Equid Teeth Research Articles

Measurements of the uranium concentration and distribution in a fossil equid tooth using fission tracks, TIMS and laser ablation ICPMS: Implications for ESR dating

Combined ESR/U-series dating of tooth enamel is important for obtaining chronological constraints on archaeological sites. The precision and accuracy of this method depend on the uncertainties associated with the equivalent dose and the dose rate. The precision of the dose rate calculation, in turn, depends on a number of parameters such as the internal radionuclide concentrations and distributions in the enamel, dentine and cementum and in the surrounding sediments as well as on the cosmogenic dose rate. Furthermore, teeth are open systems for U and therefore constraints on the U-uptake model are needed. The combination of all the uncertainties results in uncertainties of ∼ 10 % on the age at 95% confidence level. This can, however, be improved by reducing the uncertainties on the internal radionuclide concentrations and distributions. We present a comparison of three methods for measuring the U concentrations in dental tissue of a 40 ka old equid tooth. Fission tracks are used to map the U distribution with high spatial resolution. The U and Th concentrations and isotopic compositions were measured with high precision but poor spatial resolution using TIMS. It is demonstrated that in situ LA-ICPMS provides an effective method for precise measurements of the U concentration at high spatial resolution. This method also has high potential for isotopic U-series measurements using MC-ICPMS. The results of the U measurements are used to constrain the effective alpha- and beta-dose rates in enamel sampled for ESR analysis. The TIMS U–Th isotope concentrations and in particular the 230Th concentrations are used to constrain the U uptake for combined ESR/U-series dating.

Horses Through The Ages

Honoring a renowned specialist in studies of ancient horses, Equids in Time and Space: Papers in Honour of Vera Eisenmann edited by Marjan Mashkour includes 18 chapters by 22 international paleontologists, archaeologists, anatomists, historians, and other experts on living and extinct Equus and related horse relatives. There is also an introduction providing a brief review of Dr. Eisenmann’s life and career. The volume is not without some substantial flaws, but remains a useful acquisition for anyone interested in the history and prehistory of these important animals. The book is hardcover and oversize. Typeface is generally clean and sharp; figures are grayscale and of varying quality, depending upon the author(s). The volume is divided into five more or less equally-weighted sections: geographic distribution, Paleolithic horses, domestication, methodology, and anthropozoology. Equids earlier than the latest Tertiary and Quaternary are not discussed in any detail, reflecting Dr. Eisenmann’s focus on more geologically recent horses. The investigative approaches employed are extremely diverse, including historic and paleontologic literature reviews, interpretations of old and new metric indices, uniand multivariate statistical analyses, and assessments of stable isotope chemistry in fossil equid teeth. The four papers comprising the geographic distribution portion of the volume demonstrate the diversity of investigative methods employed. Deng reviews late Pleistocene Przewalski’s horses from China, using carbon isotopes from dental enamel to suggest that Przewalski’s horses, as arid-adapted animals, can be used to track the waxing and waning of the East Asian monsoon through time. Foronova examines numerous late Pleistocene sites in southern Siberia, determining through dental morphology and morphometric analyses of metapodials that caballine and hemionine horses were all present in the region, but in varying abundance through time depending upon local climate. Pichardo reviews the published literature on late Pleistocene horses from North and South America; this study is unfortunately hampered by an unfocused presentation and an incomplete selection of reference works [several important J Mammal Evol (2008) 15:147–149 DOI 10.1007/s10914-007-9063-5

