For the 31 bulk fossil samplescollected from the Dominican Republic in 2000(each with five sub-samples), the average thick-ness of samples was 0.156 m with the maximumthickness at 0.23 m.

The taphonomic condition of the fossil mate-rial is another proxy often used to assess timeaveraging (Best and Kidwell 2000). In all of thefossil samples we collected, there is little evidenceof post-mortem boring or encrusting, thus suggest-ing that the fossils did not spend much time on theseafloor surface before being buried; virtually allborings in the samples are predatory borings,(Tang 2002). Not uncommonly, articulated corbulidbivalves are found and in several cases, very deli-cate spines (for example, on spondylid bivalves)and thin tubes of scaphapods and vermetid gastro-pods are present, again suggesting limitedamounts of post-mortem exposure on the seaflooras well as limited transport. Preserved color pat-terns have been observed in associated DominicanRepublic fossil samples (Costa et al. 2001). Thus,we interpret these fossil assemblages to be para-authochthonous, or representative of disturbedneighborhood assemblages (in this paper we willrefer to these as “paleocommunities”).

The two samples analyzed for this paper—060700-2A and 052500-1C—were collected fromthe Rio Gurabo section. Our fieldwork indicatesthat these samples are roughly equivalent to sam-ples 18189 and 15907 of Saunders et al. (1986)respectively, in both stratigraphic and geographiclocation. Based on close correlation with the strati-graphic section prepared by Saunders et al.(1986), the samples are estimated to be separatedby approximately 9 m vertically, with 060700-2Alocated at approximately 136 m in the section and052500-1C at 127 m. Both are from within the NN11 zone of the Late Miocene Cercado Formation.All specimens and unwashed sub-samples aredeposited at the California Academy of SciencesDepartment of Invertebrate Zoology and Geologycollections (San Francisco, California, USA).

The two samples analyzed are composed ofskeletal grains in a matrix of poorly cemented silici-

clastic grains. This type of muddy tropical deposi-tional environment is considered to have minimaltime averaging due to a higher rate of sedimenta-tion and minimal physical reworking (Best and Kid-well 2000). Sample 060700-2A comes from blue-grey muddy sandstone horizons that contained thinshell beds, which were laterally continuous andtraceable for some distance. The foraminiferaAmphistigena was common in this sample asobserved in the field, and it was also noted to con-tain medium to large bivalves, scaphapods, andgastropods. Sample 052500-1C was collectedfrom a horizon containing an amalgamated,densely packed shell bed. In the field, it was notedthat the siliciclastic and fossil components did notappear to be well size-sorted and that delicate mol-luscs were well preserved. Both samples appearto be storm-deposited shell beds and have similarlithological and taphonomic properties.

One complete sample bag from each sampleanalyzed was washed over a 2 mm sieve; most ofthe materials are fairly unconsolidated althoughoccasional soaking and rewashing was necessaryto disaggregate some portions of some samples.To clean individual fossils for accurate identificationand morphometric analyses, specimens weresoaked overnight in water and a sharp thin pickused to gently remove matrix particles.

METHODS

Initially, all fossils and fossil fragments werepicked from the > 2 mm aliquot. For this analysis,all complete bivalve fossils and fragments thatwere (1) at least two-thirds complete and (2) wereidentifiable to at least a generic level were counted.Because almost all of the bivalves are disarticu-lated into two valves, this counting method esti-mates the maximum abundance of bivalvespresent in the assemblage. Furthermore, becausethis study is based exclusively on bivalves, therewas no attempt made at this juncture to convertthese maximum counts of bivalve individuals into anumber that can be used to compare bivalve abun-dances to those of gastropods, scaphopods, soli-tary corals or other taxa, nor with colonialorganisms like bryozoans.

Some groups of bivalves were identified togeneric and species levels based on descriptionsby Maury (1917). While we recognize that therehas been some systematic revisions and synonimi-zation of these taxa, this preliminary study wasmore concerned with the identification of species-level groupings rather than on their taxonomic revi-sion. Although some bivalves were not identified tothe generic level, they were still counted so that we


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could obtain a snapshot of the entire bivalve paleo-community and examine dominance patterns.

All 22 individuals of “Pecten” thompsoni wereselected for morphometric analysis from Sample052500-1C while 21 individuals were randomlyselected for analysis from Sample 060700-2A.Specimens were imaged with either a KodakMDS100 digital camera mounted on a Wild M400dissecting microscope or a high- resolution flatbedscanner. Digital images were then imported asjpeg files into TnImage 3.0.6 (Linux) where 10specimen landmarks were digitized (Figure 1).

The raw landmark data were used to calculatespecimen size and describe the shape of the shell.Size was calculated as centroid size, which can bedescribed independently of shape (Bookstein1991), and bears the advantage of being uncorre-lated with measures of shape in the absence ofallometry.

