Paleoclimatic Inference from Morphological Structure in the Snail Gastrocopta Procera (Gastropoda:Pupillidae)

2004·6.6 MB·English

by  Barry B Miller

KIRTLANDIA
The Cleveland Museum of Natural History
March 2004 Number 54:89—100
PALEOCLIMATIC INFERENCE FROM MORPHOLOGICAL
STRUCTURE IN THE SNAIL GASTROCOPTA PROCERA
(GASTROPODA: PUPILLIDAE)
BARRYB.MILLER,CLAYDONC.COLIRI
DepartmentofGeology
KentStateUniversity,Kent,Ohio44242-0001
DANIELAC.SMITH,MICHAELJ.S.TEVESZ,andMICHAELA.GATES
DepartmentofBiological,Geological,andEnvironmentalSciences
ClevelandStateUniversity,Cleveland,Ohio44115-2406
[email protected]
ABSTRACT
ThisstudydescribesrelationshipsbetweenmorphometricparametersofCenozoicpopulations
ofGastrocoptaprocera(Gould, 1840)andclimaticvariablesforthenorthcentralUnitedStates
overwhichthesnailiscurrentlydistributed.Weusethemultivariatetechniquesofprincipalcom-
ponentsanalysisandmultiplelinearregressiontoillustratehowtheresponseofsnailstructureto
climaticvariationinextantpopulationscanbeusedtopredictclimaticconditionsforfossilpop-
ulations.FivePliocenepopulationsofG.franzenaeTaylor, 1960,twoHolocenepopulationsofG.
riograndensis(PilsbryandVanatta, 1900)andoneofG.cristata(PilsbryandVanatta, 1900)also
werestudied.Fourclimaticvariables(latitude,longitude,totalannualprecipitation,andnumber
offrost-freedays)haveahighlysignificantpredictivevalueforthemajoraxisofmorphological
variationinextantpopulationsofG.procera.Thisaxisaccountsformorethanhalfofthestruc-
tural variation among 339 specimens sampled from 31 extant populations across the biogeo-
graphicrangeofthespecies.Extrapolationofrelationshipsbetweenmorphologicalstructureand
modemclimaticconditionstofossilpopulationsstronglysuggeststhatthepopulationsrepresent-
edbyfossilslivedinconditionssimilartothosenowpresentinthesouthernpartsoftherangeof
G.procera.Theseanalysesimplythatpopulationsoftheputativeancestralspecies,G.franzenae,
mayhavelivedunderconditionswarmerandwetterthanthoseofpresent-daypopulationsofthe
descendantspecies.
90 MILLERANDOTHERS No.54
Introduction
The primary purpose ofthis paper is to demonstrate
relationshipsbetweenmorphometricparametersofpopu-
lationsofasnailandclimaticvariablesforthecontinental
region over which the snail is distributed. We illustrate
how the response ofsnail structure to climate in extant
populationscanbeusedtopredictclimaticconditionsfor
fossilpopulationsofthissnail.
Paleoclimatic reconstructions using nonmarine mol-
luscstypicallyassumethatclimateisamajorfactorcon-
trolling theirdistributions (Hibbard andTaylor, 1960, p.
16-19;Taylor,1960,p.4-9;Sparks,1961;Lozek,1964,p.
49-53; Harris andPip, 1973; Baerreis, 1980; Millerand
Kay, 1981; Keen, 1990). Onlyrarelyhavetheseassump-
tionsbeensubjectedtoquantitativeanalysis.Weareaware
ofonly one such study in NorthAmerica, involving the
terrestrial pulmonate gastropod, Gastrocopta procera mm
1
(Gould, 1840),astatisticalanalysiswhichsuggestedthat
someofthevariationinshellmorphologyappearedtobe I 1
relatedtoclimaticvariables(Couri, 1976). Figure 1. Diagram of a Gastrocoptaprocera specimen
GastrocoptaproceraandthecloselyrelatedspeciesG. showing the primary morphometric landmarks and vari-
cristata(PilsbryandVanatta,1900)areextantspecieswith ables.A,ventralview;B,lateralview.Abbreviationsand
relativelybroaddistributionsintheeasternUnitedStates definitionsofparametersaregiveninTable2.
(Pilsbry,1948,p.907,p.913;Hubricht,1985,p.72).They
are also common as fossils in many of the Pleistocene
molluscanfaunasreportedfromthesouthernGreatPlains morphology that have been used to distinguish various
(Hibbard and Taylor, 1960, p. 10; Taylor, 1965, p. 607; members of this cluster of species. Preliminary results
Miller, 1976). Another common Holocene species, G. fromthisphaseofthestudyyieldedstatisticallysignificant
riograndensis(PilsbryandVanatta, 1900,p. 595),isalso correlations between particular morphological measure-
morphologicallysimilartoG.procera. mentswithcertainclimaticparameters, suchastempera-
Someofthemorphologicalshellcharactersthatdistin- ture and precipitation, for the collection sites (Couri,
guish G. procera and G. cristata from each other have 1976). The potential usefulness of these shell-climate
beenfoundoccurringtogetherin many individuals from interrelationships forpaleoenvironmental reconstructions
severaldifferentPliocenefossillocalities.Taylor(1960,p. ledtoanexpansionofthisstudyintothecurrentreport.