Paleoseasonality inferred from equid teeth and intra-tooth isotopic variability

This research presents a study of intra-tooth isotopic variability measured in hypsodont equid teeth as a means of determining paleoseasonality. The purpose of this investigation is to determine whether Sivapithecus, a Miocene hominoid from the Siwaliks of Pakistan, lived in habitats characterized by a seasonal rainfall regime, and if so, what kind of forest is associated with this regime. Forest type, such as evergreen, semi-evergreen, moist deciduous, and dry deciduous, is largely determined by length of the dry period. Reconstruction of the rainfall regime therefore allows inferences of forest type in the fossil record. Seasonal rainfall regimes can be determined from oxygen stable isotopes, for oxygen isotopes track amount of rainfall in tropical regions. Sampling oxygen isotopes along the length of an equid tooth, which takes over a year to develop, therefore provides a record of seasonal isotopic changes in body water, which in turn reflect drinking water and ultimately precipitation. Carbonate oxygen results from Siwalik equid teeth spanning 10.0–6.3 Ma indicate a rainfall regime similar to that experienced by southern China monsoon forests today, with a monsoonal rainy season and a five-to-six month dry season. Furthermore, results suggest a seasonal birthing pattern for Siwalik equids and therefore predictable seasonal regime. These results indicate that Sivapithecus lived under a seasonal regime with dry periods longer than the maximum of 4 months that most great apes experience today. Therefore Sivapithecus may have experienced longer periods of low fruit availability than do modern apes. Either Sivapithecus differed in habitat requirements, or the forest differed in terms of the spatio-temporal variability of fruit compared to modern analogues. Over geologic time, annual rainfall appears to have decreased in the Siwaliks, beginning around the time Sivapithecus became extinct, but annual seasonality of rainfall does not change.

Isotopic analysis of tooth enamel carbonate from modern North American feral horses: implications for paleoenvironmental reconstructions

The accuracy of paleoenvironmental reconstructions based on isotope analyses of equid teeth is currently uncertain because the exact relationship between the isotope composition of modern feral equids and their environment has not been thoroughly studied. We analyzed the carbon and oxygen isotope values (δ 13C and δ 18O) of tooth enamel carbonate and the δ 13C values of fecal samples from modern feral horses. We compared those values with the δ 13C values of local vegetation and the δ 18O values of local waters. Herds were studied in two contrasting localities: eastern Oregon, where grasslands consisted of 100% C 3 species, and New Mexico, where >95% of the grasses were C 4 species. Carbon isotope analyses of fecal material and tooth enamel suggest that horses consumed primarily grass, but some New Mexico horses also consumed significant amounts of shrubs and/or forbs. Microhistological analyses of fecal samples show that Oregon horses consumed 95% grass, and Oregon enamel δ 13C values are consistent with a diet containing 100% C 3 plants. Microhistological analyses of fecal samples from New Mexico indicate a diet averaging 75% grass, while enamel δ 13C values suggest that diets averaged 85% C 4 plants (range=72–97%). Thus, reconstructions of the C 3/C 4 ratio of grasses in ancient grasslands that are based on the δ 13C values of fossil equid teeth may underestimate the abundance of C 4 grasses. The mean δ 18O values of tooth enamel paralleled the trends observed in the mean δ 18O values of precipitation. However, the mean δ 18O values of enamel carbonate from Oregon and New Mexico differed by only 3.3‰, which is less than the difference in the mean δ 18O values of precipitation (6.5‰). In addition, the range of δ 18O values within New Mexico enamel samples (6.5‰) was greater than the difference between mean enamel δ 18O values at each site. Calculated values for the δ 18O of water ingested by horses are 2–3‰ more positive than mean δ 18O values for corresponding precipitation, suggesting that horses consumed waters that were enriched in 18O due to evaporation. While our results confirm that local climatic and hydrological conditions can influence the δ 18O values of equid enamel, they also show that the δ 18O values of equid teeth are not always a direct proxy for the isotope ratios of precipitation.