Shape was described using principal warpanalysis (Bookstein 1991), which summarizesshape as a transformation relative to a common

reference form. Specimens were first scaled to unitcentroid size and rotated to maximum interspeci-men alignment using a Procrustes iterative align-ment. Each specimen, or its map of landmarks,may then be represented in a special “shapespace,” relative to the common reference form,with each unique landmark configuration occupy-ing a unique position in the shape space. Thisresulting Kendall shape space (Kendall 1986; actu-ally, the small portion occupied by the distributionof specimens) is then projected onto a tangentialEuclidean space where the axes correspond to themajor dimensions of specimen distribution (varia-tion) in the curved shape space (principal warps)(Rohlf 1996), with specimen coordinates on theseaxes (partial warp scores) summarizing specimenshape. Procrustes alignment and principal warpanalyses were performed with tpsRelw (Rohlf2002).

Shape variation was tested with multivariateanalysis of variance (MANOVA) of partial warpscores and subsequent classification of specimens

Figure 1. “P.” thompsoni valve and landmarks used in this study; numbered landmarks correspond tothose in Figure 6.


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performed using canonical variates analysis (CVA).Intrasample allometry was tested by regression ofrelative warp scores on centroid size. Relativewarps are factors (principal components of partialwarp scores) which themselves summarize themajor vectors of shape variation within samples.Because relative warps are independent compo-nents decomposing shape, a proper depiction ofallometry is the multiple regression of size on allrelative warps, or the individual regression of rela-tive warps on size. The latter approach is usedhere. All statistical analyses were performed withSYSTAT 10 (Windows) and Stata 8.0 (Linux).

RESULTS

Bivalve paleocommunity composition

The number of counted bivalve specimenswas similar for both samples with 501 individuals inSample 52500-1C and 511 in Sample 060700-2A.Paleocommunity differences are often assessedeither with presence/absence data or with censusdata. We have compared our two samples usingthese two different metrics, and the results indicatethat census data conclusions differ from thoseobtained from presence/absence data alone. Sincenot all groups were identified in this initial studyand since substantive taxonomic revisions need tobe undertaken for the groups, we report initial com-parisons for some bivalve groups here only to pro-vide evidence for major paleocommunitydifferences between these two samples.

Arcids made up 19.2% of the bivalve individu-als in Sample 52500-1C and 12.7% of Sample060700-2A bivalves. There are five different arcidspecies present in Sample 52500-1C and nine dif-ferent species in Sample 060700-2A. One of thosespecies is Scapharca hispaniolana, and it showsmarked differences of abundance between the twosamples. In Sample 52500-1C, S. hispaniolanacomprised 18.4% of all bivalves counted and95.8% of all the arcids whereas in Sample 060700-2A, it comprised only 0.6% of all bivalves and 4.6%of the arcids. Another arcid species, Scapharcacibaoica, is not found at all in Sample 52500-1Cbut comprised 4.7% of bivalves and 36.9% of arc-ids in Sample 060700-2A.

In another example, pectinids are only 10.4%of Sample 52500-1C but make up 61.8% of Sam-ple 060700-2A. But this significant differencebetween samples is not reflected in the number ofspecies present: Sample 52500-1C has eight dif-ferent pectinid species while the other sample hasnine. They have seven species in common.“Pecten” thompsoni is one of those species but itsabundance varies greatly between samples. In

Sample 52500-1C, “P.” thompsoni comprised59.6% of pectinid individuals and only 6.2% of allbivalves. However in Sample 060700-2A, this spe-cies was 90% of pectinids and 50.7% of bivalves.

Morphology

Given the disparity in paleocommunity numer-ical composition between the two samples, themorphological results can be used to determine ifpaleoecological changes are correlated with phe-notypic differences.

“Pecten” thompsoni was selected because ofits contrasting abundance in one of the samplesand its relatively smaller role in the other assem-blage. This taxon is also easy to recognize due tothe distinctive protuberances near its byssal notch.The distinctive nature of the species eliminates anyerrors, which may occur due to taxonomic misiden-tifications. In addition, valves of “P.” thompsoni arenot highly convex and allow for digital image cap-ture using a flatbed scanner for larger individualsrather than on a conventional microscope-digitalcamera setup. This combination of factors makesit an ideal candidate for morphometric work.

Centroid size does not differ statisticallybetween the two samples (t-test, p = 0.2283),although the variance is slightly greater in Sample060700-2A (but not statistically significant, F-test, p= 0.2765) (Figure 2). Relative warps analysis of thetwo samples does, however, indicate differences inshape (Figure 3), though there is considerableoverlap. Multivariate analysis of variance and sub-sequent CVA of partial warp scores confirms thatthe samples differ significantly in shape (Wilks'Lambda , p < 0.0001) (Figure 4), with 97.7% ofspecimens correctly identified by the discriminantfunction.