67-69) considered the morphological overlap in these
Plioceneshellstojustifyanewspecies,G.franzenae,and MaterialsandMethods
suggested that G.franzenae was the probable ancestral Samples from Holocene populations of the snail
speciesthatevolvedintoG.proceraandG.cristataduring Gastrocopta procera were obtained from 31 localities
the Pleistocene. If true, this would be one of the few acrossthecurrentgeographicrangeofthespecies(Table
knownexamplesdocumentingtheevolutionofanewter- 1);atotalof614specimenswereusedinvariousanalyses.
restrialpulmonatesnailspeciesduringthePleistocene. Museumlotswereselectedforwhichtherewasreasonable
Ourinvestigationstartedasastudyofthenatureandmag- certainty that the specimens were collected alive and at
nitudeoftheevolutionarychangesinshellmorphologythat their living site. Thus, stream drift material, although
couldbemeasuredinthefossilshellsofG.procerafromthe abundant in museum collections, was excluded. In addi-
Pleistocene faunas of the southern Great Plains (Couri, tiontotheserepresentativesofGastrocoptaprocera,sam-
1976).Gastrocoptaprocerawaschosenforstudybecauseits ples were obtained of five Pliocene populations of G.
shellcharacteristicspermittedunequivocalidentificationto franzenae(atotalof141 specimens),twoHolocenepopu-
species(Figure 1) andtheavailabilityofbothmodemand lations of G. riograndensis (37 specimens), and one
fossilmaterialsforstudyinmuseumcollections. HolocenepopulationofG.cristata(29specimens).
The range of shell variation was evaluated in recent All specimens were cleaned and body remnants
populationsofG.procerafrom14localitiesovermuchof removedbysoakingin50percenthouseholdbleachsolu-
its modern range in the continental United States. tion,usingultrasonicagitationasnecessary.Onlymature
Measurements weremade oftheseveral aspects ofshell specimens, asjudgedbythepresenceoflamellaewithin
r
'
2003 PALEOCLIMATICINFERENCE 91
Table 1. Sampling localities, codes, and sample sizes. Table2. Morphometricparameters,climaticvariables,
Samples A through AE represent populations of andtheirabbreviations.
Gastrocoptaprocera\samples1-5areofG.franzenae;sam-
pbleelsonrgsatnodG.gcrbiesltoatnag.MtousGe.umriaobgbrraenvdieantsiiosn\saarnedassafmolplloews:c VHariable Umnmit haeliognhgtcooflusmheellll,afrDreaosxmcirsaippetxiotnoreflectedperistome,
KSU,KentStateUniversity,DepartmentofGeology;MCZ, W mm widthofshell,measuredperpendiculartothebody
Museum of Comparative Zoology, Harvard University; axis
UMMZ,UniversityofMichiganMuseumofZoology. WL count numberofwhorls,tonearestquarter-revolution
APW mm maximumangulo-parietallamellarwidth
Code Sample Museum Geographic Lat.(N) Long.(W) AXP anglebetweencolumellaraxisandaxisoflower
lotno. location palatallamella
A 22 UMMZ106898 OrangeCo.VA 38°15.6' 77°59.8' LCD mm lip-crestdistance,fromaperturelipedgetocrest
B 30 MCZ54573 DailsvilleMD 38°34.2' 76°04.7' BW mm ahxeiisghtofbodywhorl,fromouteredgeofreflected
C 13 MCZ95876& KnoxvilleTN 35°58.1' 83°55.1' peristometosuturebetweenbodywhorlandsecond
MCZ46852 whorl
D 30 MCZ3851& CincinnatiOH 39°06.0' 84°30.8' LAT °N latitude
UMMZ106894 LONG °W longitude
E 21 MCZ9659 AlbionIL 38°22.6' 88°03.5' FROST inches averagedepthoffrostpenetration
F 24 MCZ94679 St.LouisCo.MO 38°37.0' 90°11.6' TEMPAVE °F averageannualtemperature
G 12 UMMZ106905 CalceraAL 33°16.4' 86°42.5' TEMPJL °F averageJulytemperature
H 7 UMMZ106955 TregoCo.KS 38°54.6' 99°52.8' PRECSUM inches averagesummerprecipitation
I 18 KSU1970 EllisCo.KS 38°54.6' 99°19.5' PRECAVE inches averageannualprecipitation
J 26 KSU7460 BeaverCo.OK 36°43.7' 100°31.3' FFREE days averagenumberoffrost-freedays
K 12 UMMZ106928 BeaverCo.OK 36°43.7' 100°31.3' LASTKILL date averagedayoflastkillingfrostinSpring
L 30 UMMZ106932 WitchitaMtnsOK 34°51.0' 98°27.8' FIRSTKILL date averagedayoffirstkillingfrostinFall
M 30 UMMZ106926 DundeeTX 33°35.7' 98°41.5' CLOUDY days averagenumberofcloudydaysperyear
N 30 UMMZ165464 ComfortTX 29°57.0' 98°39.8'
P0 3110 MMCCZZ398824291 WBaesahriCnrgeteoknADLC 3328°°5230..53'' 7877°°0122..01'' tuhseed.apeCrlteuarneeda,ndmatthuerecressptecoinmetnhesewxetreernamloulnabtieudm,inwe6r0e-
Q 24 USNM474086 YorktownVA 37°14.1' 76°30.4' divisionfaunalslideswithgum-tragicanthsolution.Using
R 24 USNM448536 WilmingtonNC 34°14.2' 77°55.4' a Gaertner coordinate microscope (with “flat” optics),
S 16 USNM68744 NashvilleTN 36°09.8' 86°46.7' specimensweremountedwiththeapertureupandtherim
T 13 USNM231926 KansasCityMO 39°06.0' 94”34.7' oftheperistomeashorizontalaspossible.
