Doi:10.1016/j.gca.2005.12.022

Geochimica et Cosmochimica Acta 70 (2006) 1644–1654 Enamel diagenesis at South African Australopith sites: Implications for paleoecological reconstruction with trace elements a Department of Anthropology, University of Colorado at Boulder, Boulder, CO 80309, USA b Department of Archaeology, University of Cape Town, Rondebosch 7701, South Africa c Department of Archaeological Sciences, University of Bradford, Bradford BD7 1DP, UK Received 27 October 2005; accepted in revised form 27 December 2005 Elemental ratio data from archaeological and paleontological bone have often been used for paleoecological reconstruction, but recent studies have shown that, even when solubility profiling techniques are employed in an attempt to recover biogenic signals, boneis an unreliable material. As a result, there has been renewed interest in using enamel for such studies, as it is known to be less susceptibleto diagenesis. Nevertheless, enamel is not immune from diagenetic processes, and several studies have suggested that paleoecologicallyrelevant elements may be altered in fossil enamel. Here, we investigate Sr, Ba, Zn, and Pb compositions of enamel from South Africankarstic cave sites in an effort to ascertain whether or not this material provides reliable paleoecological information. We comparedenamel data for mammals from three fossil sites aged 1.8–3.0 Ma, all of which are on dolomites, with data from modern mammals livingon dolomitic and granitic substrates. Sr/Ca and Ba/Ca are about three times higher in enamel from modern mammals on granites thanthose living on dolomites, stressing the need for geologically appropriate modern/fossil comparisons. After pretreatment with dilute acid,we found no evidence of increased Sr/Ca, Ba/Ca, or Pb/Ca in fossil enamel. In contrast, Zn/Ca increased by over five times at one site(Makapansgat), but much more subtly elsewhere. Ecological patterning in Sr/Ca, Ba/Ca, and Sr/Ba ratios was also retained in fossilenamel. This study suggests that Sr/Ca, Ba/Ca, and Pb/Ca data likely preserve paleoecological information from these sites, but alsodemonstrates that geologically similar sites can differ in the degree to which they impart certain elements (Zn in this case) to fossils. Thus,screening is probably necessary on a site-by-site basis. Lastly, further investigation of elemental distributions in modern foodwebs isnecessary before elemental ratio analysis can become a common tool for paleoecological reconstruction.
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diagenesis as well as problems of interpretation (e.g., Elemental analysis of archaeological and paleontologi- cal bone has been a widely used tool for paleoecological reconstruction based upon the premise that systematic elemental diagenesis, Sillen developed a sophisticated variations in elemental ratio data (e.g., Sr/Ca and Ba/Ca) ‘‘solubility profiling’’ technique that aimed to extract a are related to differences in trophic behavior (e.g., near-original biogenic component based on the differing solubility properties of various apatites and carbonates that even when these painstaking techniques are used, diagenetic strontium often cannot be eradicated from bone questioned, however, due to growing concerns about This has led to recent attempts to investigate Corresponding author. Fax: +1 916 313 3226.
E-mail address: (M. Sponheimer).
paleoecology using elemental ratios in modern enamel 0016-7037/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.gca.2005.12.022 Enamel diagenesis at South African Australopith sites In fact, there have been very few studies of alteration in crystalline and ordered apatitic tissue ( the enamel elemental concentrations of terrestrial mam- is more resistant to post-mortem alteration than mals in general, much less in the karstic environments that typify the South African early hominin sites. The most comprehensive mammalian study focused largely (but not and while suggestive, might not prove illustrative of the Nevertheless, enamel is not immune from diagenetic changes that occur in the enamel of large-bodied mammals, processes. It may be structurally and isotopically altered as fossil rodent teeth have often been altered by gastric acids in the alimentary tracts of avian predators examined large mammal teeth from the early homi- nin site Allia Bay, it is on the shores of highly alkaline Lake during fossilization, although these changes are very Turkana which favors recrystallization to fluorapatite small compared to those in poorly crystalline bone and genetic processes at the South African karstic caves are similar. Indeed, there is limited evidence suggesting that this is not the case, as rodent bones from alkaline lake sites More significantly for our purposes here, in East Africa are highly enriched in Sr and Ba however, is that a few studies have suggested that elements while rodent bones from karstic cave sites in used for investigating paleoecology—both paleodiets (Sr, there is good reason to believe that enamel diagenesis in these two regions is generally dissimilar.
