new visions of dental tissue research: tooth development, chemistry ...bioanth/tanya_smith/pdf/smith...

New Visions of Dental Tissue Research: ToothDevelopment, Chemistry, and StructureTANYA M. SMITH AND PAUL TAFFOREAU

Teeth are one of the best preserved and most commonly recovered elementsin primate fossil assemblages. Taxonomic, functional, and phylogenetic hypothe-ses often rely on dental characters, despite considerable evidence of homoplasyin tooth form and large variation in tooth size within and among primates.1,2

Recent studies have led to new areas of research centered on incremental toothdevelopment, chemical composition, and internal structure. Due to rapid techno-logical developments in imaging and elemental sampling, these new approacheshave the potential to increase our understanding of developmental biology,including not only changes in the pace of growth and reproduction, but also ourassessments of diets, migration patterns, environments, and taxonomy. The inte-gration of these temporal, chemical, and structural approaches heralds a brightfuture for the role of dental tissue research in evolutionary anthropology.

It is well established that dentaltissues preserve a permanent recordof their development through time,represented by incremental features

in enamel and dentine microstruc-ture.3–8 Counts and measurements ofthese incremental features have beenused to determine the rate and dura-tion of tooth formation, stress expe-rienced during development, and theage at death of juveniles. Incremen-tal microstructure has traditionallybeen assessed through physical sec-tioning and preparation of histologi-cal sections, which reveal contrastinglinear increments when viewed withlight microscopy (Fig. 1). Develop-mental features are often classifiedas short- and long-period increments,based on their rhythmic repeat inter-vals (Box 1). Although histologicalinvestigations can yield highly accu-rate estimates of tooth formation,until recently these types of studieshave been limited to small samplesdue to their time-consuming andsemi-destructive nature.6

Because tooth growth beginsbefore birth and continues through-out adolescence, assessment of incre-mental features may permit precisereconstruction of an individual’s de-velopmental history, including birth,subsequent stress during develop-ment, and death (Fig. 2). For exam-

ple, a recent study of a young captivegorilla demonstrated a strong corre-lation between accentuated lines intooth enamel and traumatic events,including eye injury, subsequent hos-pital visits, and transfers to differentenclosures.11 Other such studies havesuggested that patterns of develop-mental stress inside teeth may corre-late with social stress, weaning, eco-logical variation, and menarche.12

Similar interpretations have beenbased on patterns of hypoplasias onexternal tooth surfaces,13–15 althoughwithout knowledge of the precisetiming of tooth formation. Unfortu-nately, relatively few data exist forindividuals with corresponding be-havioral, physiological, and ecologi-cal records, a fact that prohibits con-fident interpretations of accentuatedlines or hypoplasias in fossil prima-tes or archeological material. Thisline of research would benefit fromstudy of additional individuals withknown histories, as was done bySchwartz and colleagues.11 Onepotentially complementary approachinvolves the assessment of changesin tooth chemistry, since dietarychanges associated with birth orweaning may be integrated with thetiming of tooth development.16–19

INCREMENTAL TOOTHDEVELOPMENT

Developmental Variation

Over the past 25 years, incremen-tal features have been investigated indiverse primate taxa, particularlyhominoid primates6 (Table 1). Dur-ing the past decade, researchers havetaken a rigorous comparativeapproach through the use of large

ARTICLES

Tanya Smith is an Assistant Professor inthe Department of Anthropology atHarvard University, and an AssociatedScientist in the Department of HumanEvolution at the Max Planck Institute forEvolutionary Anthropology. Her primaryresearch centers on the fundamental na-ture of dental microstructure, includingits variation in hominoid primates, as wellas applications for understanding pri-mate ontogeny and phylogeny. E-mail:[email protected] Tafforeau is a member of the imag-ing group of the European SynchrotronRadiation Facility (ESRF). His mainresearch is on fossil and modern primatetooth structure, microstructure and de-velopment. He is also in charge of thedevelopment of synchrotron X-ray imag-ing for paleontology at the ESRF. E-mail:[email protected]

Key words: tooth microstructure; incremental

feature; isotope; diagenesis; micro-computed

tomography; micro-CT; synchrotron; phase

contrast; virtual histology

VVC 2008 Wiley-Liss, Inc.DOI 10.1002/evan.20176Published online in Wiley InterScience(www.interscience.wiley.com).

Evolutionary Anthropology 17:213–226 (2008)

samples and analyses of variationwithin particular tooth types andamong teeth of a single taxon.20–23

These studies demonstrate thatcertain aspects of dental microstruc-ture, such as the average rate ofenamel secretion, are fairly con-served among hominoids, while theduration of secretion and periodicityof long-period lines varies consider-ably. For example, hominoid primateperiodicities can range from 4 to 12days, with living humans showingthe widest-known range, 6 to 12days.6 Factors implicated in develop-mental variation include differencesin brain size, body mass, metabolicrate, diet, and life history, definedhere as the pace of growth and devel-opment, including ages at weaningand first and last reproduction, aswell as total life span.3,6,9,24,25 How-ever, these patterns become morecomplex when considered withintaxa such as humans or acrossdiverse primate taxa,25,26 necessitat-ing further study of animals withknown developmental and physiolog-ical variables, such as brain size,body mass, and metabolic rate.Modern humans are one of the

most well-documented primate taxafor which incremental data are avail-able; this is largely due to the work ofReid and colleagues,27–29 who haverecently completed analyses of eachtooth type in a relatively large sampleof northern European and southernAfrican individuals. These data revealsubstantial developmental variationwithin humans, particularly for ante-rior teeth and premolars, with south-ern Africans consistently showinglower numbers of perikymata andshorter formation times. Theseresults underscore the importance ofconsidering intraspecific variationwhen comparing fossil hominins andmodern humans.21,30,31 Modernhuman populations also display aninverse correlation between Retziusline number and periodicity within atooth and cusp type, leading to crownformation times with relatively smallstandard deviations.23,32 These find-ings on modern human developmenthave recently been applied to theinterpretation of tooth growth andlife history in Neanderthals, contrib-uting to one of the more hotly

Figure 1. Developing chimpanzee molar illustrating the scale of dental microstructurestudies, ranging from the whole crown (top), a plane of section across the mesial cusps(middle), and incremental features in the enamel observed with transmitted light micros-copy (bottom). Not to the same scale. [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com.]

214 Smith and Tafforeau ARTICLES

Box 1. Glossary of Aspects of Dental Microstructure and Tooth Development

Accentuated line—a pronouncedinternal line corresponding to the posi-tion of the developing enamel or den-

tine front; it relates to a stressor experi-enced during tooth development (asopposed to an intrinsic rhythm). The

neonatal line found in teeth developingduring birth is the most commonexample. See examples in Figure 2.

Figure B1. Incremental features in enamel (A,C) and dentine (B,D). Long-period lines are indicated by white arrows (Retzius linesin A, Andresen lines in B) and blue dotted lines (perikymata in C, periradicular bands in D). Groups of four short-period lines areindicated within each white bracket (cross-striations in A, von Ebner’s lines in B). Images are not to the same scale.

ARTICLES New Visions of Dental Tissue Research 215

debated topics in dental tissueresearch.