U 24 USNM416301 LincolnNB 40°49.4' 96”41.1 Morphometricdataforallspecimenswerecollectedby
V 15 USNM424002 TopekaKS 39°03.1' 95°40.2' asingleobserverusingaGaertnercoordinatemicrometer.
w 24 USNM520233 SayreOK 35°17.7' 99°38.4' Figure1illustrateskeystructuralfeaturesofthissnailused
X 8 USNM536571 ColemanTX 31°44.5' 99°29.1' todefinethecorrespondingmorphometricvariables.Table
Y 24 USNM448585 AmarilloTX 35°12.5' 101°50.0' 2givesthe listofmeasurements andtheirabbreviations;
Z 11 USNM102555 LeeTX 30°17.1' 96°57.7' however,threeparameters(APW,LCD,andBW)werenot
AABA 2142 UUSSNNMM511016791488 GLreaenTtXLA 3310°°3147..7' 9926°°5371..17' tm9a0eb°alescsul.roeIcndkwfoiorsrdeearlalntoodfmrteehaemsopuuorpneutleaLdtCiwDoin,tsh,sptahesecniaompteeenrdtsuinrweesruteobstrehoqetuaelteneftdt
AC 13 USNM473858 ColbertAL 34°43.9' 87°49.0' andtheaperturerimvertical(asshowninFigure IB).
AD 12 USNM448594 MobileAL 30°41.3' 88°02.6' Shellheight(H)wasmeasuredbyatraversealongthe
AE 24 USNM511230 IndianapolisIN 39°46.2' 86°09.3' columellar axis from the apex to the outer edge ofthe
1 30 UMMZ183033 MeadeCo.KS 37°00.7' 100°32.0' reflected peristome, while the height ofthe body whorl
2 30 KSU0046 MeadeCo.KS 37°00.7' 100°32.0' (BW) was measured by traversing the same axis in the
3 21 UMMZ183001 OldhamCo.TX 35°34.8' 102°29.3' reversedirectionuntilthecrosshairsrestedonthe suture
4 30 UMMZ184153 SewardCo.KS 37°03.0' 100°55.8' betweenthebodywhorlandthesecondwhorl.Theshell
5 30 UMMZ183068 MeadeCo.KS 37°14.4' 100°21.6' width (W) was measured by a traverse across the body
r 14 UMMZ106957 VallesMX 21°15.0' 99°26.0' whorl perpendicular to the body axis. Angulo-parietal
g 23 UMMZ34269 BrownsvilleTX 25°54.0' 97°30.0' width(APW)wasmeasuredbytraversingacrossthetuber-
c 29 KSU7461 SayreOK 35°17.6' 99°39.1'
92 MILLERANDOTHERS No.54
cle. The angular measurement (AXP) was obtained by principal components analysis was undertaken with all
positioning the vertical crosshair on the columellar axis fourspecies,inordertoexaminepossiblechangesinmor-
withtheoriginoverlyingthecolumellardenticle,andthen phologicalstructureovertime.Thefinalstepintheanaly-
rotatingthestageuntilthelowerpalatallamellawaspar- siswastousetheinterpretationofthemorphologicalprin-
alleltotheverticalcrosshair.Thelip-crestdistance(LCD) cipalcomponentaxesintermsofclimate(viaregression)
was obtained by traversing the remounted specimen at topredictpossibleclimaticconditionsforthefossilpopu-
rightanglestotheaxisofthecrest. lationsonthebasisoftheirphysicalstructure,asindicated
Climatic data for the sample localities were taken bytheirlocationinprincipalcomponentspace.
from the nearest reporting station ofthe United States
WeatherBureau,fortheperiod 1899-1938.Theninecli- ResultsandDiscussion
matic parameters that were used correspond to average Univariateandbivariateanalysesofextantpopulations
valuesfortheneareststation,determinedfrompublished Table3presentsthemeanvaluesforeachmeasurement
mtiaopnssa(rKeingcievre,ni1n94T1a)b.leTh2e.iWredeisncsrpiepcttieodnsthaenrdelaabtbiroenvsihai-p tatheeamcehaonfatnhneu3al1vsaalmupelseolfoceaalcithieosfotfheG.clpirmoacteircav,aarsiawbellelsaast
between each ofthe original morphometric parameters thenearestrecording station. Toinvestigatepossiblerela-
fortheextantpopulationsandtheclimaticvariablescor- tionshipsbetweenextantmorphologyandmodemclimatic
respondingtotheirsamplelocalitiesbymeansofbivari- conditions,wecalculatedthematrixofbivariatecorrelation
atelinearregression.However,onlythetwohighestcor- coefficients,basedonthesemeanvalues,betweeneachof
relationsarereported. thesevenstructuralparametersandeachofthe 11 climatic
All statistical analyses were performed using the S- variables (Table 4). Linearregression analyses were per-
PLUS2000softwarepackage(MathSoft, 1999).Twopri- formed on each ofthe 61 statistically significantcorrela-
mary analytical techniques were used. To examine the tionsaftercorrectionformultiplecomparisons(Sokaland
maindimensionsofthemultivariatestructureofthemor- Rohlf, 1995). However, because ourprimary focus isthe
phometricdata,forbothextantandfossilpopulations,we relationshipbetweenthetotalityofmorphologicalstructure,
used principal components analysis (Morrison, 1967; whichisamultivariatephenomenon,andpossibleclimatic
Gauch, 1982). Thistechniqueconstructslinearcombina- influences,herewepresentonlytheresultsofthetwomost
ptiaosnsstohfetmhueltoirviagriinaatlemveaarsiautrieomneinnttshethdaattam.axInimpaarltliycuelanrc,otmh-e saingdnilfaitciatnutdebi(vra=ria0t.e84c3o,rrPel<ati0o.n0s0,1)viazn.,dtthhaattbbeettwweeeennLACXDP
firstprincipalcomponentaccountsforthegreatestvaria- and latitude (r= -0.819, P < 0.001). Figure 2 shows the
tioninthedata;thesecondcomponentisorthogonaltothe strong linearrelationshipbetween the mean valuesforthe
firstcomponent(i.e.,isperpendiculartoitanduncorrelat- angleofthelowerpalatallamella(AXP)andforthelip-crest
edwithit)andaccountsforthesecondlargestamountof distance (LCD) foreach ofthepopulationsonwhichthey
variation; the third component is orthogonal to the first weremeasuredandthelatitudeofthesamplinglocality.As
twoandaccountsforthenextlargestamountofvariation; latitude,inturn,ishighlycorrelatedwithcertainspecificcli-
etc. The principal components provide a mathematically matic factors (e.g., TEMPAVE, r = -0.974; FFREE, r =
uniquesolutionthateffectivelyreducesthedimensionality -0.943; FIRSTKILL, r=-0.873; FROST, r=0.845), this
ofthe original data. Typically, instead ofhaving a large suggeststhevalueoflookingattotalmorphologyandover-
numberofcorrelatedmeasurementsthattogetherdescribe allclimatebymultivariatemethods.