Dating the Naisiusiu Beds, Olduvai Gorge, by electron spin resonance

The lower beds at Olduvai Gorge are well known for containing early hominid fossils and Oldowan stone tools, and their ages have been established by 40Ar/ 39Ar dating and paleomagnetic stratigraphy. Ages are generally less certain for the upper deposits at Olduvai Gorge because of the scarcity of datable tuffs. The youngest archaeologically significant site at Olduvai is microlithic LSA, which lies in the type section of the Naisiusiu Beds. The age for the site is controversial, with 14C dates of 17,000–17,550 (Hay, R.L., 1976 Geology of Olduvai Gorge, University of California Press, Berkeley) and >42,000 BP (Manega, P.C., 1993. Geochronology, geochemistry, and isotopic study of the Plio–Pleistocene Hominid sites and the Ngorongoro Volcanic Highland in Northern Tanzania. Unpublished Ph.D. Thesis, University of Colorado, Boulder, CO). The tuff bed in the zone with artifacts does not contain materials datable by 40Ar/ 39Ar, and some other dating method was needed. In the summer of 2001, five equid teeth were collected from the type Naisiusiu site. Another tooth had previously been collected. ESR ages have been determined for three teeth from the archaeological level and their ages cluster around 62±5 ka, assuming linear uranium uptake. Another tooth from a level without artifacts and believed to be significantly younger dated to 39±5 ka, again assuming LU. These dates are considerably older than previous estimates and suggest that the East African MSA/LSA transition occurred very early.

Oxygen isotope evidence for progressive uplift of the Cascade Range, Oregon

Oxygen isotope compositions of fossil equid teeth in the Cascade rainshadow reveal a ∼5‰ decrease in mean δ 18O since 27 Ma. Isotopic changes are inconsistent with expected effects from global climate change because: (a) the expected isotopic shift to tooth δ 18O values due to global climate change (∼1‰) is much smaller than the observed shift, (b) predicted and observed isotopic trends are opposite for Oligocene vs. Miocene samples, and (c) average compositions and ranges in compositions remained unchanged for samples from before and after major global cooling in the mid-Miocene. Accounting for a decrease in relative humidity of at least 15%, we infer a topographically driven secular shift in the δ 18O value of rainwater of 6–8‰ since the late Oligocene, which is approximately equivalent to the modern-day difference in δ 18O values of precipitation and surface waters across the central Cascades. Rise of the central Cascades apparently occurred monotonically over the last 27 Ma, with a hiatus between ∼15.4 and 7.2 Ma, possibly related to eruption of the Columbia River Basalts. Progressive volcanic accumulation over tens of millions of years best explains the data, rather than a short-lived uplift event. Paleoseasonality, as inferred from isotope zoning and intertooth variability, decreased dramatically from 7–9‰ at 15.4–7 Ma to ∼3‰ at 3 Ma, then increased to 6–8‰ today. The cause of the decrease in seasonality at 3 Ma may reflect either brief warming during the mid-Pliocene within the context of global tectonic reorganization, or consumption by equids of water from an isotopically buffered Lake Idaho.

Oxygen isotopic composition of fossil horse tooth phosphate as a record of continental paleoclimate

Oxygen isotopic, elemental, and X-ray data are presented for a suite of 24 fossil horse teeth from Nebraska ranging in age from 18.2 to 8.5 Ma, to test the use of δ 18O of enamel phosphate ( δ 18O P04) as a quantitative record of continental climate. Modern equid teeth were analyzed to estimate a relationship between δ 8O P04 and environment water. Multiple samples of seven different fossil species from Burge Quarry, a ∼ 12 Ma attritional fossil deposit, indicate the diagenetic overprints exist but can be detected by decreased P concentration and increased crystallinity relative to modern enamel. Isotopic variation for the pristine samples from Burge Quarry is ±1.5% (1 σ, n=9), which may represent the resolution of the procedure within a stratigraphic horizon. There are no apparent correlations with body size, hypsodonty, or phylogeny. A range of 7%0 in δ 18O P04 occurs over the 10 m.y. interval. A trend towards depleted δ 18O P04 of about 4% corresponds to a depletion of up to 6%0 in δ 18O of precipitation between 18.2 and 8.5 Ma, but the range of variation of Burge is large relative to the climate signal. Our results demonstrate that δ 18O P04 should be useful in quantitatively reconstructing Cenozoic continental paleoclimate on 10 6-year timescales. Isotopic variation due to taphonomic bias and the terrestrial rock record will likely obscure higher-order climate signals.