Hominoid and HomininLife History

Dental microstructure analysis hasfrequently been employed in studiesof the evolution of hominoid andhominin life histories.6,33 A keyadvantage of using incremental fea-tures is their ability to yield chrono-

logical age estimates of specificdevelopmental stages, such as firstmolar eruption, which are correlatedwith other developmental varia-bles.10,25,33,34 These studies suggestthat, relative to other catarrhineprimates, a prolonged hominoid life-history pattern may have arisen inthe early Miocene, as shown by longcrown formation times in Proconsulnyanzae and Afropithecus turkanen-sis, slow rates of root extension in P.

nyanzae, and a chimpanzee-like ageat first molar eruption in A. turka-nensis.33–36 However, determinationof an ancestral great ape pattern hasbeen complicated by recent docu-mentation of substantial variation inextant great ape crown formationtimes and molar eruption ages,11,22,37

as well as life-history parameters.38

Additional data on developmentalparameters in other early and middleMiocene taxa, as well as in addi-

Andresen lines—long-period(greater than circadian) incrementalfeatures in dentine representing thesuccessive positions of the dentine-forming front, and corresponding toperiradicular bands on the root sur-face. They are temporally equivalentto Retzius lines and perikymata inenamel. See examples in Figure B1B.Cross-striations—short-period in-

cremental features in enamel runningat right angles to enamel prisms.Cross-striations represent a 24-hourcycle of ameloblast activity. Thus,the distance between adjacentcross-striations yields the daily rateof enamel secretion. These striationsare temporally equivalent to vonEbner’s lines in dentine. See exam-ples in Figure B1A.Crown formation time—the dura-

tion of tooth crown secretion, eitherfor a single cusp or the entire crownof multicusped teeth, which is typi-cally assessed through counts andmeasurements of cross-striationsand Retzius lines.Dental development—the contin-

uous process of tooth initiation, ma-trix secretion, crown mineralization,dental eruption, and root completion.Primate dental development beginsbefore birth with initiation of the de-ciduous dentition, followed by initia-tion of the permanent dentition.Dental eruption—the process of

tooth crown emergence; the toothmust move past the bone margin(alveolar eruption) and the gum (gin-gival eruption) in order to emergeinto the oral cavity and eventuallyinto functional occlusion.Dentine development—dentine is

formed when odontoblasts secretea collagenous matrix (predentine),

which rapidly undergoes mineraliza-tion to form primary dentine. Den-tine formation begins at the dentinehorn underlying the future cusp tipand progresses inward throughsecretion and downward throughextension until it reaches the apexof the root.Enamel development—enamel is

formed when ameloblasts secreteenamel matrix proteins that mineral-ize into long, thin bundles of hy-droxyapatite crystallites known asenamel prisms. As the secretorycells progress outward toward thefuture tooth surface, additional adja-cent cells are activated throughextension until the forming frontreaches the cervix of the crown.Hypoplasia—feature formed when

hard tissue formation has been dis-rupted by a stressor experienced dur-ing development, which results in ananomalous depression (furrow or pit)on the surface of a tooth. These exter-nal features are often associated withaccentuated lines internally.Incremental features—micro-

scopic markings that representintrinsic temporal rhythms in dentalhard tissue secretion. These can beannual (cementum annulations),long-period (for example, Retziuslines in enamel), or short-period (forexample, cross-striations in enamel).See examples in Figure B1.Laminations—daily incremental

features in enamel running parallel tothe developing enamel front andRetzius lines. Laminations are tempo-rally equivalent to cross-striations butcan be distinguished from them bytheir relatively oblique orientation.Perikymata—external ridges and

troughs encircling the tooth crown

that are formed by Retzius lines asthey reach the enamel surface. Thisregion of the crown is often calledthe imbricational enamel. See exam-ples in Figure B1C.Periodicity—the number of days

between long-period lines (Retziuslines or perikymata in enamel, Andre-sen lines or periradicular bands indentine). Periodicity typically isassessed by counting daily cross-striations between pairs of Retziuslines. The value for long-period lineperiodicity is consistent within all teethin an individual’s dentition, but mayvary among individuals in a taxon.Periradicular bands—external

ridges and troughs encircling thetooth root that are temporally equiv-alent to the Andresen lines in thedentine (in addition to the long-pe-riod lines in enamel). See examplesin Figure B1D.Retzius lines—long-period incre-

mental features in enamel that repre-sent the position of the developingenamel front at successive points intime. They are manifest on the toothsurface as perikymata and are analo-gous to the Andresen lines in dentine.The temporal repeat interval of Retziuslines is known as their periodicity. Seeexamples in Figure B1A.von Ebner’s lines—short-period

incremental features in dentine thatreflect a 24-hour cycle of dentinesecretion and are temporally equiva-lent to cross-striations and lamina-tions in enamel. See examples inFigure B1B.For additional information and

illustrations, see Dean,3 FitzGerald,4

Smith,5,6 Bromage,7 Tafforeau andcolleagues,8 Dean and Scandrett,9

and Smith and colleagues.10


tional extant wild individuals, wouldhelp in further clarifying the evolu-tion of great ape life histories.Dental microstructure studies of

fossil hominins have almost com-pletely replaced determinations ofdental eruption sequences or com-parisons of relative degrees of devel-opment. Dental eruption sequencesare known to be polymorphic withintaxa,39 while comparisons of thedegree of dental developmentrequires an assumption of develop-mental time or chronological age,leading to circular reasoning.40

Bromage and Dean41 first applieddental microstructure analysis toestimate the age at first molar emer-gence in several juvenile Plio-Pleisto-cene hominins, which appeared to beseveral years younger than modernhuman children at a similar stage ofdental development. This and severalsubsequent studies have led to a gen-eral consensus that the prolongedchildhood period and slow life his-tory that are characteristic of livinghumans originated fairly recently,most likely within later members ofthe genus Homo.6,33

Unfortunately, little is knownabout incremental tooth growth inMiddle Pleistocene hominins, savefor counts of perikymata on anteriorteeth of Homo antecessor and Homoheidelbergensis.42 As reviewed bySmith and colleagues10 and Guatelli-Steinberg,101 data on Neanderthal

tooth growth are conflicting. Forexample, counts of perikymata havebeen interpreted as suggestingrapid development relative to recenthumans42 or as indicating substan-tial overlap between Neanderthalsand modern humans.29,31 However,since perikymata numbers do notdirectly yield formation times, infor-mation from internal features is criti-cal to determine precise crown for-mation and eruption ages. Recenthistological studies of Neanderthalpermanent molar formation illustratesimilarities in certain aspects of in-ternal development, such as dailyrates of enamel secretion,43 but dif-ferences in cuspal enamel thicknessand rates of crown extension, leadingto shorter molar formation times.10

Building on these differences, astudy of the developmental status ofthe nearly complete dentition of a ju-venile Belgian Neanderthal suggestedthat Neanderthals did not share theprolonged life history of living andfossil Homo sapiens.10 Exactly whereand when this transition took placewithin the genus Homo remainsunresolved.

Virtual Histology

A promising new area of incremen-tal feature research is the applicationof virtual histology via propagation-phase contrast X-ray synchrotronmicrotomography,44,45 which has

greatly benefited from recent advan-ces in technology. A synchrotron is amachine that produces beams oflight from accelerated electrons devi-ated by magnetic fields. Dependingon the energy, the light spectrummay range from radio frequencies tohigh-energy X-rays (hard X-rays).This type of light source facilitatesmore efficient and diverse imagingpossibilities than do conventional lab-oratory X-ray sources.45 This is a con-sequence of specific physical proper-ties such as high flux, monochroma-ticity, parallel geometry, and spatialcoherence (defined in Box 2). The firstthree of these special properties pro-duce high-quality absorption scanswith gray levels that can be used toquantify mineral densities.8

A unique and critical aspect ofsynchrotron imaging for dental tis-sue research is based on phase-con-trast techniques, which reveal subtlevariations in tissue structure thatare invisible with other absorptioncontrast imaging techniques such asradiography or conventional micro-tomography. Over the past fewyears, improvements in the micro-tomography beamline ID19 at theEuropean Synchrotron RadiationFacility in Grenoble, France, havepermitted rapid imaging of micro-structure in fully mineralizeddental tissues at submicron resolu-tion.8,44–47 This nondestructive tech-nique yields images of incremental

Figure 2. Illustration of histological reconstruction of an individual chimpanzee’s developmental history from birth (green line on left) todeath (blue arrow on right). The colored lines indicate unspecified postnatal developmental stress, with the corresponding age in daysgiven. The age at death determined from field notes was 1,372 days, or 3.76 years, suggesting that histologically derived times of stressevents are likely to be less than 2% different from the actual timing.


features in tooth enamel and den-tine, as well as the neonatal line,which can be seen in highly miner-alized fossils that are millions ofyears old.44 Highly precise assess-ments of tooth formation are nowpossible though the nondestructivevirtual determination of long-periodline periodicity. In combination withvirtual data on cuspal enamel thick-ness and the developmental statusof unerupted teeth, this approach

can be used to characterize dentaldevelopment and age at death inmaterial that is unavailable fortraditional physical sectioning.47 Itmay also be used to provide novelinternal information from previouslystudied fossils. Beyond revealing de-velopmental information in fossil-ized dentitions nondestructively, itis hoped that virtual histology willeventually facilitate precise under-standing of the three-dimensional

growth of a tooth at the cellularlevel.