themorphologicalstructureofanorganism,thefirstfew
principal components often are sufficient to encompass Principalcomponentsanalysisofmorphologicalstruc-
mostofthevariationamongtheindividualsintheseveral tureinextantpopulations
sampledpopulations.Thesesyntheticvariablesthusrepre- Principal components analysis of the morphological
sentuncorrelatedaspectsoforganismalstructure.Because data,basedonthematrixofcorrelationsamongtheseven
two ofthe original morphological variables represented variates,wassuccessfulincapturingmostofthevariation
angularmeasurements,whiletheotherswerelinearmeas- amongall339specimens(thosewithnomissingvaluesfor
uresofthesizeofvariousaspectsoftheshell,ourprinci- all seven variates) in the first few principal components.
pal components analyses were based on the correlation Table5givesthe“loadings,”orcomposition,ofeachofthe
matrixamongthevariables. firstthreecomponentsintermstheirindividualcorrelation
Theprincipalcomponentsforthemorphologicalstruc- coefficients witheach ofthe original variables. Principal
tureofG.procerathenwererelatedtolatitude,longitude, component1(PCA1),whichaccountedfor52.8percentof
andtheninemorespecificclimaticvariablesforthesam- thetotalvariationinthedata,loadedpositivelyonallvari-
ple locations by using multiple linear regression tech- ables,especiallytheheight,basalwidth,andtotalwidthof
niques (Gauch, 1982; Sokal and Rohlf, 1995). Another the shell; asiscommoninmorphometric applicationsof
2003 PALEOCLIMATICINFERENCE 93
Table3.Meanvaluesforeachlocalityforthemorphometricandclimaticvariables,andforthescoresonthefirstfourprin-
cipalcomponents.Alllinearmorphometricparametersweremeasuredinhundredthsofmillimeters. Standarddeviationsof
themorphometricvariablesarereportedbeloweachmean.
Code H W WL APW AXP LCD BW LAT LONG FROST TEMPAVE TEMPJL
A 236.9 104.4 5.5 13.9 85.3 16.6 125.8 38.26 78.00 9.0 56.0 76.0
12.3 1.9 0.2 1.2 5.4 3.1 3.6
B 254.4 111.1 5.6 16.5 87.3 18.4 132.3 38.57 76.08 8.0 57.0 76.0
12.4 2.8 0.3 1.7 9.1 2.8 5.5
C 243.0 109.3 5.6 14.8 68.5 16.3 126.5 35.97 83.92 9.0 57.5 76.0
14.0 3.8 0.3 0.8 12.7 2.5 4.4
D 261.7 110.1 5.7 15.4 77.3 14.3 135.0 39.10 84.51 9.5 54.5 75.5
9.9 3.6 0.3 1.5 8.3 3.1 4.6
E 255.0 108.9 5.6 15.0 74.8 14.0 131.6 38.38 88.06 10.0 56.0 77.5
9.5 2.6 0.2 1.1 7.8 2.8 3.3
F 252.0 108.0 5.6 14.0 74.1 14.9 129.9 38.62 90.19 15.0 56.0 77.5
11.5 2.5 0.3 1.7 6.9 2.4 4.5
G 252.7 109.7 5.8 13.5 60.9 18.4 126.9 33.27 86.71 2.5 64.0 80.5
9.2 2.3 0.3 1.2 4.3 2.8 3.7
H 243.4 102.4 5.7 14.0 76.1 16.1 123.7 38.91 99.88 16.0 54.0 79.0
5.9 4.3 0.2 1.8 5.9 1.7 5.0
I 238.8 101.7 5.6 13.9 71.7 15.1 122.1 38.91 99.33 16.0 54.0 79.0
13.8 4.9 0.3 2.0 7.9 2.6 6.9
J 240.2 102.3 5.7 15.8 79.0 16.0 119.2 36.73 100.52 10.0 56.0 80.0
10.5 2.4 0.2 1.6 7.5 3.1 3.6
K 235.7 98.3 5.7 15.2 74.3 18.3 119.6 36.73 100.52 10.0 56.0 80.0
10.5 2.6 0.3 0.6 6.0 2.5 4.6
L 259.4 107.7 5.9 16.3 61.9 20.7 131.0 34.85 98.46 7.5 61.5 81.5
14.4 4.3 0.3 1.7 9.0 4.0 6.8
M 241.1 103.9 5.7 14.2 68.2 15.7 124.8 33.60 98.69 6.5 63.0 82.0
15.6 5.4 0.3 1.7 6.3 2.6 6.0
N 282.9 120.2 6.0 17.1 60.9 25.0 146.0 29.95 98.66 2.5 68.0 84.0
17.9 6.0 0.3 1.7 7.7 3.9 6.8
O 245.5 107.8 5.7 14.1 62.0 18.2 125.5 32.34 87.20 2.0 65.5 81.0
10.4 3.1 0.3 1.5 11.7 6.2 3.9
P 234.5 104.1 5.5 14.6 86.6 18.0 123.2 38.89 77.03 11.0 54.0 75.5
6.5 2.7 0.2 1.0 7.0 4.6 3.3