There are also a few methodological issues that make previous studies of enamel diagenesis difficult to interpret one extent or another, over geological time. For example, from a paleoecological context. Firstly, few multiple-ele- ment studies pretreated fossil enamel to remove diagenetic from a variety of African fossil sites was enriched in Sr contaminants, which is standard procedure for paleoeco- (as well as Fe) compared to modern enamel; and logical studies using fossil enamel (e.g., argued that Ba, and possibly Sr, were diagenet- ically enriched in enamel from the Kenyan early hominin Nonetheless, the degree to which these findings can be were present either within inclusions or incorporated with- generalized to enamel at other fossil sites remains uncer- in the apatite itself, this might not tell us about the levels of tain. Others have found little or no evidence that enamel is enriched in either Sr or Zn even after more than 60 mil- found, for instance, that up to 50% of the carbonate in fos- sil enamel specimens can be diagenetic, but that even under or that enamel Sr-isotope compositions are signifi- these extreme circumstances, it is usually bound in second- ary carbonate minerals or in highly soluble apatitic do- mains that are completely removed with 0.1 M acetic acid tance of the taphonomic history, burial environment, and that the principle elemental contamination in their fossil surrounding sediment in determining the nature of alter- samples was in the form of oxyhydroxides, which are likely ation in any given set of fossils (e.g., removed using standard pretreatment protocols ( is the degree to which previous studies of enamel diagenesis A second, and we believe, crucial issue is that the inher- appertain to enamel from large mammals interred in South ent variability in trace element distributions in calcified tis- African early hominin sites, as material from such sites has sues has been largely unappreciated. reported been used in a series of elemental paleodietary studies ( coefficients of variation (CVs) for Sr/Ca for a single species in a single location (Nagapande, Zimbabwe) of up to 71%, because much of our previous work has focused on recon- although CVs of 30–40% are more typical for individual structing the diets of hominins from these sites using stable ). So the question we are asking here is: what ing studies intraspecific CVs tend to be about 20% ( evidence is there that paleoecologically relevant enamel ele- mental compositions are altered during fossilization at in mammalian elemental compositions is such that large numbers of samples are required to adequately characterize M. Sponheimer, J.A. Lee-Thorp 70 (2006) 1644–1654 a species, much less entire mammalian communities. Yet, most previous studies looked at very small numbers ofmodern and fossil mammals. for in- Enamel powder (3 mg) was removed from permanent, stance, examined only five modern and five fossil teeth.
late-forming teeth (e.g., bovid M3s) using a rotary drill with Lastly, there are often problems of geographic compara- a diamond-tipped dental burr. Enamel was collected from bility between the modern and fossil samples. as large an area of the tooth as possible to ensure that the sample contained enamel laid down over many months, France, Morocco, Algeria, and Tanzania to fossil rodent and in some cases years. The enamel powder was pre- enamel from South Africa (as well as other places). While treated with 0.1 M acetic acid for 10 min to remove con- this study had large sample sizes, it is not at all clear taminants and rinsed to neutrality with triply distilled, that comparisons between disparate locations are appro- priate given the importance of local geology in governing was then dissolved in 1 ml of 40% HF: 65% HNO3 (4:1) in mammalian elemental concentrations (e.g., closed teflon beakers. After complete dissolution the beak- ers were opened and the samples evaporated to dryness on a hotplate. The residue was then dissolved in 0.5 ml of 65% Here, we provide new elemental ratio data for modern HNO3 and evaporated to dryness. After further dissolution and fossil enamel from South Africa in an effort to address in 0.5 ml of 65% HNO3, the samples were dried and dis- the above concerns, and in so doing, better our understand- solved in 10 ml of 5% HNO3 solution and finally analyzed ing of diagenesis at South African early hominin sites. As it along with several multi-element standard solutions for Sr, is our aim to investigate enamel diagenesis as it directly Ba, Zn, Pb, and Ca on a Perkin-Elmer Elan 6000 ICP-MS.