DEVELOPMENTAL TIME ANDTOOTH CHEMISTRY

Studies of tooth chemistry, partic-ularly the proportions of specific ele-ments such as strontium and cal-cium or ratios of elemental isotopessuch as 13C/12C, 18O/16O, and

Box 2. Glossary of X-ray and Synchrotron Imaging Terms

Absorption—the amount ofincoming X-rays stopped by a mate-rial for a given thickness (expressedin cm21). The different tissues of atooth have different absorptions,largely depending on their densityand chemical composition.Beam geometry—the shape of the

x-ray beam, which affects the imageresolution and the way that cross-sectional slices are reconstructedfrom radiographs. Most conventionalmicrotomographs are based on conebeam geometry, as the beamdiverges from a microfocus X-raysource. The resolution limit is deter-mined by the source size. Changingthe position of the sample in the conemodifies the magnification factor ofthe radiographs. Synchrotron beamshave such small divergence that theyare considered to be parallel. The re-solution is limited only by the detec-tor. In parallel geometry, the algo-rithms used for slice reconstructionare exact, in contrast to the approxi-mated algorithms used for cone beamreconstruction.Beam hardening—differential

absorption of the beam by the sam-ple leading to artifacts in recon-structed slices, such as brighteningof the tissue borders and false bridg-ing of dense structures. Beam hard-ening occurs when the beam leavingthe sample has relatively more hardX-rays than does the beam enteringthe sample. When micro-tomographyis performed using a white X-raybeam (polychromatic continuous X-ray spectrum), the low energies of

the beam are absorbed by the sam-ples more than the high energies(the hard X-rays); this increases withsample thickness and density.Beamline—experimental station

where experiments are conducted,which uses the very narrow conicalbeam of synchrotron light generatedat each position where acceleratedelectrons are deviated by bendingmagnets or insertion devices (periodicmagnetic devices that cause elec-trons to undulate). A large synchrotroncan have more than 40 beamlinesdevoted to diverse techniques.Coherence—a physical property

of synchrotron radiation related to X-ray wave geometry, which facilitatesphase contrast imaging. Third-gener-ation synchrotron radiation facilitiesexhibit small (<100 lm) X-ray sourcesizes and long source-to-sample dis-tances (in the 50–150 m range). Insuch conditions, the spherical X-raywaves originating from the nearlypunctual source can be considered,at the level of the sample, as quasi-plane waves, with an extended lat-eral coherence. Structures imagedwith this coherent light produceinterferences when the beam propa-gates, leading to the propagationphase contrast effect.Flux—essentially, the number of

photons for a given surface during agiven time. Synchrotrons have farhigher flux than do conventionalsources (up to 1014 times). In addi-tion to rapid scans and very highdata quality, high flux allows the useof monochromators.

Monochromaticity—selection ofonly a narrow part of the incomingwhite X-ray (polychromatic) beamspectrum produced by the sourceby using a monochromator. A mono-chromatic beam produces imageswithout beam hardening artifactsand allows quantitative imaging.Propagation phase contrast—

imaging technique based on thedetection of interference patternsgenerated by interfaces in the sam-ple, seen when imaged with a par-tially coherent X-ray beam. Increas-ing the sample-to-detector distanceincreases the phase contrast effect.This technique often reveals struc-tures that are invisible with absorp-tion contrast.Resolution and voxel size—re-

solution is the size of the smallestdetail that can be detected with agiven setup (see ‘‘beam geometry’’).Voxel size corresponds to the sizeof each elemental component ofthe 3D volume. It is related to thenumber of pixels of the detectorand the magnification factor (in thecase of conical or fan beam geom-etry). For an equivalent voxel size,the resolution would be better witha synchrotron than with a conven-tional microtomograph due to thebeam geometry.Tomography—computerized re-

construction of virtual slices throughan object from a set of radiographstaken during rotation of the sample.For additional information, see

Tafforeau and colleagues.45


87Sr/86Sr, have expanded over thepast decade to integrate informationon the pattern and duration ofdental development. Initial analysesrequired bulk samples taken from alarge proportion of the tooth crownor root to infer an organism’s dietgeographic location, or climate ofthe region in which the individuallived during tooth formation48,49

(Fig. 3). In contrast to bone, dentaltissues mineralize over a relativelyshort time without subsequentremodeling,50 providing a narrowerwindow of recorded time. Isotopicstudies of dental tissues may provideinformation about young individuals;once the tooth roots are complete,relatively little tissue is added laterin life (in the form of small amountsof secondary/tertiary dentine andcementum).

Variation Within Individuals

Several recent studies have usedinformation on tooth growth to lookat migration patterns and dietarychanges in human archeologicalcontexts.51 Sealy, Armstrong, andSchrire52 examined skeletal and den-tal samples from five unknownindividuals, including pairs of ear-lier- and later-forming teeth, andconcluded that differences in carbon,nitrogen, and strontium isotope

ratios revealed dietary shifts andmigration events. Wright andSchwarcz18 examined changes in car-bon and oxygen isotopes across sev-eral teeth of the same individualsspanning the time from birth to ado-lescence. Their results suggested thatinfant diets in their archeologicalpopulation were supplemented withsolid food by two years of age,although breastfeeding appeared tocontinue for an extended period.Building on this bulk samplingapproach, Fuller, Richards, andMays19 used the internal spatial pat-tern of tooth formation to trackchanges in nitrogen and carbon iso-topes in serial sections of succes-sively forming tooth crowns androots, demonstrating differencesbetween earlier- and later-formedregions within a tooth, which wereinterpreted as pre- and postweaningdietary signals.

Most recently, Humphrey and col-leagues16,102 used laser ablationinductively coupled plasma massspectrometry to sample strontium/calcium ratios of pre- and postnatalenamel in histological thin sectionsof teeth from modern humans. Thistechnique involves focusing a thinlaser beam on a small area of a toothsurface or thin section that trans-forms (ablates) the solid into aplasma, which is then analyzed with

a mass spectrometer to yield the ele-mental composition. The results ofthis study demonstrated thatdecreasing strontium/calcium ratioscould be tracked through time fromthe inner to the outer enamel, reveal-ing dietary shifts associated withbirth and the beginning of the post-natal diet. Laser ablation is a promis-ing new approach, as certain traceelements16,17,53 and carbon withincarbonate54 can be detected inmicroscopic samples and related toincremental features. However, thistechnique is restricted to measure-ments of a limited number of ele-ments and is typically performed oninorganic material.

The Problem of EnamelMineralization

Isotopic studies of dental enamelhave recently underscored the impor-tance of considering the complextooth mineralization process foraccurate temporal interpretationof chemical signatures.8,16,46,51,55–58

Mineralization proceeds in a seriesof stages as apatite crystallites growin length and thickness, with the ma-jority of mineral incorporationoccurring some time after the end ofsecretion.8,16,55 Isotopic studies ofintratooth variation often sampleenamel serially from one end of thecrown to the other, assuming a rateof constant growth and mineraliza-tion, which allows isotopic values tobe regressed against sampling posi-tion as a proxy for a specific point indevelopmental time (see, for exam-ple, Fig. 3, p. 208, in Balasse and col-leagues59). However, the final miner-alization process does not follow thepattern of matrix secretion, which isrepresented by incremental fea-tures.8,60,61 This is particularly criti-cal for studies that attempt to relateisotopic information to local incre-mental features.8,16,17,54 It is stillunclear when particular elements areincorporated into enamel, and thusif the recovered signal has been‘‘dampened’’ (shifted in time or modi-fied in degree) by subsequent miner-alization during maturation.16,51 Fur-ther complicating the situation,many studies of tooth mineralization

Figure 3. Stable isotopes studied in dental tissues (reviewed in Mays49). *Enamel proteinsmay one day yield isotopic data from individual amino acids.48


have been performed on nonprimatemammals,8,55,57–59 and thus may notclarify how primate teeth mature,since tooth mineralization and matu-ration processes vary among mam-mals.50,62

Passey and Cerling55 attempted tomodel the dampening effect that sub-sequent mineralization may have onthe original ‘‘input signal’’ recordedas a tooth develops. By measuringphosphorous, an indicator of hy-droxyapatite content, they found thattooth mineralization occurred in aspatially diffuse pattern that differedamong teeth and continued after ma-trix secretion was finished. Based onthis information, they proposed amodel to predict the original inputsignal from knowledge of the‘‘mature (measured) signal’’ and theprocess of amelogenesis for teethgrowing at a constant rate through-out the life of the animal, with theaim of interpreting temporal infor-mation present in the pattern ofchemical variation. However, thismodel appears to be of limited utilityfor hominoid teeth, as rates ofenamel secretion and extension varyfrom the beginning to the end ofcrown formation (reviewed inSmith6). Additional study is of criti-cal importance to document the tim-ing and relative degree of incorpora-tion of particular elements inprimate tooth enamel.Tafforeau and colleagues8 applied