Q 225.1 104.4 5.2 79.3 37.24 76.51 4.0 58.0 77.0
11.5 2.3 0.2 7.9
R 243.2 106.4 5.3 73.9 34.24 77.92 1.5 63.0 79.0
6.2 2.5 0.1 7.4
S 247.3 105.4 5.5 72.6 36.16 86.78 4.5 58.0 78.0
10.5 2.8 0.2 10.2
T 233.1 99.9 5.3 81.7 39.10 94.58 5.0 55.5 78.0
13.5 3.9 0.2 11.7
U 236.5 103.3 5.4 79.0 40.82 96.69 29.0 51.0 77.0
7.0 2.5 0.2 9.1
V 230.8 101.3 5.3 80.1 39.05 95.67 17.5 54.0 77.5
w 25122..30 1073..13 05..25 7140..11 35.30 99.64 8.0 60.0 81.0
17.6 4.1 0.3 14.4
X 275.3 114.3 5.6 63.8 31.74 99.49 4.0 65.0 83.0
6.5 3.5 0.1 4.3
Y 262.0 110.4 5.6 58.2 35.21 101.83 6.0 57.5 78.0
13.5 3.8 0.2 5.2
Z 231.0 103.6 5.3 66.0 30.29 96.96 1.0 69.0 83.5
10.4 1.7 0.3 7.8
AA 237.5 102.7 5.4 70.3 30.29 96.96 1.0 69.0 83.5
14.3 3.4 0.2 11.8
AB 243.8 107.2 5.4 75.3 31.58 92.52 1.5 67.5 82.5
8.7 3.7 0.2 8.9
AC 244.5 107.1 5.5 74.0 34.73 87.82 3.5 61.0 80.0
11.0 2.1 0.3 8.5
AD 243.3 106.0 5.4 78.7 30.69 88.04 1.0 67.0 81.5
15.7 5.6 0.2 7.5
AE 258.6 107.7 5.7 75.8 15.3 39.77 86.16 20.0 52.5 75.0
8.5 2.6 0.2 5.4 2.0
94 MILLERANDOTHERS No.54
Table3.continued
Code PRECSUM PRECAVE FFREE LASTKILL FIRSTKILL CLOUDY PCA1 PCA2 PCA3 PCA4
A 13.5 40.0 200.0 110.0 298.0 110.0 1.05 -0.70 -0.05 -0.13
B 13.0 40.0 200.0 100.0 307.0 110.0 -0.30 -1.30 0.59 -0.08
C 13.0 50.0 200.0 106.0 295.0 120.0 -0.27 0.41 0.21 -0.46
D 12.0 40.0 180.0 110.0 293.0 135.0 -0.95 -0.64 0.10 0.13
E 10.0 42.0 187.0 105.0 292.0 118.0 -0.45 -0.22 0.25 0.12
F 11.5 40.0 178.0 100.0 293.0 117.0 -0.28 -0.11 0.12 0.06
G 13.0 48.0 220.0 377.0 312.0 110.0 -1.14 0.77 -0.20 -0.33
H 10.0 24.0 170.0 117.0 285.0 75.0 0.41 -0.15 -0.83 0.02
I 10.0 24.0 170.0 117.0 285.0 75.0 0.67 0.37 -0.74 0.01
J 9.5 21.0 190.0 108.0 293.0 65.0 0.64 -0.33 -0.79 -0.08
K 9.5 21.0 190.0 108.0 293.0 65.0 1.03 0.19 -1.34 0.05
L 8.0 30.0 220.0 89.0 303.0 85.0 -1.33 0.58 -0.66 0.01
M 7.0 27.5 220.0 87.0 314.0 88.0 0.17 0.52 -0.65 -0.18
N 7.0 29.0 240.0 79.0 322.0 92.0 -3.55 0.02 0.63 -0.21
O 13.7 48.0 235.0 77.0 316.0 107.0 -0.62 0.84 -0.33 -0.42
P 13.0 40.0 200.0 100.0 103.0 110.0 1.14 -0.81 -0.18 -0.26
Q 16.0 45.0 220.0 95.0 309.0 105.0 1.79 0.17 0.60 -0.25
R 18.0 50.0 240.0 69.0 324.0 90.0 0.63 0.28 0.58 0.20
s 12.0 50.0 200.0 100.0 295.0 125.0 0.25 0.22 0.03 0.21
T 12.0 36.0 182.0 108.0 293.0 102.0 1.75 -0.13 -0.27 0.29
U 11.2 30.0 165.0 119.0 283.0 105.0 1.04 -0.13 -0.17 -0.02
wV 128..00 3247..00 212800..00 19152..00 238083..00 10800..00 -01..1718 -00..0064 00..0176 00..1274
X 6.5 25.0 230.0 384.0 314.0 85.0 -2.00 0.33 0.82 0.49
Y 7.0 20.0 180.0 74.0 329.0 55.0 -1.31 1.03 0.32 0.26
z 8.0 31.0 260.0 79.0 324.0 95.0 1.16 1.15 0.24 -0.06
AA 8.0 31.0 260.0 79.0 324.0 95.0 0.97 0.67 -0.08 0.18
AB 13.0 55.0 240.0 71.0 316.0 105.0 0.42 0.05 0.46 0.03
AC 13.5 52.0 205.0 89.0 303.0 115.0 0.24 0.09 0.26 -0.04
AD 20.0 62.0 280.0 69.0 339.0 98.0 0.63 -0.19 0.31 0.11
AE 11.0 40.0 170.0 115.0 285.0 132.0 -0.61 -0.40 -0.14 0.24
principal components analysis, component 1 can be andM,respectively).Sincethiscomponentcorrespondsto
regardedas ageneral “sizefactor.”The secondprincipal ageneralsizefactor,snailsfromComfort,TXare,onaver-
component (PCA2), accountingforanother 15.0 percent age,thelargestinourdataset,whilethosefromDundee,
ofthe variation, involved acontrastbetween bothAPX, TXarethesmallest(ascanbeseenfromthemeanvalues
APWandWL(inthesensethatloadingsonthefirsttwo givenforallmeasurementsinTable3).
