Fossil Tooth Enamel Research Articles

Grassland development under glacial and interglacial conditions in southern Africa: review of pollen, phytolith and isotope evidence

Pollen evidence suggests that grasslands were well established in southern Africa by the Late Tertiary. Evidence for grassland composition in the region during the Quaternary includes published accounts of isotopes, grass phytoliths and pollen of both grasses and woody plants from a wide range of different environments. Isotope data were derived from speleothems (stalagmites), fossil bones, and fossil tooth enamel and plant material in fossil hyrax dung. The different data types suggest that, with perhaps the exception of the dry southern Kalahari region, temperate grassland consisted of a relatively increased C 3- to C 4-grass ratios during the Last Glacial Maximum (LGM). Cold winter temperature extremes in the southern high latitude and altitude regions and a persistent winter rainfall pattern over the Cape region during the LGM probably limited the distribution of C 4 grasses and canceled out any advantages gained from lowered CO 2 concentrations in the atmosphere. In contrast, in the tropics where marked seasonal temperature fluctuations were lacking, C 4-grass growth was favored.

High-resolution electron microscopy of the crystallites of fossil enamels obtained from various geological ages.

To elucidate the stability of the central dark line (CDL) in biologically induced hydroxyapatite crystals, we examined the diagenetic changes on the microstructures of the crystallites during the course of fossilization. Using transmission electron microscopy, we investigated the enamel crystallites of fossil animals of various geological ages ranging from Pleistocene to Cretaceous. Electron micrographs indicated that the microstructures and lattice images of each crystallite in fossil enamels were well-preserved regardless of the thickness of the enamel layer, and the presence of CDLs in fossil enamel crystals was also confirmed. The results indicated that the microstructure of hydroxyapatite crystals containing lattice images of CDLs appear stable during long geological periods. In addition, we conclude that the existence of lattice images in apatite with CDLs may be an indicator for the assessment of the evolution of dental enamel from fossil remains.

Cosmogenic 3He in igneous and fossil tooth enamel fluorapatite

Igneous fluorapatite samples from a suite of six granitic rocks from the Transantarctic Mountains have high ^3He concentrations (to 5×10^9 atoms g^(−1)) and high ^3He/^4He ratios (to 9×10^(−7)). These values are far higher than those found in several hundred igneous apatites from elsewhere around the world and are higher than can be attributed to nuclear reactions on ^6Li. This ^3He is almost certainly derived from cosmic ray reactions in rocks with high exposure ages at high latitude and elevation. Several samples of fossil tooth enamel fluorapatite from the Turkana Basin of Kenya are similarly rich in ^3He, with up to 1×10^7 atoms ^3He g^(−1) and ^3He/^4He ratios up to 4×10^(−6). Again, this ^3He is most logically attributed to cosmic ray reactions. Provided that cosmogenic ^3He, like radiogenic ^4He, is quantitatively retained in fluorapatite under Earth surface conditions, routine ^3He exposure dating of this common phase may be possible. Based on its chemical composition, the ^3He production rate in fluorapatite is about 100 atoms g^(−1) yr^(−1) at sea level and high latitude. Using this rate the apatites from the Transantarctic Mountains have apparent exposure ages of 0.5–6.2 Myr, in agreement with values elsewhere in the range. The fossil tooth enamel samples have apparent exposure ages ranging from a few up to 130 kyr. Such high exposure ages suggest some of these fossils may be lag deposits with a very long residence time at or near the Earth’s surface. ^3He exposure ages can provide insights to the depositional and reworking history of enamel-bearing fossils. At present the major limitations to ^3He exposure dating of fluorapatite are purification of sufficient amounts of material and measurement of small amounts of ^3He in the presence of large quantities of ^4He. In addition, further work is necessary to establish the nucleogenic ^3He background in fluorapatite.