X-ray synchrotron microtomographyto the study of tooth mineralization,finding that rhinoceros enamelshows a 20-micron thick zone nearthe enamel-dentine junction thatappears to mineralize rapidly aftersecretion. This suggests that isotopicanalyses of intratooth variation mayrecover a near-primary signal fromsampling along this zone (also seeBalasse56 and Zazzo, Balasse, andPatterson58). Even when the spatialpattern of mineralization is takeninto account, it is unlikely that a sin-gle isotopic sample will represent adiscontinuous or instantaneous pointin time because some degree of timeaveraging will occur during minerali-zation.8,56,57 However, a recent studyof strontium/calcium ratios inhuman molar enamel suggests thatthe effect of time averaging may be

minor for particular elements.16 Thisstudy demonstrated a consistentchange in ratios sampled before theneonatal (birth) line and after thisline, suggesting that physiologicaldifferences in strontium discrimina-tion were recorded at the time ofmatrix secretion and were not lostduring subsequent mineralization.Humphrey and colleagues17 alsorecently applied laser ablation todevelopmentally overlapping teeth intwo baboon individuals and foundsome correspondence in ratiochanges between simultaneouslyforming enamel (between teeth).This provides additional evidencethat subsequent mineralization doesnot completely dampen the original(biogenetic) signal.

Fossil Hominins and ToothDiagenesis

Isotopic analyses of fossil hominindental tissues have primarily cen-tered on elucidating southern AfricanPlio-Pleistocene hominin diets54,63,64

and potential environmental shifts,65

and these studies are beginning toprovide insight into hominin mobil-ity.53 Dental tissues are of particularimportance for older time periodsbecause little organic material (col-lagen) remains in fossil bone anddiagenetic processes are highly likelyto have modified the inorganic com-ponent.66 Although most dietarystudies have used bulk sampling, arecent approach by Sponheimer andcolleagues54 related sequential iso-topic laser ablation samples from thetooth surface to counts of periky-mata in four Paranthropus teeth.Their results showed a high degreeof variation within and betweenteeth. This, based on estimates of thetime represented by perikymata, wasinterpreted as seasonal and interan-nual variation, implying that Para-nthropus was not a dietary specialist.Less research has been done onhominin migration. Richards andcolleagues53 recently presented iso-topic evidence of Neanderthal mobil-ity based on strontium isotopes indental enamel. Differences in thestrontium detected in the internaltooth enamel relative to the local ge-

ology were used to suggest that theindividual had migrated from aregion at least 20 kilometers away.An advantage of using tooth

enamel for isotopic studies is itsgreater resistance than bone to dia-genetic change.67,68 Diagenesis, orthe process of chemical change afterdeath (including fossilization) ishighly variable and still not com-pletely understood in hard tis-sues.51,54,66 However, diagenesis canstrongly affect the assessment oftooth chemistry in archeological orfully fossilized tissues. Becauseenamel is approximately 95% miner-alized by the end of formation, incontrast to dentine or bone atapproximately 70%–75%, the chemi-cal composition of mature enamel isbelieved to provide a more faithfulrecord of the original mineralizationprocess. Furthermore, given thehighly inorganic composition ofenamel, it has been suggested to ap-proximate a ‘‘closed system’’ that ismore impermeable to alteration fromthe postmortem burial environment(but see evidence of enamel diagene-sis below and in Wang and Cerling,68

Kohn, Schoeninger, and Barker,69

and Schoeninger and colleagues70).However, one limitation of inorganiccomponent analyses is the lack of in-ternal verification of results; in cer-tain cases it is difficult to excludeenvironmental sources of particularelements that may bias results.In a study of modern and fossil

southern African fauna, Sponheimerand colleagues64 argued that similar-ity in strontium/calcium ratiosbetween modern and fossil animalsdemonstrated that diagenesis wasunlikely to have a major impact onthese ratios in their sample of fossil-ized dental enamel. However, othershave abandoned the use of trace-ele-ment ratios such as that of strontiumto calcium, in part because it is diffi-cult to detect when elements haveleeched in or out of a tooth orbone.71,72 Because even slightamounts of particular elements or iso-topes will be detected in most cases, aratio is always obtained, but it is oftenquite difficult to verify that it repre-sents the original signal. Hydroxyapa-tite, the major mineralized compo-nent of enamel, dentine, and bone,


can become unstable after death andmay undergo chemical change (forexample, ionic substitution, such asCa2þ by Sr2þ) with or without funda-mental structural change.66,73 Fur-thermore, as Humphrey, Dean, andJeffries16 note, subsurface enamel isknown to exchange mineral ions suchas fluoride74 with the oral environ-ment. This is a potential complicationfor isotopic studies that attempt touse incremental features on the tooth

surface to infer the timing of elemen-tal incorporation.54

In summary, new isotopicapproaches concerned with identify-ing dietary, environmental, and geo-graphic transitions within the periodof individual tooth formation or thedevelopment of the entire dentitionmay allow insight into the timing ofweaning, the seasonality of particu-lar environments, and behavioraladaptations. While evidence derived

from small samples of living humansand baboons supports the utility ofisotopic ratios such as strontium/cal-cium in enamel for identifying die-tary transitions, additional study isneeded to validate the utility of thisapproach for archeological or fossilmaterial. Additional experimentalwork on the timing of elementalincorporation and the effects of min-eralization would be particularlybeneficial. For example, comparisonsof simultaneously forming regions ofthe enamel within a tooth (that is,along the plane of a Retzius line)could be illustrative of the minerali-zation process in different regions ofthe tooth crown. Additional compari-sons of temporal signals in slow-mineralizing enamel and fast-miner-alizing dentine may allow betterinterpretation of previous analyses.Although dentine often is not idealfor studies of archeological and fossilmaterial due to diagenesis and doesundergo slight changes in minerali-zation through sclerosis, it could beused to cross-check the fidelity ofelemental incorporation in theenamel of extant taxa.75 Future stud-ies of primate dental tissue chemis-try may also benefit from the devel-opment of additional screening pro-cedures, such as virtual sectioning,quantitative mineralization mapping,and refined chemical tests. Theseprocedures could reveal changes inmineral density or composition sincediagenetic patterns can vary dramati-cally within and between sites, aswell as within a dentition or evenwithin an individual tooth (Fig. 4).

THREE-DIMENSIONAL ANALYSESOF TOOTH STRUCTURE

The recent development andincreasing availability of nondestruc-tive high-resolution micro-computedtomography (micro-CT), based eitheron conventional or synchrotron X-ray sources, has facilitated evolution-ary and clinical studies of internaltooth structure and dental tissue vol-umes (reviewed in Tafforeau andSmith,44 Hayakawa and colleagues,76

Olejniczak, Grine, and Martin77 andOlejniczak, Tafforeau, and Feeney78).Micro-CT imaging is an improve-

Figure 4. Examples of diagenetic patterns in fossil primate teeth imaged using monochro-matic absorption synchrotron micro-CT (Box 2). A) Modern human tooth showing normal(homogenous) tissue contrast; enamel is white and dentine is gray. B) Pleistocene homi-nin tooth with good preservation; only slight heterogeneity is apparent within each tissue.C) Miocene hominoid tooth with similar preservation. D) Miocene hominoid tooth fromthe same site as C with good preservation of enamel but poor preservation of dentine.E) Eocene primate with inverse contrast between tissues. F) Pleistocene hominin toothshowing very strong alteration of enamel and dentine. G) Same individual as F (adjacenttooth) showing a complex gradient from moderate tissue contrast (top left) to no con-trast (top right). H) Miocene hominoid with little to no tissue contrast and enamel demin-eralization (left). I) Eocene primate with no tissue contrast and infilling of metallic oxides(bright cracks). Patterns A through C would facilitate full 3D segmentation of enameland dentine (semi-automatic discrimination of corresponding pixel values). Pattern Dwould permit only enamel segmentation. Patterns E and F would permit only measure-ments on virtual slices or fully manual slice-by-slice 3D segmentation. Pattern G wouldpermit only partial assessments of enamel thickness. Patterns H and I prohibit accurateidentification of internal dental tissue structure (position of the enamel-dentine junction).


ment over medical CT imagingbecause fine anatomical details canbe resolved (Fig. 5) and accuratemetric assessment can be done onrelatively small dental samples.46,77,79

This is particularly important forstudies of enamel thickness, giventhe limitations of conventional radi-ography or tomography,79 as well aslimitations in physically sectioninginvaluable fossil material. Aspects ofmicroscopic dental tissue structurecan also be investigated nondestruc-tively with high precision; this is oneof the main features of synchrotronimaging. Despite these advantages,more commonly available conven-tional microtomographs may yieldabsorption data quite similar to syn-chrotron data for voxel sizes between5 and 50 microns. For standardabsorption imaging of well-preserved

teeth (for example, cases A to C inFigure 4), there is little advantage inusing a synchrotron. However, prop-agation phase-contrast X-ray syn-chrotron microtomography oftenreveals the enamel-dentine junctionin strongly modified teeth even whenthere is little or no absorption con-trast.45,46 This is a particularly valua-ble tool for imaging diageneticallyaltered fossil teeth, especially tissuestructure in older hominins and ear-lier primate fossils.