pertains to paleoecological reconstruction, we have limited The precision of this instrument for these elements is better our analysis to four elements (Sr, Ba, Zn, and Pb) that oth- than 3%. Elemental data are presented as ratios (e.g., (Ba/ ers have suggested provide paleoecological information Ca) * 1000) as is typical for paleoecological applications We looked for differences in Sr/Ca, Ba/Ca, Zn/Ca, and Pb/Ca between sites/areas using analysis of variance (AN-OVA), and where significant differences were found we per- formed pairwise comparisons using Fisher’s PLSD test. Alldata were log-transformed to obtain normal distributions prior to statistical analysis, but non-transformed data arepresented in the tables and figures to maximize comparabil- Modern teeth from two areas were analyzed. We sam- ity between this and previous studies (e.g., pled 39 teeth from animals on granitic substrates in the Greater Kruger National Park (KNP), South Africa, as well as 12 teeth from animals on dolomitic substratesaround the well-known South African early hominin sites.
We also sampled fossil teeth from Swartkrans (Member 1,1.8 Ma; 29 teeth), Sterkfontein (Member 4, 2.5 Ma; 19 3.1. Modern and fossil elemental ratio data teeth), and Makapansgat Limeworks (Member 3, 3.0 Ma;50 teeth), all of which are karstic cave sites formed in the ANOVA reveals highly significant differences in Sr/Ca, 2.5–2.6 Ga Malmani Dolomites of South Africa. Some of Ba/Ca, and Zn/Ca between sites/areas (P < 0.0001) ( the first australopith specimens were discovered in these ; all data discussed herein can be found in caves (more than a decade before similar finds were un- earthed in East Africa), and they are also among the these differences are between the mammals on granitic sub- world’s most fossiliferous hominin sites with hundreds of strates (KNP) and those on dolomites (all others), with thousands of specimens retrieved to date mean Sr/Ca and Ba/Ca at least 2.5 times higher on the gra- Hence, they are very important sites for students of human nitic substrates. This observation demonstrates, again, the evolution. All of the specimens were sampled at either the importance of local geology in determining mammalian Transvaal Museum or the Bernard Price Institute for elemental compositions. In contrast, there are no signifi- Palaeontological Research, both of which are located in cant differences in the Pb/Ca of the modern or fossil sites Gauteng, South Africa. We restricted this study to late- except between the modern granites and the fossil site forming teeth, as enamel from early-forming teeth is Swartkrans (P < 0.01). Notably, Pb/Ca for animals on formed partially during infancy when mammals may not the modern dolomites and all three fossil sites are statisti- have fully developed their capacity to discriminate against cally indistinguishable (P > 0.48); thus, there is no evidence that Pb has been altered in any way through time at these fossil sites. This fidelity, or consistency, is significant for Enamel diagenesis at South African Australopith sites Table 1Elemental ratio means, standard errors, and sample sizes for modern tooth enamel from two geological substrates and from three fossil sites in SouthAfrica The data distributions are shown in We did not obtain Pb data for some teeth, so the Pb/Ca sample sizes for the modern granites and Makapansgatare reduced to 25 and 41, respectively.