Climate and atmosphere interact to drive vegetation change

About seven Mya, the carbon isotope composition of fossil tooth enamel and paleosol carbonates from Pakistan, North America, Africa and South America shifted to higher values. The shift recorded a transition from ecosystems dominated by plants using the C3 photosynthetic pathway (most plants, including all woody species and many grasses) to those using C4 (including some grasses). C3 plants have lower carbon isotope values than do C4 plants, and this difference is reflected in the soil in which they grow and the herbivores that eat them. Because C4 plants are competitively favored under warm, dry conditions, the ecological shift was initially attributed to climate change. However, the global near-synchronism of the isotopic shift led to the hypothesis that the C3–C4 transition was driven by falling atmospheric CO2 concentrations. New carbon isotope records from Africa and Asia suggest that patterns of regional vegetation change might be controlled by a combination of factors.

The hunters and the hunted revisited

The dietary niches of extinct animals, including hominids and predators, may be constrained using stable carbon isotope ratios in fossil tooth enamel.13C/12C ratios of many of the primates abundant in the faunal assemblages of Members 1 and 2 at Swartkrans, including cercopithecoids and Australopithecus (Paranthropus) robustus, and a range of other possible prey species, have been reported previously. Resulting suggestions of a mixed, or omnivorous, diet for A. robustus raise questions about niche overlap with coeval, larger brained Homo. Here we present13C/12C data from Homo and several large predators including Panthera pardus, Dinofelis sp., Megantereon cultridens and Chasmoporthetes nitidula in Member 1, and P. pardus and P. leo in Member 2, in order to compare the two hominid species and to determine likely predators of the various primates and other macrovertebrates. Results for three Homo cf. ergaster individuals are indistinguishable from those of A. robustus, showing that proportions of C3- and C4-based foods in their diets did not differ. P. pardus, Megantereon and Crocuta are shown to be likely predators of the hominids and Papio baboons in Member 1, while the Dinofelis individual concentrated on prey which consumed C4grasses. The hunting hyaenid C. nitidula preyed on either mixed feeders or on a range of animals across the spectrum of C3and C4variation. The data from Members 1 and 2 confirm a shift in leopard diets towards animals that consumed C4grasses.

A method for identification of interfering signals in EPR dating of tooth enamel

In this paper, a method for identifying interfering EPR signals in fossil tooth enamel is presented. This method consists of optimizing EPR parameters to enhance spectral resolution of the fossil signals followed by a g-factor and intensity normalized subtraction of a high-dosed, modern tooth enamel sample spectrum. By scanning the modern tooth enamel sample with the same parameters as the fossil sample, the difference spectrum of the fossil and modern irradiated tooth enamel samples can resolve numerous hyperfine and other interfering signals. This method was successfully applied to a fossil sample studied elsewhere (Grün, R., 1998a. Ancient TL 16, 51–55; Grün, R., Clapp, R., 1996. Ancient TL 14, 1–5; Martin Jonas, 1997. Ph.D. Thesis, Cambridge University; Jonas, M., Grün, R., 1997. Radiat. Meas. 27, 49–58; Vanhaelewyn et al., 2000. Appl. Radiat. Isot. 52, 1317–1326). This sample has shown discrepancies in dose estimations obtained from the power absorption curve versus those obtained from the first derivative spectra (Grün, R., 1998b. Radiat. Meas. 29, 177–193). The reason for this, and other discrepancies, are accounted for by the signals resolved using the method presented here.

EPR spectrum deconvolution and dose assessment of fossil tooth enamel using maximum likelihood common factor analysis

In order to determine the components which give rise to the EPR spectrum around g = 2 we have applied Maximum Likelihood Common Factor Analysis (MLCFA) on the EPR spectra of enamel sample 1126 which has previously been analysed by continuous wave and pulsed EPR as well as EPR microscopy. MLCFA yielded agreeing results on three sets of X-band spectra and the following components were identified: an orthorhombic component attributed to CO − 2 , an axial component CO 3− 3 , as well as four isotropic components, three of which could be attributed to SO − 2 , a tumbling CO − 2 and a central line of a dimethyl radical. The X-band results were confirmed by analysis of Q-band spectra where three additional isotropic lines were found, however, these three components could not be attributed to known radicals. The orthorhombic component was used to establish dose response curves for the assessment of the past radiation dose, D E. The results appear to be more reliable than those based on conventional peak-to-peak EPR intensity measurements or simple Gaussian deconvolution methods.