Tooth Crown and Root Structure

Most anthropological applicationsof micro-CT techniques have exam-ined internal tooth structure fromvirtual two-dimensional planes ofsections.46,80–83 Reiko Kono’s land-mark study84 firmly established the

utility of micro-CT for assessment ofthree-dimensional (3D) molar enamelthickness and enamel distribution.Three-dimensional enamel thicknessdata are now available for a broadsample of extant primates,46,78,84 thefossil ape Gigantopithecus blacki,85

Neanderthals,43,86 australopiths,103

and Middle Stone Age Homo sapi-ens.87 New 3D data have confirmedbroad trends in primate enamelthickness derived from earlier two-dimensional studies78 and have alsodemonstrated that thick molarenamel is not found in all fossil hom-inin taxa.86 Future applications ofmicro-CT are needed to clarify phylo-genetic or functional aspects of hom-inin enamel thickness, particularlygiven the diverse range of measure-ment techniques employed in previ-ous studies. In particular, such data

Figure 5. Developing chimpanzee tooth (shown in Figs. 1 and 2) imaged with an isotropic voxel size of 13.8 lm using a conventionalmicro-CT before sectioning. A) 3D rendering of the occlusal surface. B) Virtual vertical cut through the mesial cusps. C) 3D model of thetooth after segmentation showing the enamel cap (transparent yellow), EDJ (red), and dentine (transparent blue). D) Isolated enamelcap used for volume measurement (178.6 mm3). E) EDJ used for surface area measurement (188 mm2). F) Dentine used for volumemeasurement (293.3 mm3). The volume of dentine and pulp in the enamel cap delimited by the basal plane (following Olejniczak andcolleagues86) is 239 mm3, yielding a 3D average enamel thickness of 0.95 mm and a 3D relative enamel thickness of 15.31 (followingTafforeau,46 Olejniczak and colleagues,78 and Kono.84)


are lacking for early and middlePleistocene Homo.Another recent application of com-

puted tomography is the characteri-

zation of primate tooth roots. Wood,Abbott, and Uytterschaut88 originallydemonstrated the taxonomic value ofhominin mandibular postcanine root

number, size, and morphology usingphysical specimens and radiography.Nondestructive micro-CT imagingcan now be applied to visualize toothroots in situ, permitting identifica-tion of postcanine root number,shape, and size (linear dimensions,surface area, volume) in primate fos-sil material.46,81,89,90 This approachmay serve in further investigations ofreported differences in root lengthsbetween Neanderthals and contem-poraneous modern humans.91 Tomo-graphic studies of living human andprimate root size have also demon-strated a correlation with functionaldemands.90,92 Kupczik and Dean90

recently used computed tomographyto document the relatively large post-canine root surface areas of G.blacki, which they suggest may havebeen required to sustain large forcesduring mastication of highly resist-ant food items such as bamboo. Ex-perimental and descriptive microto-mographic studies are just now be-ginning to investigate phylogenetic,taxonomic, and functional aspects ofprimate tooth roots.

Enamel-Dentine Junction Shape

Studies of dental morphology havelong regarded the internal interfacebetween coronal dental tissues, theenamel-dentine junction (EDJ), as ahighly conserved and developmen-tally informative character. Thisinterface represents the initial posi-tion of the cells that are responsiblefor enamel and dentine secretion andis believed to be heavily influencedby embryological enamel knots thatform before hard-tissue secretion(reviewed in Avishai and colleagues93

and Skinner and colleagues94). Olej-niczak and colleagues,85,95–97 havedemonstrated that the primate EDJhas a taxon-specific shape, despitesome intra-taxon variation, and thatcomponents of this shape, such asrelative dentine horn height, mayserve to group members of the Pon-ginae. Before the recent applicationof micro-CT (Fig. 5), EDJ shape wasassessed from impressions of enamelcaps,98 acid etching of completeteeth,99 or physical sections.95,96

Skinner and colleagues94 recentlyconsidered the EDJ from a nondes-

Figure 6. Chimpanzee tooth germ showing integration of multiple technical approaches.Developing chimpanzee cusp after (A) and before (B–F) physical sectioning. A) Light micro-graph of 100-lm-thick histological section. B) Synchrotron microtomographic virtual 50.6-lmthick section (derived from 3D data with an isotropic voxel size of 5.06 lm) imaged in mono-chromatic absorption mode, showing the quantitative degree of mineralization. C) Virtual50.3-lm thick section (derived from 3D data with an isotropic voxel size of 5.03 lm) imagedwith propagation phase contrast mode, illustrating incremental lines in the enamel and den-tine. D) Virtual color-coded absorption and phase contrast composite; area enlarged in E)indicated in white box. E) Three-dimensional model showing enamel mineralization and devel-opmental pattern. Scale indicates quantitative mineral density in hydroxyapatite g/cm3. F) 3Drendering of isodensity segments extracted from the absorption scan data showing the 3Dmineralization pattern of the enamel. Left, densities from 0.5 to 1.3; middle, densities from 1.3 to1.8; right, densities from 1.8 to 2.2. Scale bar in B) is 1 mm, 0.5 mm for E), and 10 mm for F).


tructive 3D perspective, demonstrat-ing that dental traits commonly iden-tified on the outer enamel surfacemay result from different EDJ con-figurations. It appears that in somecases these traditional ‘‘discretetraits’’ may not be developmentallyhomologous.94 This study also dem-onstrated another potential advant-age of micro-CT studies: Teeth thatshow marked attrition that obscuresor obliterates external morphologycan still be included in analyses ofEDJ shape, underscoring the value ofnondestructive studies of internaltooth structure. Similar research onexternal dental tissue structure mayone day allow worn teeth to be rebuiltvolumetrically to better understandtheir original form, as well as thecomplex process of tooth attrition.

A WAY FORWARD: INTEGRATIONOF TEMPORAL, CHEMICAL, AND

STRUCTURAL INFORMATION

One challenge of having new toolsis learning how to best apply them.It is clear that decades of studies ontooth development, chemistry, andstructure have provided valuableinsight into the evolution of primatelife history, paleodiets, and taxon-omy. Future studies of dental tissueswill undoubtedly benefit fromincreased technological efficiency inchemical sampling, tomographicscanning resolution and sensitivity,and increased computing power fordata processing. We suggest thatadditional advancements may befound through the synergy of theseseemingly disparate research areas.For example, studies of toothchemistry may be better informedby complementary applications ofnondestructive microtomography tocharacterize mineral density and theduration of mineralization8 (Fig. 6).Furthermore, absorption micro-CTimaging of teeth before samplingmay yield information about internaldiagenetic modification, leading tomore efficient selection of materialfor chemical analyses. Similarly, theintegration of incremental develop-ment, maturation patterns, andisotopic sampling strategies mayfacilitate more precise assessments

of dietary or environmental changewhile providing the potential foridentifying life history events such asbirth or weaning.8,16,17

Other examples of this synergisticapproach include studies of hominindental development that have usedmicro-CT imaging to characterizeaspects of internal structure or thedevelopmental status of uneruptedteeth in situ.10,47,87 Structural fea-tures of developmental significance,such as the thickness of cuspalenamel, the length of the EDJ, andthe length of tooth roots, can beaccurately characterized nondestruc-tively. Data obtained from conven-tional or virtual histological investi-gations can then be used to estimatethe duration of formation alongthese virtual growth axes, leading tomore precise estimations of toothgrowth and age at death in rare fos-sil juveniles.