Fig. 1. Sr/Ca (a), Ba/Ca (b), Zn/Ca (c), and Pb/Ca (d) for mammals from the granites of Kruger National Park (MG), dolomites in the vicinity of fossilhominin sites (MD), Swartkrans Member 1 (SK), Sterkfontein Member 4 (ST), and Makapansgat Limeworks Member 3 (LW). The boxes represent the25th–75th percentiles (with the medians as horizontal lines) and the whiskers show the 10th–90th percentiles.
although Pb/Ca ratios are not generally believed to reveal least partly due to the preponderance of browsing taxa at much about paleodiet (but see ), lead iso- the site, since they tend to have low Sr/Ca (see below and topes can be important for determining ‘‘local’’ and ‘‘immi- grant’’ individuals at archaeological and paleontological ogy in the immediate vicinity of the site supports vegetation with anomalously low Sr/Ca. Regardless, there is no evi- and thus can help answer questions about distances trav- dence that exogenous Sr ions have become incorporated eled by species across the paleolandscape.
to a significant/detectable extent in fossil enamel at this Likewise, there is no evidence that diagenetic Sr or Ba have become structurally incorporated within the fossil It is not the case, however, that no elemental concentra- enamel analyzed here. None of the fossil fauna are signifi- tions have increased at these sites. At Makapansgat, cantly different from fauna from the modern dolomites in enamel Zn/Ca ratios are nearly six times greater than those Ba/Ca (P > 0.16). Moreover, the Sr/Ca of the Swartkrans from the modern dolomites (P < 0.0001), and at least three and Sterkfontein fauna is not different from that of fauna times greater than those from the other fossil sites on the modern dolomites (P > 0.81). Makapansgat, in con- (P < 0.0001). In fact, linear regression reveals a significant trast, has lower Sr/Ca (P < 0.001); however, this result is at temporal trend with Zn/Ca becoming greater over time M. Sponheimer, J.A. Lee-Thorp 70 (2006) 1644–1654 (P < 0.0001; R2 = 0.48); this is at least partially an artifact ecological diversity from the KNP, and from the combined of the exceptional values at Makapansgat, however, as Sterkfontein Valley sites (Swartkrans and Sterkfontein are Swartkrans enamel Zn/Ca is not significantly different only 1 km from each other and are on identical geological from that of the modern dolomites (P = 0.12). It may be substrates), to attempt such an analysis.
that Zn contamination is a common problem at these sites, Yet, we must first address the question of what type of but it appears to be particularly egregious at Makapansgat.
ecological patterning to expect? Mammals discriminate This is not surprising, for fossils from Makapansgat (par- against Ba and Sr with respect to Ca in the digestive tract ticularly Member 3) are commonly stained by manganese tion metal with a smaller ionic radius (0.46 A ˚ ). Our data cannot be used to pinpoint the location tissues have lower Ba/Ca and Sr/Ca ratios than the plants in which the diagenetic Zn resides in the Makapansgat fos- that they eat, and carnivores in turn have lower Ba/Ca and sils (within the apatite itself or within secondary minerals), Sr/Ca than the herbivores they consume (e.g., but Zn2+ substitution for larger Ca2+ ions within the crys- tal lattice is known to occur in synthetic apatites (LeGeros, variation in Ba/Ca and Sr/Ca within trophic levels is also 1991), so incorporation of exogenous Zn within enamel apatite due to dissolution/reprecipitation phenomena at crystal surfaces and in highly soluble microdomains is to the distributions and the mechanisms responsible for them ). Zn substitution for Sr at Ca-sites might also con- remain poorly understood. Our data for plants and animals tribute to the lower Sr/Ca at this site, and might signal in the KNP might exemplify and clarify these patterns.
moderate leaching of native Sr from enamel therein.