Altered states: effects of diagenesis on fossil tooth chemistry

Investigation of modern and fossil teeth from northern and central Kenya, using the ion microprobe, electron microprobe, and transmission electron microscope, confirms that fossil tooth chemistry is controlled not only by the diagenetic precipitation of secondary minerals but also by the chemical alteration of the biogenic apatite. Increases in the concentrations of Fe, Mn, Si, Al, Ba, and possibly Cu in fossil vs. modern teeth reflect mixtures of apatite and secondary minerals. These secondary minerals occur in concentrations ranging from ∼0.3% in enamel to ∼5% in dentine and include sub-μm, interstitial Fe-bearing manganite [(Fe 3+, Mn 3+)O(OH)], and smectite. The pervasive distribution and fine grain size of the secondary minerals indicate that mixed analyses of primary and secondary material are unavoidable in in situ methods, even in ion microprobe spots only 10 μm in diameter, and that bulk chemical analyses are severely biased. Increases in other elements, including the rare earth elements, U, F, and possibly Sr apparently reflect additional alteration of apatite in both dentine and enamel. Extreme care will be required to separate secondary minerals from original biogenic apatite for paleobiological or paleoclimate studies, and nonetheless bulk analyses of purified apatite may be suspect. Although the PO 4 component of teeth seems resistant to chemical alteration, the OH component is extensively altered. This OH alteration implies that bulk analyses of fossil tooth enamel for oxygen isotope composition may be systematically biased by ±1‰, and seasonal records of oxygen isotope composition may be spuriously shifted, enhanced, or diminished.

Oxygen Isotopes in Enamel Carbonate and their Ecological Significance

Stable carbon isotope analysis of fossil tooth enamel carbonate, and oxygen isotope analysis of bone or enamel phosphate, are established tools for palaeodietary and palaeoclimatic reconstruction, respectively.13C/12C ratios provide evidence of an animal’s diet and phosphate-based18O/16O values are used to establish palaeotemperature proxies. Recent studies of fossil enamel suggest that biogenic18O/16O signals are also retained in the carbonate compartment, despite assumptions that18O/16O ratios from apatite carbonate are highly susceptible to exchange during fossilization. Here, we re-examine existing enamel carbonate18O/16O data from the rich fossil assemblages of Swartkrans and Equus Cave. We find patterns that can be interpreted in terms of drinking behaviour, diet, and physiology. In general, herbivores that drink little are more enriched than those that drink frequently, while carnivores are depleted compared with herbivores. Thus, we can increase our knowledge of the ecology of fossil taxa by coupling carbon and oxygen isotope ratio data.

Alteration of Enamel Carbonate Environments during Fossilization

An understanding of enamel diagenesis is necessary to ensure sound isotopic palaeodietary and palaeoenvironmental reconstructions. Although carbon isotope signals of browsing and grazing herbivores remain distinct in enamel even after millions of years, subtle alteration of isotopic signatures does occur. To better understand this change we analysed modern and fossil enamel from a number of South African sites using Fourier Transform Infra-Red spectroscopy. Our results indicate that while there is little evidence of increased crystallinity in fossil enamel, there is a small but significant change in the proportion of carbonate ions occupying hydroxyl and phosphate sites. This seems to occur early in the process of fossilization, after which there is no noticeable change. It is also important to note that the degree of alteration varies significantly within and between sites. We suggest that this change results from one or some combination of three mechanisms: exogenous carbonate incorporation, endogenous carbonate loss, and endogenous carbonate reorganization. Determining which mechanism(s) contribute to this alteration is important because all three are likely to affect biogenic carbon isotope ratios differently. FTIR spectroscopy promises to increase our knowledge of diagenesis, and in so doing, should improve our palaeodietary and palaeoenvironmental reconstructions.