A final example of research inte-gration is the quantification of volu-metric growth rates, which can becalculated as overall crown averagesusing knowledge of incremental de-velopment and 3D dental tissue met-rics. For example, a chimpanzee firstmolar tooth was subjected to micro-CT scanning before histological sec-tioning and analysis (Figs. 1, 2, 4),yielding the volume of enamel (178.6mm3) and dentine (293.3 mm3) andthe maximum formation time ofeach tissue (722 and 1427 days,respectively). This information wasused to estimate minimum averagevolumetric crown growth rates(247.4 lm3/day for enamel and 205.5lm3/day for dentine), along with theminimum average coronal extensionrate (260.4 lm2/day, using a mea-sured EDJ surface area of 188 mm2).Volumetric rates of primate toothgrowth can be compared to rates ofbrain growth100 or other aspects ofsomatic development, as well as tocharacterize volumetric increases inthe formation of the deciduous denti-tion or the successive formation ofthe permanent dentition. This analy-sis can also be extended to volumet-ric quantification at the microstruc-tural level with propagation phasecontrast X-ray synchrotron microto-mography, as prisms may be virtu-ally extracted in 3D and the rate of

secretion determined along the prismlength from the cross-striation spac-ing. These examples of research inte-gration represent a few promisingmeans to advance our understandingof primate developmental biology,and to initiate the formulation ofmore refined areas of dental tissueresearch over the coming years.

ACKNOWLEDGMENTS

Special thanks to John Fleagle forpatience with this manuscript, and toJean-Jacques Hublin for support ofthis research. Comments on themanuscript were kindly provided byBen Fuller, Vaughan Grimes, DonaldReid, Chris Dean, Mary Maas, AllisonCleveland, and one anonymousreviewer. We thank Ben Fuller,Vaughan Grimes, and Mike Richardsfor illuminating discussions of iso-topes; Louise Humphrey and WendyDirks for access to unpublished data;Bernard Wood for assistance with anearlier version of Box 1; Jose Baruchelwith Box 2; and Christophe Boesch,Lawrence Martin, and Michel Tous-saint, who kindly provided material.The Max Planck Society and Euro-pean Synchrotron Radiation Facilityprovided financial support.

REFERENCES

1 Collard M, Wood B. 2000. How reliable arehuman phylogenetic hypotheses? Proc NatlAcad Sci USA 97:5003–5006.

2 Hooijer DA. 1948. Prehistoric teeth of manand of the orang-utan from central Sumatra,with notes on the fossil orang-utan from Javaand southern China. Zool Mededeelingen29:175–301.

3 Dean MC. 1995. The nature and periodicityof incremental lines in primate dentine andtheir relationship to periradicular bands in OH16 (Homo habilis). In: Moggi-Cecchi J, editor.Aspects of dental biology: paleontology, an-thropology and evolution. Florence: Interna-tional Institute for the Study of Man. p 239–265.

4 FitzGerald CM. 1998. Do enamel microstruc-tures have regular time dependency? Conclu-sions from the literature and a large-scalestudy. J Hum Evol 35:371–386.

5 Smith TM. 2006. Experimental determinationof the periodicity of incremental features inenamel. J Anat 208:99–114.

6 Smith TM. 2008 Incremental dental develop-ment: methods and applications in hominoidevolutionary studies. J Hum Evol 54:205–224.

7 Bromage TG. 1991. Enamel incremental peri-odicity in the pig-tailed macaque: a polychromefluorescent labeling study of dental hard tis-sues. Am J Phys Anthropol 86:205–214.


8 Tafforeau P, Bentaleb I, Jaeger J-J, Martin C.2007. Nature of laminations and mineralizationin rhinoceros enamel using histology and X-raysynchrotron microtomography: potential impli-cations for palaeoenvironmental isotopic stud-ies. Palaeogeogr Palaeoclimatol Palaeoecol246:206–227.

9 Dean MC, Scandrett AE. 1996. The relationbetween long-period incremental markings indentine and daily cross-striations in enamel inhuman teeth. Arch Oral Biol 41:233–241.

10 Smith TM, Toussaint M, Reid DJ, OlejniczakAJ, Hublin J-J. 2007. Rapid dental developmentin a Middle Paleolithic Belgian Neanderthal.Proc Natl Acad Sci USA 104:20220–20225.

11 Schwartz GT, Reid DJ, Dean MC, ZihlmanAL. 2006. A faithful record of stressful lifeevents preserved in the dental developmentalrecord of a juvenile gorilla. Int J Primatol22:837–860.

12 Dirks W, Reid DJ, Jolly CJ, Phillips-ConroyJE, Brett FL. 2002. Out of the mouths ofbaboons: stress, life history, and dental develop-ment in the Awash National Park hybrid zone,Ethiopia. Am J Phys Anthropol 118:239–252.

13 Guatelli-Steinberg D. 2001. What can devel-opmental defects of enamel reveal about physi-ological stress in nonhuman primates? EvolAnthropol 10:138–151.

14 Skinner MF, Hopwood D. 2004. Hypothesisfor the causes and periodicity of repetitive lin-ear enamel hypoplasia in large, wild African(Pan troglodytes and Gorilla gorilla) and Asian(Pongo pygmaeus) apes. Am J Phys Anthropol123:216–235.

15 Katzenberg MA, Herring DA, Saunders SR.1996. Weaning and infant mortality: evaluatingthe skeletal evidence. Yearbook Phys Anthropol39:177–199.

16 Humphrey LT, Dean MC, Jeffries TE. 2007.An evaluation of changes in strontium/calciumratios across the neonatal line in human decid-uous teeth. In: Bailey SE, Hublin J-J, editors.Dental perspectives on human evolution: stateof the art research in dental paleoanthropology.Dordrecht: Springer. p 301–317.

17 Humphrey LT, Dirks W, Dean MC, JeffriesTE. 2008. Tracking dietary transitions in wean-ling baboons (Papio hamadryas anubis) usingstrontium/calcium ratios in enamel. Folia Pri-matol 79:197–212.

18 Wright LE, Schwarcz HP. 1998. Stable car-bon and oxygen isotopes in human toothenamel: identifying breastfeeding and weaningin prehistory. Am J Phys Anthropol 106:1–18.

19 Fuller BT, Richards MP, Mays SA. 2003. Sta-ble carbon and nitrogen isotope variations intooth dentine serial sections from WharramPercy. J Archeol Sci 30:1673–1684.

20 Schwartz GT, Reid DJ, Dean C. 2001. Devel-opmental aspects of sexual dimorphism inhominoid canines. Int J Primatol 22:837–860.

21 Guatelli-Steinberg D, Reid DJ, Bishop TA,Larsen CS. 2005. Anterior tooth growth periodsin Neanderthals were comparable to those ofmodern humans. Proc Natl Acad Sci USA102:14197–14202.

22 Smith TM, Reid DJ, Dean MC, OlejniczakAJ, Martin LB. 2007. Molar development incommon chimpanzees (Pan troglodytes). J HumEvol 52:201–216.

23 Smith TM, Reid DJ, Dean MC, OlejniczakAJ, Ferrell RJ, Martin LB. 2007. New perspec-tives on chimpanzee and human dental devel-opment. In: Bailey SE, Hublin J-J, editors. Den-tal perspectives on human evolution: state ofthe art research in dental paleoanthropology.Dordrecht: Springer. p 177–192.

24 Macho GA. 2001. Primate molar crown for-mation times and life history evolution revis-ited. Am J Primatol 55:189–201.

25 Dirks W, Bowman JE. 2007. Life historytheory and dental development in four speciesof catarrhine primates. J Hum Evol 53:309–320.

26 Godfrey LR, Schwartz GT, Samonds KE,Jungers W, Catlett KK. 2006. The secrets oflemur teeth. Evol Anthropol 15:142–154.

27 Reid DJ, Dean MC. 2000. Brief communica-tion: the timing of linear hypoplasias on humananterior teeth. Am J Phys Anthropol 113:135–139.