measures were high and negative, while the loading on TheG.procerapopulationfromDailsville,MD(letter
WL was positive). The third component (PCA2) com- B) is distinct in lyingnearthe low endofPCA2, corre-
prisedanadditional11.7percentofthevariation,andcon- sponding,e.g.,tohighvaluesoftheangularmeasurement
sistedofacontrastbetween,ontheonehand,LCDandthe andangulo-parietalwidth.Incontrast,thepopulationfrom
angulo-parietal width (APW), and on the other hand, BearCreek,AL(letterO)attheotherextremeofthePCA2
height,theangularmeasurement(AXP),andwhorlheight spectrum,haslowvaluesforthesemeasures(Table3).
(BW)andshellwidth(W).Thus,thefirstthreeprincipal TheplotofPCA2versusPCA3 (Figure3B)reiterates
componentstogetheraccountfornearly80percentofthe thedistinctnessofthetwolatterpopulations(lettersBand
variationinthemorphologicalstructureofG.procera,as O),aswellasshowingacontinuumalongPCA3fromthe
captured in our seven measurements of 339 specimens Cincinnati,OHpopulation(letterD)atthelowendtothe
from31 localitiesacrosstherangeofthespecies. WitchitaMountains,OKpopulation(letterL)atthehigh
Inspectionofthelocationoftheindividual specimens end.Thisrangecorrespondstoincreasingvaluesof,e.g.,
inthethreemostimportantdimensionsofprincipalcom- theratioofLCDtoheight.Thisisnotsurprising,giventhe
ponentsspace(Figure3)revealsthat,asonewouldexpect, highcorrelationbetweenshellheightandothersize-relat-
the majority of individuals cluster tightly in a central edparameters(inotherwords,shellheightisanobvious
region, correspondingtothe average morphology ofthis and trivial surrogate for “size”) and given the important
snail. However, certain populations consistently deviate correlationbetweenLCDandclimaticfactorssuchaslat-
from this central tendency. Forexample, the G. procera itude(Figure2B).
populationfromComfort,TX,clustersatthehighendof
PCA1,whiletheG.procerapopulationfromDundee,TX Relationships between multivariate morphological
clustersatthelowendofPCA1 (seeFigure3A,lettersN structureandclimaticfactors
r
2003 PALEOCLIMATICINFERENCE 95
Table4. Pairwisecorrelationcoefficientsbetweenmor- Table5. Loadingsofthefirstthreeprincipalcomponents
phometricparametersandclimaticvariables. (PCA1-PCA3) on the morphometric measurements of
Gastrocoptaprocera.
H W WL APW AXP LCD BW
LAT -0.483 -0.573 -0.771 -0.180 0.843 -0.819 -0.415 PCA1 PCA2 PCA3
TTFLFPPLFEAERRRORMSOMEENETPSCPCGEKJTAASILVVULEEML----00000000........553105315412217218529044 ---00000000........662406036351057857992474 ---00000000........371765144704575753497404 -----00000000........022132146409954915994000 ------00000000........507622379927934788406924 ----00000000........277736109096734013960624 ----00000000........421424317058073757742904 WHwAALBXCPLWPDW -0000000.......44342335187400 -----0000000.......30105711354462 -----0000000.......28121227788326
FIRSTKILL0.610 0.727 0.674 0.388 -0.541 0.799 0.565
CLOUDY 0.378 0.559 -0.101 0.133 0.031 -0.251 0.438 Standarddeviation 1.92 1.02 0.90
Percentvariance 52.77 14.99 11.67
Cumulativepercentvariance 52.77 67.76 79.43
Table6. Correlationcoefficientsbetweenmeanvaluesfor
thespecimensinalocalityoneachofthefirstthreeprin-
cipalcomponentsandtheclimaticvariables.
PCA1 PCA2 PCA3
LAT 0.3451 -0.5792 -0.3055
LONG -0.1785 0.4642 -0.4045
FROST 0.1853 -0.3708 -0.3653
TEMPAV -0.2958 0.5246 0.3547
TEMPJL -0.2692 0.5809 0.0160
PRECSUM 0.3965 -0.3052 0.2470
PRECAVE 0.1079 -0.1426 0.4463
FFREE -0.1059 0.3636 0.3835
LASTKILL -0.3206 0.1254 0.0834
FIRSTKILL -0.2417 0.4382 0.2333
CLOUDY -0.0027 -0.3624 0.3578
Latitude
Table6givesthecorrelationsbetweenthemeanvalues
foreachlocalityofeachofthefirstthreeprincipalcompo-
nentsandthevaluesforeachofthe 11 climaticfactorsfor
eachlocality.Itisclear,forexample,thatPCA1ispositive-
lycorrelatedmosthighlywithtotalprecipitation(r=0.397)
andwithlatitude(r=0.345)andnegativelycorrelatedmost
highly with the date ofthe last killing spring frost (r =
-0.321)andwithaverageannualtemperature( =-0.296).
ThissuggeststhatthegeneralsizeoftheshellofG.procera
(PCA1) will be larger (and therefore the organism will
attainlargersizes) athigherlatitudes, which aretypically
colderandwetterwithin thecontinental UnitedStates. In
thisrespect,G.proceraappearstofollowBergman’srule.