Allometric development was measured foreach sample by examining the correlation betweencentroid size and relative warp factors, revealingthat there is an allometric relationship apparent inone population but not in the other. In Sample060700-2A, there is no shape change through thedifferent size classes. Correlations between rela-tive warps I, II, and IV are not significant (Figure5A; linear regression of relative warp I on centroidsize, F-ratio, p = 0.4811). A significant relationshipbetween relative warp III and size (Pearson's cor-relation coefficient = -0.4563, p = 0.0376) is thespurious result of a single outlying specimen and isnot considered further.

The first relative warp of Sample 052500-1Cdisplays significant allometry (Pearson's correla-tion coefficient = -0.4676, p = 0.0282), but a plot ofthis warp and centroid size (Figure 5B) shows veryclearly that the relationship between size and


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shape is not linear. There is an obvious change inthe relationship when specimens attain a centroidsize of approximately 5.5 mm. A regular (firstorder) linear regression of relative warp I on size,while statistically significant (F-ratio, p = 0.0282),produces highly irregular residuals. A betterregression relationship is found with a secondorder polynomial regression (F-ratio, p = 0.0069),which produces normally distributed residuals.

An examination of specimens from Sample052500-1C suggests that the allometric develop-ment is due mainly to a widening of the byssalnotch and a change in the shape of the anteriorauricle. This change is confirmed by thin-platespline visualization of the allometry, which wasdone by constructing splines of predicted specimenshapes, on relative warp I, at two centroid sizes, 4and 7 mm (Figure 6). The shapes were predictedusing the regression functions of relative warp I oncentroid size. Figure 6 illustrates the shapechanges that occur in specimens, from both Sam-ples 060700-2A and 052500-1C, in this size range.Specimens from the former sample develop iso-metrically, whereas specimens from Sample052500-1C exhibit a displacement largely localized

Figure 2. Histograms of centroid sizes in 060700-2A (upper graph) and 052500-1C (lower graph). There is no statistical difference between the two populations (t-test, p = 0.2283).

Figure 3. Plot of relative warp scores (I and II) of 060700-2A in blue squares and 052500-1C in red circles. Although overlap does occur between the two populations, they differ from one another statistically.


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between landmarks 3 and 7. This displacement isthe area of the byssal notch.

DISCUSSION

In this preliminary paleoecological analysis,the abundance of one bivalve family, the arcids,exhibited little difference between two sampleswhile another bivalve family, the pectinids, exhib-ited great variability in dominance. However, thepresence and absence of species within the arcidsand their individual abundances did vary greatlybetween samples. Among pectinids, the speciespresence/absence data alone would not have dif-ferentiated the two samples but the species abun-dances would have been informative. Thus, thesedifferent data sets may provide non-overlappinglines of evidence for subtle differences in paleo-community structure. In this case, the differencesin species compositions and abundances betweensamples in stratigraphic proximity may not haveclear significance in terms of the reconstruction offood webs or paleoecological roles but the differ-ences may suggest subtle paleoenvironmental orpaleoecological conditions useful for reconstructingthe context for evolutionary change. In addition,

the paleocommunity analysis allows for the identifi-cation of species that may be sensitive to ecologi-cal changes over short time intervals and thus maywarrant additional evolutionary analysis. Otherproxies examined in Dominican Republic RioGurabo samples have suggested similar paleoeco-logical differences between other closely spacedstratigraphic samples (Tang et al. 2003; Chan et al.2003).

Our preliminary paleocommunity analysissuggested that “P.” thompsoni may be a good can-didate for looking at morphological differences,which may occur when paleoecological conditionschange significantly. In the stratigraphically lowersample, “P.” thomsponi comprised only 6.2% of thetotal bivalves present whereas in the upper sam-ple, it comprised 50.7% of the total bivalvescounted. This difference suggests a significantchange in ecological conditions as well as a signifi-cant change in the ecological role for “P.” thomp-soni, making this species a good candidate forinvestigating whether there are measurable mor-phological responses correlated with these paleo-ecological changes.

Figure 4. Distributions of canonical variate scores in 060700-2A (upper graph) and 052500-1C (lower graph). There is a highly significant difference between the two populations (Wilks' Lambda, p < 0.0001).


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The results indicate that there is a significantdifference in morphological patterns between thetwo samples. Although there is no size differencebetween the two populations of “P.” thompsoni,detailed landmark analysis indicates marked differ-ences of shape between the samples and moresurprisingly, a change in growth patterns as well. Inthe lower interval, we documented a clear differ-ence between smaller and larger individualswhereas in the upper sample, there is no allometricgrowth signal.