28 Reid DJ, Dean MC. 2006. Variation in mod-ern human enamel formation times. J HumEvol 50:329–346.

29 Reid DJ, Guatelli-Steinberg D, Walton P.2008. Variation in modern human premolarenamel formation times: implications for Nean-dertals. J Hum Evol 54:225–235.

30 Mann A, Monge JM, Lampl M. 1991. Investi-gation into the relationship between perikymatacounts and crown formation times. Am J PhysAnthropol 86:175–188.

31 Guatelli-Steinberg D, Reid DJ. 2008. Whatmolars contribute to an emerging understand-ing of lateral enamel formation in Neandertalsvs. modern humans. J Hum Evol 54:236–250.

32 Reid DJ, Ferrell RJ. 2006. The relationshipbetween number of striae of Retzius and theirperiodicity in imbricational enamel formation.J Hum Evol 50:195–202.

33 Dean MC. 2006. Tooth microstructure tracksthe pace of human life-history evolution. ProcR Soc B 273:2799–2808.

34 Kelley J, Smith TM. 2003. Age at first molaremergence in early Miocene Afropithecus turka-nensis and life-history evolution in the Homi-noidea. J Hum Evol 44:307–329.

35 Beynon AD, Dean MC, Leakey MG, Reid DJ,Walker A. 1998. Comparative dental develop-ment and microstructure of Proconsul teethfrom Rusinga Island, Kenya. J Hum Evol35:163–209.

36 Smith TM, Martin LB, Leakey MG. 2003.Enamel thickness, microstructure and develop-ment in Afropithecus turkanensis. J Hum Evol44:283–306.

37 Zihlman A, Bolter D, Boesch C. 2004. Wildchimpanzee dentition and its implications forassessing life history in immature hominin fos-sils. Proc Natl Acad Sci USA 101:10541–10543.

38 Robson SL, van Schaik CP, Hawkes K. 2006.The derived features of human life history. In:Hawkes K, Paine RP, editors. The evolution ofhuman life history. Santa Fe: School of Ameri-can Research Press. p 17–44.

39 Smith BH, Garn SM. 1987. Polymorphismsin eruption sequence of permanent teeth inAmerican children. Am J Phys Anthropol74:289–303.

40 Smith BH. 2004. The paleontology of growthand development. Evol Anthropol 13:239–241.

41 Bromage TG, Dean MC. 1985. Re-evaluationof the age at death of immature fossil homi-nids. Nature 317:525–527.

42 Ramirez Rozzi FV, Bermudez de Castro JM.2004. Surprisingly rapid growth in Neander-thals. Nature 428:936–939.

43 Macchiarelli R, Bondioli L, Debenath A,Mazurier A, Tournepiche J-F, Birch W, Dean C.2006. How Neanderthal molar teeth grew.Nature 444:748–751.

44 Tafforeau P, Smith TM. 2008. Nondestruc-tive imaging of hominoid dental microstructureusing phase contrast X-ray synchrotron micro-tomography. J Hum Evol 54:272–278.

45 Tafforeau P, Boistel R, Boller E, Bravin A,Brunet M, Chaimanee Y, Cloetens P, Feist M,Hoszowska J, Jaeger J-J, Kay RF, Lazzari V,Marivaux L, Nel A, Nemoz C, Thibault X,Vignaud P, Zabler S. 2006. Applications ofX-ray synchrotron microtomography for non-destructive 3D studies of paleontological speci-mens. Appl Phys A 83:195–202.

46 Tafforeau P. 2004. Aspects phylognetiques etfonctionnels de la microstructure de l’emaildentaire et de la structure tridimensionnelle desmolaires chez les primates fossiles et actuels:apports de la microtomographie a rayonnementX synchrotron. PhD Thesis, Universite de Mont-pellier II, France. http://www.paleoanthro.org/dissertation_list.htm

47 Smith TM, Tafforeau PT, Reid DJ, Grun R,Eggins S, Boutakiout M, Hublin J-J. 2007. Ear-liest evidence of modern human life history inNorth African early Homo sapiens. Proc NatlAcad Sci USA 104:6128–6133.

48 Schoeninger MJ. 1995. Stable isotope stud-ies in human evolution. Evol Anthropol 4:83–98.

49 Mays S. 2000. New directions in the analysisof stable isotopes in excavated bones and teeth.In: Cox M, Mays S, editors. Human osteologyin archaeology and forensic science. Cam-bridge: Cambridge University Press. p 425–438.

50 Boyde A. 1997. Microstructure of enamel.In: Chadwick DJ, Cardew G, editors. Dentalenamel. Chichester: Wiley. p 18–31.

51 Montgomery J, Evans JA. 2006. Immigrantson the Isle of Lewis: combining traditionalfunerary and modern isotope evidence to inves-tigate social differentiation, migration and die-tary change in the Outer Hebrides of Scotland.In: Gowland R, Knsel C, editors. Social archae-ology of funerary remains. Oxford: OxbowBooks. p 122–142.

52 Sealy J, Armstrong R, Schrire C. 1995.Beyond lifetime averages: tracing life historiesthrough isotopic analysis of different calcifiedtissues from archaeological human skeletons.Antiquity 69:290–300.

53 Richards M, Harvati K, Grimes V, Smith C,Smith T, Hublin J-J, Karkanas P, PanagopoulouE. 2008. Isotope evidence of Neanderthal mobil-ity. J Archeol Sci 35:1251–1256.

54 Sponheimer M, Passey BH, de Ruiter DJ,Guatelli-Steinberg D, Cerling TE, Lee-Thorp JA.2006. Isotopic evidence for dietary variabilityin the early hominin Paranthropus robustus.Science 314:980–982.

55 Passey BH, Cerling TE. 2002. Tooth enamelmineralization in ungulates: implications forrecovering a primary isotopic time-series. Geo-chim Cosmochim Acta 66:3225–3234.

56 Balasse M. 2003. Potential biases in sam-pling design and interpretation of intra-toothisotope analysis. Int J Osteoarcheol 13:3–10.

57 Hoppe KA, Stover SM, Pascoe JR, Amund-son R. 2004. Tooth enamel biomineralization inextant horses: implications for isotopic micro-sampling. Palaeogeogr Palaeoclimatol Palaeoe-col 206:355–365.

58 Zazzo A, Balasse M, Patterson WP. 2005.High-resolution d13C intratooth profiles in bo-vine enamel: implications for mineralizationpattern and isotopic attenuation. Geochim Cos-mochim Acta 69:3631–3642.

59 Balasse M, Smith AB, Ambrose SH, LeighSR. 2003. Determining sheep birth seasonalityby analysis of tooth enamel oxygen isotoperatios: the Late Stone Age site of Kasteelberg(South Africa). J Archeol Sci 30:205–215.

60 Suga S. 1983. Comparative histology of theprogressive mineralization pattern of develop-


ing enamel. In: Suga S, editor. Mechanisms oftooth enamel formation. Tokyo: QuintessencePublishing. p 167–203.

61 Wilson PR, Beynon AD. 1989. Mineraliza-tion differences between human deciduousand permanent enamel measured by quantita-tive microradiography. Arch Oral Biol 34:85–88.

62 Kohn MJ. 2004. Comment: tooth enamelmineralization in ungulates: implications forrecovering a primary isotopic time-series, byB.H. Passey and T.E. Cerling (2002). GeochimCosmochim Acta 68:403–405.

63 Lee-Thorp JA, van der Merwe NJ, Brain CK.1994. Diet of Australopithecus robustus atSwartkrans from stable carbon isotopic analy-sis. J Hum Evol 27:361–372.

64 Sponheimer M, de Ruiter D, Lee-Thorp J,Spath A. 2005. Sr/Ca and early hominin dietsrevisited: new data from modern and fossiltooth enamel. J Hum Evol 48:147–156.

65 Lee-Thorp JA, Sponheimer M, Luyt J. 2007.Tracking changing environments using stablecarbon isotopes in fossil tooth enamel: anexample from the South African hominin sites.J Hum Evol 53:595–601.

66 Nielsen-Marsh C, Gernaey A, Turner-WalkerG, Hedges R, Pike A, Collins M. 2000. Thechemical degradation of bone. In: Cox M, MaysS, editors. Human osteology in archaeology andforensic science. Cambridge: Cambridge Uni-versity Press. p 439–454.

67 Lee-Thorp JA, van der Merwe NJ. 1991.Aspects of the chemistry of modern and fossilbiological apatites. J Arch Sci 18:343–354.