Although there is a great deal of inter- and intraspecificvariation in plant Ba/Ca ratios in KNP, there is a system- 3.2. Modern and fossil ecological patterning atic difference in the Ba/Ca of grasses and browse plants(forbs and trees), with the former having higher Ba/Ca Given that the traditional paleodietary elements (Ba and than the latter (P < 0.01) ). This distinction is ulti- Sr) have been minimally altered at these sites, we should mately passed down to the herbivores within KNP, as Ba/ then expect the same ecological patterning of elemental ra- Ca is significantly higher in grazers than in browsers tios in the modern and fossil faunas. This cannot be tested (P = 0.01), and as expected, carnivore Ba/Ca is lower than using our modern dolomite specimens as they are too few, that of both herbivore groups (P < 0.02) (; or with our 3 Ma Makapansgat specimens, as they are see observed ratios in A similar pattern is also evi- dominated by one ecological type (browsing herbivores); dent in the Sr/Ca of plants and animals in Kruger, however, we do have sufficient numbers of specimens and although the distinctions between groups are not always Table 2Elemental ratio means, standard errors, and sample sizes of grazers, browsers, carnivores, molerats, browse plants, and graze plants in the KrugerNational Park The same data are also provided for fossil browsers, carnivores, and grazers from the Sterkfontein Valley. Data distributions are shown in Zn/Ca and Pb/Ca data are not included as they manifest no ecological patterning in our modern and fossil datasets. Observed Ratioenamel-diet (OR) values forthe KNP browsers, grazers, and carnivores are 0.13, 0.23 (herbivore mean = 0.18), and 0.53, respectively, for Sr, and 0.14, 0.10 (herbivore mean = 0.12),and 0.26 for Ba. We caution, however, that given the massive inter- and intraseasonal variability in plant elemental compositions (e.g., our small dry season plant sample is only sufficient to show general patterns, not properly calculate OR values. Carnivore OR values were estimatedassuming grazers were their principle foods (Sponheimer, unpublished data). Once again, however, these numbers mean little except when calculated usingknown prey species, which was not possible in this case.
Enamel diagenesis at South African Australopith sites Fig. 2. Ba/Ca patterns for modern (a) and fossil (b) mammals. The boxes Fig. 3. Sr/Ca patterns for modern (a) and fossil (b) mammals. The boxes represent the 25th–75th percentiles (with the medians as horizontal lines) represent the 25th–75th percentiles (with the medians as horizontal lines) and the whiskers show the 10th–90th percentiles. Note the nearly identical and the whiskers show the 10th–90th percentiles. Note the similar ecological patterning for both datasets. Absolute values are expected to ecological patterning for both datasets. Absolute values are expected to differ for the modern and fossil specimens because they are from different differ for the modern and fossil specimens because they are from different statistically significant ; The KNP browse is not as exact as that for Ba/Ca (; In both plants have slightly, but not significantly lower Sr/Ca than cases, grazers have higher Sr/Ca than browsers (P < 0.01), grasses (P = 0.14), and, concomitantly, browsers have low- which are in turn slightly elevated compared to carnivores er Sr/Ca than grazers (P < 0.01); however, the Sr/Ca ratios (but only significantly so for the fossils, P = 0.04); and once of the KNP browsers and carnivores are indistinguishable again the magnitude of change is similar, as browser Sr/Ca (P = 0.40). Thus, Ba/Ca may be a more sensitive paleodi- is 43% and 51% of grazer Sr/Ca for the modern and fossil Given the above, it is not surprising that Sr/Ba ratios are in this section as they display no statistically significant also patterned similarly in modern and fossil enamels.
ecological patterning in our modern or fossil datasets.) Although rarely used, this ratio strongly distinguishes be- Can we identify the Ba/Ca pattern of grazers > brows- tween herbivorous and carnivorous fauna. As Sr and Ba ers > carnivores in the Sterkfontein Valley fossil fauna? concentrations tend to covary in foodwebs served modern ecological pattern is indeed preserved in ), grazing and browsing herbivores in Kruger, despite the Sterkfontein Valley fossils. The Sterkfontein Valley having very different Sr/Ca and Ba/Ca, have similar Sr/Ba grazers have higher Ba/Ca than browsers (P < 0.01), which ratios (P = 0.66) ; Carnivores, in contrast, are in turn are significantly elevated compared to carni- have higher Sr/Ba than both grazers and browsers vores (P < 0.01). Furthermore, the magnitudes of change (P < 0.01). The same pattern holds for the Sterkfontein in both the modern and fossil faunas are similar. For in- Valley fossils ; b), as grazer and browser stance, in both the modern and fossil datasets, browser Sr/Ba are indistinguishable (P = 0.09), but carnivore Sr/ Ba/Ca is about 40% of grazer Ba/Ca (44% and 37%, Ba is significantly higher than both herbivore groups respectively). The Sr/Ca patterning is also the same for (P < 0.01). This trophic level effect is a natural consequence the modern and fossil faunas, although the concordance of the higher bioapatite/diet observed ratio (OR) for Sr M. Sponheimer, J.A. Lee-Thorp 70 (2006) 1644–1654 but it does show that the assemblages on the whole, espe- cially those from Swartkrans and Sterkfontein, appear to have maintained much of their paleoecological integrity.