Thesecondprincipalcomponentismoststronglyposi-
tivelycorrelatedwiththeannualaverageandthemidsum-
Latitude mertemperature (r=0.525 and r=0.581, respectively)
andnegativelycorrelated(r=-0.579)withlatitude(Table
Figure2.Linearregressionfitsofthetwostrongestbivari- 6).Sincehigherlatitudescorrespondwithlowertempera-
atecorrelationsforGastrocoptaprocera.A,Theincrease tures overthe biogeographic range ofG. procera, these
ofAXP, the angleofthelowerpalatallamella, withlati- relationships imply that, based on our interpretation of
tude.B,ThedecreaseofLCD,thelip-crestdistance,with PCA2 as determined primarily by a contrast between
latitude.SymbolsrefertothelocalitycodesofTable 1. measurementsoftheaperture (APWandAXP) andWL,
r
96 MILLERANDOTHERS No.54
Table7. Resultsofmultiplestepwiselinearregressionof
the firsttwoprincipalcomponentofmorphometric vari-
ables for Gastrocopta procera (PCA1) on the climatic
variables.
CoefficientsforPCA1:
Value Std.error t-value Prob.(>Itl)
(Intercept) 1.0362 10.7264 -3.8257 0.0007
LAT 0.5889 0.1642 3.5874 0.0014
LONG 0.0830 0.0322 2.5773 0.0160
PRECSUM 0.1531 0.0822 1.8633 0.0738
FFREE 0.0519 0.0176 2.9459 0.0067
Residualstandarderror:0.9107on26degreesoffreedom
MultipleR-squared:0.4823
F-statistic:6.056on4and26degreesoffreedom(P-valueis0.001394)
CoefficientsforPCA2:
Value Std.error t-value Prob.(>Itl)
(Intercept) 3.2880 5.2289 0.6288 0.5348
LONG 0.0568 0.0207 2.7495 0.0105
TEMPAVE 0.1268 0.0498 2.5439 0.0170
TEMPJL 0.2009 0.1217 -1.6507 0.1104
Residualstandarderror:0.4107on27degreesoffreedom
MultipleR-squared:0.4850
10 20 F-statistic:8.475on3and27degreesoffreedom(P-valueis<0.001)
lycorrelatedwithlongitude(r=-0.405).Ourinterpretation
ofPCA3asastructuralfactorrelatedtotherelativesizeof
thelip-crestdistance(LCD)comparedwiththeheightofthe
1_UK Ik shellsuggeststhatsnailswithrelativelylargeLCDaremore
L likelytobefoundunderthewetterconditionsthatarecom-
monlyfoundintheeasternpartoftherangeofG.procera.
These and other piecemeal interpretations ofthe rela-
tionshipsbetweenmorphologicalstructureandgeneralcli-
matic factors are intriguing. However, amore satisfactory
approachistofindthebestfitbetweenvaluesofmorpho-
logicalfactors(inourcase,principalcomponents)andthe
entiresetofclimaticvariables.Table7givestheresultsof
the best multilinear regressions between each ofthe first
threeprincipal components andthe 11 climatic variables.
Thus,PCA1isfitverywell(multipleR-squared=-0.482,P
-0.1 0.0 0.1 0.2 <0.01)byalinearcombinationoflatitude,longitude,total
Component2 annualprecipitation,andnumberoffrost-freedays,withthe
highestloadingsonlatitudeandnumberoffrost-freedays.
Figure 3. Location of specimens belonging to various Inotherwords,thesefourclimaticvariableshaveahighly
populations of Gastrocoptaprocera (indicated with the significantpredictivevalueforthemajoraxisofmorpho-
locality codes of Table 1) in the morphometric space logicalvariationinextantpopulationsofG.procera,anaxis
definedbythefirstthreeprincipalcomponentsbasedon thataccountsformorethanhalfofthestructuralvariation
thesevenmeasurementsofTable2. among339specimenssampledfrom31extantpopulations
acrossthebiogeographicrangeofthespecies.
Similarly,PCA2,thenextmostimportantgeneralmor-
snails belonging to this species exhibit a relationship phologicalfactorcanbepredictedquitewell(multipleR-
betweenclimaticfactorsandfunctionalmorphologyofthe squared= 0.485, P<0.001) by alinearcombination of
aperturethatisnotexplainedbygeneralsizealone. longitude and the two measures ofaverage temperature
The third principal component is positively correlated (positivelywithrespecttoTEMPAVEforannualandneg-
withaverageannualprecipitation( =0.446)andnegative- ativelywithrespecttoTEMPJLforJuly).
2003 PALEOCLIMATICINFERENCE 97
Table8. Loadingsofthefirstthreeprincipalcomponents
on the morphometric measurementsoffourGastrocopta
species.
PCAl PCA2 PCA3
wH 00..5417 00..0117 -00..4110
WL 0.35 0.25 -0.76
APW 0.02 0.68 0.26
AXP -0.20 0.55 -0.13
LCD 0.33 -0.37 -0.21
BW 0.50 0.07 0.33
Standarddeviation 1.83 1.21 0.87
Percentvariance 47.71 20.97 10.88
Cumulativepercentvariance 47.71 68.68 79.56
Morphological structure of fossil and congeneric
populations
-0.10 -0.05 0.00 0.05 0.10 0.15 Datafor141 specimensbelongingtofivepopulations
Component1 presumedtobelongtothepaleospeciesG.franzenae,as
well as 37 specimens from two populations of the
Holocene species G. riograndensis and 29 specimens
fromonepopulationoftheHolocenespeciesG.cristata,
werecombinedwiththe339specimensofG.procerain
a separate analysis. Figure 4 shows the disposition of
both the fossil and extant specimens in the first three
dimensions of Gastrocopta morphological space, and
Table8givesthecompositionofthefirstthreeprincipal
components, which together account for 80 percent of
thetotalvariationinthedatasetof546Gastrocoptaspec-
imens.Whatis evidentisthat, while someofthe fossil
populations (e.g., population 4, from Seward County,
KS) correspond to the size and shape oftypical extant
populations, some (population 5, from Meade County,
KS) arelocatedatthelowendandsome(population3,
fromOldhamCounty,TX)arelocatedatthehighendof
PCA1, which accounts for48 percent ofthe total mor-
phologicalvariation.SincePCA1 againcanbeinterpret-
edasageneralsizefactor(althoughthistimenegatively
correlatedwiththeangulardimension),thisimpliesthat
the fossil populations span the same range of sizes as
-0.10 -0.05 0.00 0.05 0.10 modernpopulations.