68 Wang Y, Cerling TE. 1994. A model of fossiltooth and bone diagenesis: implications forpaleodiet reconstruction from stable isotopes.Palaeogeogr Palaeoclimatol Palaeoecol 107:281–289.

69 Kohn MJ, Schoeninger MJ, Barker WW.1999. Altered states: effects of diagenesis onfossil tooth chemistry. Geochim CosmochimActa 63:2737–2747.

70 Schoeninger MJ, Hallin K, Reeser H, ValleyJW, Fournelle J. 2003. Isotopic alteration ofmammalian tooth enamel. Int J Osteoarch13:11–19.

71 Burton JH, Price TD. 2000. The use andabuse of trace elements for paleodietaryresearch. In: Ambrose SH, Katzenberg MA, edi-tors. Biogeochemical approaches to paleodiet-ary analysis. New York: Kluwer Academic/Plenum Publishers. p 159–171.

72 Fabig A, Herrmann B. 2002. Traceelements in buried human bones: intra-popula-tion variability of Sr/Ca and Ba/Ca ratios—diet or diagenesis? Naturwissenschaften 89:115–119.

73 Zazzo A, Lecuyer C, Sheppard SMF, Grand-jean P, Mariotti A. 2004. Diagenesis and thereconstruction of paleoenvironments: a methodto restore original d18O values of carbonate andphosphate from fossil tooth enamel. GeochimCosmochim Acta 68:2245–2258.

74 Fejerskov O, Larsen MJ, Richards A, BaelumV. 1994. Dental tissue effects of fluoride. AdvDent Res 8:15–31.

75 Fisher DC, Fox DL. 1998. Oxygen isotopes inmammoth teeth: sample design, mineralizationpatterns, and enamel-dentine comparisons. JVert Paleo 18:41A–42A.

76 Hayakawa T, Mishima H, Yokota I, Sakae T,Kozawa Y, Nemoto K. 2000. Application ofhigh resolution microfocus X-ray CT for theobservation of human tooth. Dent Mat J 19:87–95.

77 Olejniczak AJ, Grine FE, Martin LB. 2007.Micro-computed tomography of the post-caninedentition: methodological aspects of three-dimensional data collection. In: Bailey SE,Hublin J-J, editors. Dental perspectives onhuman evolution: state of the art research indental paleoanthropology. Dordrecht: Springer.p 103–116.

78 Olejniczak AJ, Tafforeau P, Feeney RNM, Mar-tin LB. 2008. Three-dimensional primate molarenamel thickness. J Hum Evol 54:187–195.

79 Olejniczak AJ, Grine FE. 2006. Assessmentof the accuracy of dental enamel thicknessmeasurements using micro-focal X-ray com-puted tomography. Anat Rec 288A:263–275.

80 Chaimanee Y, Jolly D, Benammi M, Taffor-eau P, Duzer D, Moussa I, Jaeger J-J. 2003.Middle Miocene hominoid from Thailand andorangutan origins. Nature 422:61–65.

81 Brunet M, Guy F, Pilbeam D, LiebermanDE, Likius A, Mackaye HT, Ponce de Leon MS,Zollikofer CPE, Vignaud P. 2005. New materialof the earliest hominid from the upper Mioceneof Chad. Nature 434:752–755.

82 Suwa G, Kono RT, Katoh S, Asfaw B,Beyene Y. 2007. A new species of great apefrom the late Miocene epoch in Ethiopia.Nature 448:921–924.

83 Kunimatsu Y, Nakatsukasa M, Sawada Y,Sakai T, Hyodo M, Hyodo H, Itaya T, NakayaH, Saegusa H, Mazurier A, Saneyoshi M, Tsuji-kawak H, Yamamoto A, Emma Mbua. 2007. Anew Late Miocene great ape from Kenya andits implications for the origins of African greatapes and humans. Proc Natl Acad Sci USA104:19220–19225.

84 Kono R. 2004. Molar enamel thickness anddistribution patterns in extant great apes andhumans: new insights based on a 3-dimensionalwhole crown perspective. Anthropol Sci112:121–146.

85 Olejniczak AJ, Smith TM, Wei W, Potts R,Ciochon R, Kullmer O, Schrenk F, Hublin J-J.2008. Molar enamel thickness and dentine hornheight in Gigantopithecus blacki. Am J PhysAnthropol 135:85–91.

86 Olejniczak AJ, Smith TM, Feeney RNM,Macchiarelli R, Mazurier A, Bondioli L, RosasA, Fortea J, de la Rasilla M, Garcıa-TaberneroA, Radovcic J, Skinner MM, Toussaint M,Hublin J-J. 2008. Dental tissue proportions andenamel thickness in Neandertal and modernhuman molars. J Hum Evol 55:12–23.

87 Smith TM, Olejniczak AJ, Tafforeau P, ReidDJ, Grine FE, Hublin J-J. 2006. Molar crownthickness, volume, and development in SouthAfrican Middle Stone Age humans. S Afr J Sci102:513–517.

88 Wood BA, Abbott SA, Uytterschaut H. 1988.Analysis of the dental morphology of Plio-Pleis-

tocene hominids. IV. Mandibular postcanineroot morphology. J Anat 156:107–139.

89 Chaimanee Y, Yamee C, Tian P, KhaowisetK, Marandat B, Tafforeau P, Nemoz C, JaegerJ-J. 2006. Khoratpithecus piriyai, a late Miocenehominoid of Thailand. Am J Phys Anthropol131:311–323.

90 Kupczik K, Dean MC. 2008. Comparativeobservations on the tooth root morphology ofGigantopithecus blacki. J Hum Evol 54:196–204.

91 Bailey SE. 2005. Diagnostic dental differen-ces between Neandertals and Upper Paleolithicmodern humans: getting to the root of the mat-ter. In: Zadzinska E, editor. Current trends indental morphology research. Lodz: Universityof Lodz Press. p 201–210.

92 Kupczik K. 2003. Tooth root morphology inprimates and carnivores. Ph.D. Thesis, Univer-sity of London.

93 Avishai G, Muller R, Gabet Y, Bab I, Zilber-man U, Smith P. 2004. New approach to quan-tifying developmental variation in the dentitionusing serial microtomographic imaging.Microsc Res Tech 65:263–269.

94 Skinner MM, Wood B, Boesch C, OlejniczakAJ, Rosas A, Smith TM, Hublin J-J. 2008. Den-tal trait expression at the enamel-dentine junc-tion of lower molars in extant and fossil homi-noids. J Hum Evol 54:173–186.

95 Olejniczak AJ, Gilbert CC, Martin LB, SmithTM, Ulhaas L, Grine FE. 2007. Morphology ofthe enamel-dentine junction in sections ofanthropoid primate maxillary molars. J HumEvol 53:292–301.

96 Olejniczak AJ, Martin LB, Ulhaas L. 2004.Quantification of dentine shape in anthropoidprimates. Ann Anat 186:479–485.

97 Smith TM, Olejniczak AJ, Reid DJ, FerrellRJ, Hublin J-J. 2006. Modern human molarenamel thickness and enamel-dentine junctionshape. Arch Oral Biol 51:974–995.

98 Korenhof CAW. 1961. The enamel-dentineborder: a new morphological factor in the studyof the (human) molar pattern. Proc KoninklNederl Acad Wetensch 64B:639–664.

99 Kono RT, Suwa G, Tanijiri T. 2002. A three-dimensional analysis of enamel distributionpatterns in human permanent first molars.Arch Oral Biol 47:867–875.

100 Leigh S. 2004. Brain growth, life history,and cognition in primate and human evolution.Am J Phys Anthropol 62:139–164.

101 Guatelli-Steinberg D. n.d. Recent studies ofdental development in Neandertals: Implica-tions for Neandertal life histories. Evol Anthro-pol In Press.

102 Humphrey LT, Dean MC, Jeffries TE, PennM. 2008. Unlocking evidence of early diet fromtooth enamel. Proc Natl Acad Sci USA 105:6834–6839.

103 Olejniczak AJ, Smith TM, Skinner MM,Grine FE, Feeney RNM, Thackeray FJ, HublinJ-J. 2008. Three-dimensional patterning andthickness of molar enamel in Australopithecusand Paranthropus. Biol Lett 4:406–410.

VVC 2008 Wiley-Liss, Inc.


new visions of dental tissue research: tooth development, chemistry ...bioanth/tanya_smith/pdf/smith...

Documents