Whereas it is impossible without further studyto determine the selection pressures that may haveled to the different morphologies of these two “P.”thompsoni populations, the shape changes areconsistent with biomechanical interpretations of the

paleoenvironmental conditions of the two sam-ples. The analysis suggests that both the shapedifference and the allometry are related to mor-phological landmarks surrounding the byssalnotch. Bivalves pass bundles of byssal threadsthrough the byssal notch to attach to hard sub-strates, and these morphological structures arerelated to life habits (such as swimming in pec-tinids), water energy, and even substrate composi-tion (Waite et al. 2002). Based on sedimentaryinformation, it does appear that sample 052500-1C—which is an amalgamated shell bed—wouldhave experienced higher energy levels than theother sample. This interpretation is consistent withthe hypothesis that the byssal notch is wider in052500-1C so that individuals have a larger bundleof byssal threads as holdfasts attached to the sub-strate.

This difference in hydraulic energy regimesbetween the two paleoenvironments may alsoexplain the significant change in growth patternsexhibited by the two populations. While it is difficultto differentiate between genetically based differ-ences and ecophenotypic variation in fossil popula-tions, the difference in growth patterns stronglysuggests a genetic or developmental change. Inpectinids, byssal threads are usually lost duringontogeny so that adults are free-lying on the seafloor, although given “P.” thompsoni's small size,the byssus might have been retained throughoutlife. The fact that there are differences in the byssalnotch between small and large individuals in sam-ple 052500-1C suggests that there are size- orage-dependent factors reflecting a greater need forphysical attachment to a hard substrate at largersize. Perhaps larger individuals in this sample withhigher water energy were subjected to dispropor-tionately greater hydrodynamic forces than in theother population and hence required relativelydenser byssal bundles, with greater surface areasof attachment (Vogel 1988). Also suggestive is thefact that this species was much less abundant in052500-1C where hydraulic energy is inferred tobe higher.

As this study has shown, morphologicalchange need not be interpreted solely as a spe-cies-level phenomenon, but can and should beconsidered in a paleocommunity context. The geo-graphic mosaic theory of co-evolution suggeststhat microevolutionary variation is influenced bylocal conditions and biotic interactions, and theDominican Republic material represents a primeopportunity to test this assertion. A major directionof further research should therefore be the charac-terization of morphological evolution in multiple co-occurring lineages, for example among pectinids

Figure 5. Plots of shape (relative warp score I) and size in 060700-2A (A; upper graph) and 052500-1C (B; lower graph). A. Linear regression indicates no change in shape with increasing sizes. B. A second-order polynomial regression indicates an allometric relationship with shape changing with size and is a better fit than a linear regression.


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and arcids, coupled with increased study of thepotential direct and indirect biotic interactionsamong those lineages. The use of both quantitativemorphometric techniques to evaluate population-level differences and quantitative paleoecologicaltechniques to evaluate community-level differ-ences can provide a powerful approach for charac-terizing these linkages within the context ofevolutionary paleoecology.

CONCLUSIONS

1. Paleocommunity analysis at the family, spe-cies, and abundance levels yielded differentpatterns of paleoecological information.

2. Among populations that are closely spacedstratigraphically, significant differences inshape and growth patterns were detected in“P.” thompsoni using landmark morphomet-rics. These differences are consistent withchanges in paleoenvironmental conditions.

3. Simple morphological analysis (for example,size only) would not have been sufficient todetect the morphological differences docu-mented between samples.

4. Paleocommunity analysis complements mor-phological studies of microevolutionary pro-cesses by (a) providing a context forevolutionary change and (b) indicating spe-cies that are ideal candidates for examininghow paleoecological changes may bereflected in morphology.

ACKNOWLEDGMENTS

This fieldwork and research is funded byNational Science Foundation grants (EAR-0307063 & EAR-9903151 to CT). Funding for J.Pantel’s participation came from the Summer Sys-tematics Institute at the California Academy of Sci-ences, which is supported by NSF-REU program(DBI-0139215). P. Roopnarine (CAS) provided

Figure 6. Thin-plate spline diagrams of predicted specimen shapes (relative warp I) at centroid size 4 mm and 7 mm for sample 060700-2A (upper diagram) and 052500-1C (lower diagram). Note the lack of significant changes in 060700-2A and the difference around landmarks 3 and 7 for sample 052500-1C. Pixelation of the figure is a function of the analysis routine's output.


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invaluable analytical assistance and discussionswhile L. Anderson (LSU), R. Nehm, J. Freiheit, A.Skaff, and V. Hwang provided field assistance. Weare grateful to J. Agnew and especially T. Carpen-ter for washing and processing the samples in thisstudy, to J. DeMouthe and A. Grimes for curatorialassistance, and P. Roopnarine and J.W. Hagadornfor editorial patience.

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