Component2 Evenmoreinteresting,mostofthefossilpopulations,
aswellastheothertwoextantspecies, lieatoneendof
Figure4. Locationofspecimensbelongingtovariouspop- the spectrum along PCA2 (Figure 4A), while none are
ulationsoffourspeciesofGastrocopta(indicatedwiththe locatedattheotherendofthisspectrum.PCA2isprima-
localitycodesofTable1)inthemorphometricspacedefined rilyacontrastbetweenAPWandAPX,ontheonehand,
bythefirstthreeprincipalcomponentsbasedontheseven andLCD,ontheother(Table8).Thismightbeinterpret-
measurementsofTable2.PopulationsofHoloceneG.pro- edasashiftoverevolutionarytimeinthisorganismfrom
cera are denoted with uppercase letters; Pliocene G. populationsofsnailswithprimarilyoneformofaperture
franzenae populations are indicated with numerals; two morphologytothe moretypical morphology seenatthe
HoloceneG.riograndensispopulationsaredenotedwiththe presenttime.However,sincewehavedataonlyforafew
lowercaseletter“r”and“g”,whileasingleHolocenepopu- fossilpopulationsfromoneparticulargeographicregion,
lationofG.cristataisdenotedwiththelowercaseletter“c.” suchaconclusionisprematureatthistime.
98 MILLERANDOTHERS No.54
What is most clear from the principal component
analysisofallfourGastrocoptaspeciesisthatthefossil
species,G.franzenae,liesatoneendofthemorphologi-
calspectrum,inanareathatoverlapswithG.riogranden-
sisandG.cristata,aswellassomepopulationsofG.pro-
cera. This supports the possibility that the evolution of
morphologicalstructureinthisgastropodgenushasbeen
unidirectionalovertime,andthatthatdirectionispartial-
ly captured by PCA2. If so, G. riograndensis and G.
cristata are more primitive than G. procera. This view
wouldbestrengthenedbyanalysesofadditionalpopula-
tionsoffossilandextantspecies.
Gastrocopta riograndensis appears to occur in one
restrictedregionofthemorphologicalspacedefinedbythe
firstthreeprincipalcomponents,whileG.cristataliesina
different corner; the presumed ancestral species, G.
franzenae, spans this region. However, only two popula-
tionsofthefirstandoneofthelatterspeciesweresampled,
sothisconclusionistantalizingbutnotdefinitive.Clearly,
moreworkonthesystematicsofthesespeciesiswarranted.
Predictedclimaticconditionsforfossilpopulations
Whatthemorphologicaldistinctnessofseveralofthe
fossil populations can tell us with some certainty are
inferences about possible climatic conditions that were
presentatthetimethatthefossilpopulationswerealive.
These inferences rely on the assumption that the fossil
populationsbelongeithertothesamebiologicalclusterof
species as the living ones (continuity ofdescent) or to
speciessufficientlysimilarinbiologythattheyrespondin
thesamewaytomajorenvironmentalfactors. PCA2
Our results suggest that we may use the location of
fossil populations in morphological space and the rela- Figure 5. Location ofthe population means ofall four
tionshipsbetweenextantmorphologyandmodernclimat- speciesofGastrocoptainthemorphometricspacedefined
ic conditions to predict aspects ofpaleoclimatic condi- byHoloceneG.procerapopulations,asgiveninFigure3.
tions. Inparticular,thelocationofthefossilpopulations AbbreviationsareasinTable 1 andFigure4.
atthehighendofPCA2(Figure5)stronglysuggeststhat
theylivedinconditionssimilartothosenowpresentinthe
southernpartsoftherangeofG.procera,corresponding intheirshellmorphology,similarlytolong-termclimat-
topopulationsY,Z,AA,andO(fromTexasandAlabama: icconditions,wecanusethesestronglinearrelationships
seeTable 1). Unfortunately, there are no climatic infer- inextantpopulationstopredictplausibleclimaticcondi-
encedataforthespecificfossillocalitiessampled. tions for the fossil populations of G. franzenae, G.
Thecorrespondencebetweenshellstructuralparame- riograndensis, and G. cristata. Table 10 gives the pre-
tersandclimaticvariablescanbeusedtopredicttheval- dicted values for these populations for these two vari-
ues ofmajorclimatic variables fromthe morphological ables, along with their 95 percent confidence limits.
principal components, using regression analysis. For These results suggest, for example, that populations of
example, Figure 6 andTable 9 illustrate the very good the putative ancestral fossil species, G.franzenae, may
predictiveabilityofthefirstthreeprincipalcomponents havelivedunderconditionsevenwarmerandwetterthan
ofmorphologyfromG.procerainpredictingtheaverage those of present-day populations of the descendant
annualtemperature(TEMPAVE)andtheaverageannual species,G.procera, fromTexasandAlabama.
precipitation (PRECAVE) of the locations from which These inferences from our morphological analyses
thepopulationsweresampled.Assumingthatwearecor- couldbetestedfurtherbymoredirectanalysesofpaleo-
rectinregardingall Gastrocopta species asresponding. climate information for the fossil strata from which G.