We have also shown that grazers, browsers, and carnivorescan be distinguished using alkaline earth ratio data in both modern and fossil savanna ecosystems in South Africa, which further suggests that diagenesis has not obscured the relevant paleoecological signal. We stress, however,that this ecological patterning may not be relevant in all ecosystems, as each of these broad dietary categories sub- sume animals with quite different diets and habitat prefer- ences. For instance, some browsers eat nothing but tree leaves and forbs, while others may eat a great deal of fruit;and some grazers are found in sere grasslands, while others feed exclusively near the water’s edge. Such dietary and habitat differences may well influence mammalian trace ele- ment compositions, so ecological patterning may often bemore complicated than is apparent here. Nonetheless, the congruence here in modern and fossil ecological patterning suggests that paleoecological studies using Sr, Ba, or Pb, whether investigating paleodiets or paleolandscape use, are likely to prove fruitful at these and karstic cave sites It is possible that these promising results may not per- tain under the very different depositional and chemical cir- cumstances of the East African early hominin sites.
However, while it is likely that previous reports of increases Fig. 4. Sr/Ba patterns for modern (a) and fossil (b) mammals. The boxes in metals, alkaline earths, and other elements at these sites represent the 25th–75th percentiles (with the medians as horizontal lines) and the whiskers show the 10th–90th percentiles. Note the nearly identical there remains at least some grounds to surmise that ecological patterning for both datasets.
useful trace element distribution data might yet be obtain-able from these fossil enamel specimens. For instance, while it was argued that fossil enamel exhibited increased nate more strongly against dietary Ba than Sr, and thus Ba concentrations in that study, for modern and fossil Ba must become relatively less abundant every step up specimens combined (n = 10), falls comfortably within the food chain. Therefore, Sr/Ba provides further support the range of modern herbivores from the KNP. Thus, it for trophic level differences, but more importantly for our is probably premature to make strong statements about purposes here, confirms that modern ecological patterning increases in alkaline earth concentrations in enamel from has not been obscured by diagenesis in the Sterkfontein Allia Bay and other East African sites.
Valley fossils. We have also found it particularly useful Ultimately, studies of post-depositional alteration of for increasing ecological resolution when plotted against paleoecologically relevant elements should be carried out on a site-by-site basis, even when studies of fossil diagenesisare available for other sites in the region. This is under- scored by the results from Makapansgat Limeworks, whichshow that this site, although geologically similar to both 4.1. Enamel diagenesis at South African Australopith sites Swartkrans and Sterkfontein, differs in the degree to whichit imparts Zn to enamel over time. This finding also indi- This study has shown that although enamel Zn levels rectly demonstrates why analysis of REE and U concentra- have increased dramatically at the 3 Ma Makapansgat tions is inadequate for gauging the impact of diagenesis on Member 3, there is no evidence for such an increase in Sr/Ca, Ba/Ca, or Pb/Ca at any of the three fossil sites, aged 1.8–3 Ma. This does not rule out the possibility of signifi- in a given site, even if they occupy identical sites within cant alteration of any given fossil (which might be profit- the crystal lattice as a result of the unique taphonomic his- ably investigated using other tools including electron tories, burial environments, hydrological conditions, and microprobe analysis and scanning electron microscopy as sedimentary matrices of fossil assemblages as well as the Enamel diagenesis at South African Australopith sites properties of the metals themselves. Thus, while enhanced The enriched d13C of this taxon could also levels of REEs and U in fossil enamel undoubtedly demon- be taken as evidence of this trend, as it necessitates the con- strate that diagenesis has occurred, one cannot ipso facto sumption of C4 plant foods such as grasses and sedges or conclude anything about the suitability of the material for paleoecological applications using, for instance, the alkaline earth elements. In short, diagenesis is only an ene- ). Yet, the similarity to carnivores is superficial, as my when it obfuscates the sought after paleoecological sig- the Australopithecus fossils are characterized by high Sr/ nal (e.g., d13C or Ba/Ca browser/grazer patterning).