Oxygen isotopic composition of mammalian skeletal phosphate from the Natufian Period, Hayonim Cave, Israel: Diagenesis and paleoclimate

Recent and fossil (prehistoric, Natufian) gazelle bones, dentin and enamel were analyzed for their oxygen isotopic composition (δ18O) of the phosphate and carbonate, as well as their crystallinity. Fossil bones and dentin have better crystallinity than recent specimens, indicating diagenetic change. Fossil enamel, on the other hand, is identical in crystallinity to recent enamel, indicating the lack of diagenetic alteration. Comparison of δ18O of carbonate and phosphate of the skeletal elements suggests that the coexisting phosphate and carbonate of both the recent and fossil specimens are close to isotopic equilibrium. This might suggest that both phases were affected by the same degree of diagenetic alteration, and that comparison of their δ18O is not useful for the selection of pristine material for paleoclimatic reconstruction purposes. Oxygen isotope analysis of gazelle enamel from the Natufian period from Hayonim Cave, Israel, show depletion in δ18O in comparison with recent enamel. This depletion is interpreted to represent a colder and/or wetter climate in the Natufian of northern Israel. © 1999 John Wiley & Sons, Inc.

Reproducibility measurements for ESR signal intensity and dose determination: high precision but doubtful accuracy

Reproducibility measurements on fossil tooth enamel show that the intensity of the main signal can be measured with high precision (<1%). Dose response curves based on conventional peak-to-peak measurements yield doses with errors in the range of 1–3.5%. The most appropriate weighting model for exponential regression is the experimentally determined variance; a model using weights inversely proportional to intensity squared also gives sensible results. Dose versus magnetic field plots on derivative and absorption spectra yield dose values with similar high precisions. However, these plots cast doubt on the general validity of ESR dose assessments from complex ESR spectra such as fossil tooth enamel. Meaningful spectrum deconvolution is urgently required for the estimation of reliable dose values.

Dietary and environmental reconstruction with stable isotope analyses of herbivore tooth enamel from the Miocene locality of Fort Ternan, Kenya.

Tooth enamel of nine Middle Miocene mammalian herbivores from Fort Ternan, Kenya, was analyzed for delta 13C and delta 18O. The delta 18O values of the tooth enamel compared with pedogenic and diagenetic carbonate confirm the use of stable isotope analysis of fossil tooth enamel as a paleoenvironmental indicator. Furthermore, the delta 18O of tooth enamel indicates differences in water sources between some of the mammals. The delta 13C values of tooth enamel ranged from -8.6(-)-13.0/1000 which is compatible with a pure C3 diet, though the possibility of a small C4 fraction in the diet of a few of the specimens sampled is not precluded. The carbon isotopic data do not support environmental reconstructions of a Serengeti-typed wooded grassland with a significant proportion of C4 grasses. This study does not preclude the presence of C3 grasses at Fort Ternan; it is possible that C3 grasses could have had a wider geographic range if atmospheric CO2 levels were higher than the present values.

Global vegetation change through the Miocene/Pliocene boundary

Between 8 and 6 million years ago, there was a global increase in the biomass of plants using C4 photosynthesis as indicated by changes in the carbon isotope ratios of fossil tooth enamel in Asia, Africa, North America and South America. This abrupt and widespread increase in C4 biomass may be related to a decrease in atmospheric CO2 concentrations below a threshold that favoured C3-photosynthesizing plants. The change occurred earlier at lower latitudes, as the threshold for C3 photosynthesis is higher at warmer temperatures.

Teneur en uranium et datation U-Th des tissus osseux et dentaires fossiles de la grotte du Lazaret

Fossil bone and dental tissues from Lazaret cave and modern ones are here the subject of a comparative microscopical study. Porous tissues such as dentine and bone have retained their Haversian and Tomes canals respectively. However, cracked areas with calcite were detected, indicating a water percolation within porous tissues and an alteration of tissue in places. In addition, compact fossil enamel is particularly well preserved. These results are essential for U-Th and ESR dating application. Uranium contents, U-Th ages of two fossil mandibular tissues, two tibias and of six burnt fossil bones are presented and discussed.

Comparison of ESR signal dose-responses in modern and fossil tooth enamels

The dose response of ESR signals in enamels extracted from young archaeological (modern) teeth aged less than 200 y were compared with fossil teeth ranging in age from about 50 to 110 ka. We aimed to test the accuracy of equivalent dose (ED) determinations on modern teeth with a known (near zero) ED because zeroing the signal in tooth enamel is not possible. The modern teeth showed departure from true single saturating exponential dose response, but generally gave very good agreement with the expected near zero ED (±5–10% in the worst cases). Linear fitting was better than SSE fitting within the lower dose ranges. However, the fossil teeth showed significant variability and departure from the well-behaved dosing characteristics of modern enamels. When the natural signal was excluded, EDs were found to be strongly dependent upon the choice of the first added dose in both cases studied using a modulation amplitude of 0.32 mT, and strongly dependent upon choice of modulation amplitude in another case. These latter observations, when considered in relation to modern enamel behaviour, raise questions about the possibility of diagenetic change in the fossil enamel.