Oxygen isotopic composition of fossil equid tooth and bone phosphate: an archive of difficult interpretation

Variation in the concentration of the stable isotopes of hydrogen, oxygen and carbon have been tested in continental records as tools for quantitative or semi-quantitative paleoclimatological studies. Among the different methods, the potential use of oxygen isotopes in mammal bone and tooth phosphate has recently been recognized. Measurements carried out on fossil mammal bones of Holocene age and their paleoclimatological interpretation corresponded well with paleontological and paleobotanical records. In the case of considerably older fossils, diagenetic processes may change the primary oxygen isotopic composition of phosphate. Fossil horse bones and teeth (principally of the species Equus stenonis) were studied to ascertain how far back in time fossil mammal remains may be considered reliable material for paleoclimatological studies. The samples come from 13 different locations in southeastern Spain, their age ranging from Maspinian (late Pleistocene) to Middle Villafranchian (Pliocene). Apart from the variations of the δ 18O(PO 4 3−) values which may be related to climatic changes, it is apparent that the isotopic composition of bones and teeth from the same deposit are frequently rather different from one another. The isotopic differences range from a few tenths of one % to several %. This suggests a strong influence of taphonomy over the measured isotopic values of fossils in each deposit. Time can be considered neither the only variable nor the most important one responsible for the change of the primary isotopic composition of fossils. Under these conditions it is rather difficult to establish lower or upper limits for the age of fossils to be studied for reliable paleoclimatological information since the limits are directly related to taphonomic history. This, in turn, is related to local environmental conditions and not only to the age of fossils.

Isotopic analysis of equid teeth from Wonderwerk Cave, northern Cape Province, South Africa

Stable carbon and nitrogen isotope ratios have been determined from equid teeth from a well-dated sequence of Holocene deposits at Wonderwerk Cave, situated in a summer-rainfall area of the northern Cape Province of South Africa. Stable carbon isotope ratios confirm the existence of a C 4 grassland habitat throughout the Holocene. Nitrogen isotope ratios are assessed in relation to data from modern equid samples from southern Africa. Temporal variability in both carbon and nitrogen isotope ratios of Holocene equid samples from Wonderwerk appears to be analogous to spatial variability of such ratios determined for faunal samples from modern environments. Relationships between isotope ratios and data from modern environments are used to obtain rainfall indices.

The origins of horseback riding

The horse is ridden by means of the bit, and the bit leaves its trace on the horse's teeth. The beginnings of horse-riding are here identified by defining and detecting microscopic bit wear on equid teeth, using comparative samples from 4 countries and 25,000 years of prehistory. Scanning electron microscope analysis demonstrates that bit wear is clearly distinguishable from other tooth damage. It first occurs in the Ukraine, USSR, at about 4000 BC. Soon thereafter, mobility became a cultural advantage that transformed Eurasian societies. Riding now appears to be the first major innovation in land transport, pre-dating the wheel.

Abnormal Enamel Morphology in Fossil Equid Teeth

Abnormal Enamel Morphology in Fossil Equid Teeth

Paleocene Hyracothere from Polecat Bench Formation, Wyoming

A lower jaw of an eohippus (Hyracotherium cf. H. angustidens) from late Paleocene strata in Wyoming has extended the geological record of fossil horses into pre-Eocene time and suggests that the order Perissodactyla had an origin earlier than that heretofore conjectured. This specimen, together with equid teeth also possibly of late Paleocene age from Baja California, indicates that early perissodactyls were widespread on the North American continent before the Eocene epoch. Late Paleocene and early Eocene deposits of northwestern Wyoming have yielded many vertebrate rarities and "first or earliest occurrences" which require reappraisals of inter-and intracontinental dispersal patterns of the vertebrate grolups involved.