Ba that is quite distinct from all other fossil specimenswe have analyzed, including carnivores. This suggests the 4.2. A brief look at a potential application possibility that they consumed very different foods thanall of these groups—foods with unusually high Sr and rel- This study suggests that elemental ratio analysis, at cer- tain sites at least, might allow us to address a variety of One food that meets this requirement is grass seed, which highly debated paleoecological questions, such as ‘‘Did has Sr/Ba ratios 3–4 times higher than grass straw (data from Neanderthals have diets dominated by meat?’’ (e.g., Another potential food is underground stor- age organs, although the evidence for this is indirect. We ana- did early primates first adopt frugivorous, rather than lyzed three African mole rats (Cryptomys hottentotus) from the KNP, a species which is known to consume underground immediately relevant to the present study, ‘‘What was the diet of the 2.5 Ma hominin Australopithecus africanus from found them to have the highest Sr/Ba of any animal we have Sterkfontein?’’ In regard to this last question, if we produce studied (). As a result, the possibility of grass seed and a bivariate plot (Ba/Ca and Sr/Ba) of data from this study underground storage organ consumption, both of which (), it is evident that A. africanus had a different diet have been bruited as possible early hominin foods than contemporaneous grazers, browsers, and carnivores, which are in turn highly different from each other. Thus, once again, there is very good reason to believe that ele- ), requires further consideration. Both of these foods mental ratio data are providing valid paleoecological infor- are also consistent with the stable isotope evidence showing mation. Interpretation of these data, however, remains that A. africanus derived considerable dietary carbon from difficult. We can state with confidence that A. africanus had a diet fundamentally different than that of browsing and grazing herbivores, but it is much harder to make affir-mative statements about its trophic behavior. It is clearly most similar to its carnivorous coevals, so one might sug-gest that this taxon had begun to increase the amount of We have shown that enamel Sr/Ca, Ba/Ca, and Pb/Ca animal foods in its diet, which many believe was a key step ratios have not been significantly increased during fossiliza- towards developing the unusually large brains that are the tion at three karstic cave sites, aged 1.8–3.0 Ma, in South Africa. Zn/Ca ratios, in contrast, were highly altered atone site, but only moderately at others. It is likely that each site, and in fact each relevant elemental ratio, should beconsidered sui generis where diagenesis is concerned. Nev- ertheless, we have demonstrated the biogenic ecologicalpatterning has been retained in enamel from Swartkransand Sterkfontein, which may allow us to test hypotheses about the diets of early hominins as well as other mamma-lian taxa at these and similar sites. But despite the potential of this technique for improving our understanding of mam- malian paleoecology, its proper application requires con- siderable baseline work in modern ecosystem elemental distributions as well as in diagenesis. The latter require- ment has been well-recognized but the first has been all but ignored, in spite of the fact that a sounder understand- ing of modern elemental distributions is required for distin- guishing diagenetic from ‘‘real’’ ecological patterns. And Fig. 5. Bivariate log (Ba/Ca) and log (Sr/Ba) plot for fossil grazers, while some promising work has been carried out mapping browsers, carnivores, and A. africanus. The dots are mean values and the elemental distributions in North America foodwebs ( whiskers represent standard deviations. The early hominins do not groupclosely with any of these groups with known diets, and these data suggest a of the world. This problem is especially acute in African M. Sponheimer, J.A. Lee-Thorp 70 (2006) 1644–1654 savanna ecosystems, from which we know of no published Brown, A.B., 1974. Bone strontium as a dietary indicator in human data on plant elemental compositions. It is only after com- skeletal populations. Contrib. Geol. 13, 47–48.
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