Q-band ESR studies of fossil tooth enamel: Implications for spectrum deconvolution and dating

Q-band ESR spectra of eight samples of powdered fossil tooth enamel from the Florisbad archaeological site in South Africa have been measured. On one of these, spectrum deconvolution of the integrated spectrum has been performed. The results are consistent with a model that describes the ESR signal used in dating studies as a superposition of an axialsymmetrically split signal and two isotropic Gaussian signals. A narrow additional signal with higher radiation sensitivity than the main signal has been observed and this line may affect the dose determination in dating if peak-to-peak amplitudes are used to generate dose response curves.

Stable carbon and oxygen isotope analysis of fossil tooth enamel using laser ablation

A technique is described whereby the δ13C and δ18O values of fossil tooth enamel can be measured in situ using laser ablation techniques. The laser heats the sample and forms CO2 from structural carbonate apatite. The δ18O values obtained with this method are equal to those of the phosphate oxygen due to the high temperature of reaction during ablation. Analytical precision is approximately 0.5‰ for both δ13C and δ18O. The spatial resolution is 200 μm or less, which may be reduced as the technical modifications are made. This method allows the analysis of samples that previously could not be analyzed due to difficulty with sample preparation, size, or rare nature. The apparent oxygen isotope fractionation between the carbonate and laser derived oxygen (αCO3-laser) is 1.0071 for biogenic apatite at 38°C determined using the in situ laser. Comparison of δ18O(CO3) and δ18O(laser) analyses with δ18O(laser) and δ18O(PO4) results suggest a fractionation factor α(CO3)-PO4) of 1.0083. Variations in the δ18O and δ13C values of single teeth can be easily determined with the in situ technique. This will allow the study of changes in diets or water sources during the period of tooth growth, which for larger mammals can be several years for the tooth row. The δ13C and δ18O values are important in paleodiet and paleoecology studies. They can be used to study resource partitioning within an ecosystem and can shed light on the life history strategies within living and extinct species.

Biologic and climatic signals in the oxygen isotopic composition of Eocene-Oligocene equid enamel phosphate

Abstract Oxygen isotope results for tooth enamel-phosphate (δP) from late Eocene-early Oligocene fossil horses are presented to determine if paleobiologic and paleoclimatologic information is recorded in fossil tooth enamel chemistry. Teeth from jaws of Mesohippus and Miohippus from the White River Formation (or Group) in the western Great Plains are well preserved and have excellent geochronologic control. Although there is clear evidence for post-depositional alteration of the enamel, a hydroxylapatite mineralogy is preserved and isotopic exchange of oxygen does not appear to be significant. There are distinctive patterns of δP variation among individual teeth from the same jaw. These patterns reflect season of birth and timing of enamel mineralization. Most of the horses were born in the spring, and mineralization of the enamel is complete after 1–1.5 years. These results show that paleoclimate reconstruction from enamel δP must account for tooth position and timing of mineralization. The observation that there are different intrajaw patterns of δP variation among Mesohippus and Miohippus may provide a basis to reconstruct changes in climate seasonality in ancient environments.

EPR dating CO 2− sites in tooth enamel apatites by ENDOR and triple resonance

In this work we combine electron paramagnetic resonance (EPR), high-resolution electron nucleus double resonance (ENDOR) and general triple resonance (GTR) spectroscopies, to study the local environment of the CO 2 − groups created by ionizing radiation in fossil tooth enamel. We demonstrate that the CO 2 − groups occupy slightly modified phosphate sites in the hydroxyapatite lattice. In quaternary shark enamel we found these groups to be interacting with water molecules in the apetite channels. The absence of water molecules as first neighbors in mammalian samples indicate, however, that these molecules are not significantly responsible for the stabilization of CO 2 − dating centers in enamel.