By Sam Reynolds
A friend of mine produces and sells scrimshaw, engraved artwork on ivory or bone. He makes most of his money at craft fairs. There he patiently explains again and again to mortified fair-goers that the tusks he works on are not from elephants, but from mammoths. “Mammoths that lived at least several thousand years ago”, he politely clarifies—”Not the modern mammoth of today.”
Mammoth ivory—which is sometimes mislabeled fossil-ivory—is by no means ubiquitous. However there is a continuing supply of it, especially from the permafrosts of Siberia, where tusks are often treated as a raw material commodity, not as paleoecological specimens. Mammoth molars and tusks are essentially unregulated largely because they are distinct from contemporary elephant ivory, which is illegal save for a few exceptions. Though still recognizable as ivory, the teeth have often undergone partial diagenesis, consisting of both original and some fossilized material. Sometimes they are preserved nearly perfectly, with little more than staining. In either case, they tend are original material—and provide plentiful and accurate chemical snap-shots of the lives of their proboscidean owners. From a site in Switzerland, Mammoth teeth dated 45,000 years old contained oxygen-18 isotopes that indicated the average air temperature there was 4ºC cooler than today (Heuser, 2010). Teeth can also be analyzed for dietary information through values of carbon-13 and nitrogen-15.
Mammoth tusking showing characteristic banded patterning. Taken from the blog of Charlotte Bailey, a fossil-trader and educator; http://www.rocks-fossils.com.
What makes mammoth teeth and tusks so exceptional as an archive is their growth pattern. Molars grow continuously through much of the mammoth lifetime, and tusks grow the entire lifetime, incorporating many years worth of dietary and environmental traces as they do so. The tusks of mammoths grow much the same as the tusks of modern elephants; they build up by accretion, causing the dentin to form rings around the outer surface of the tusk. Similar to trees, there are distinct annual rings that consist of a seasonal light to dark pattern. In turn the extremely thin dark rings inside of the annual rings represent about a days growth. These bands change in color depending on the nutritional state of the animal. Interestingly, there are often particularly dark daily rings occurring about once a week. Here the similarities to dendrochronology end, as there are two large differences. First, the shade of rings has little to do with the width of cells, and more to do with the degree of mineralization of the dentin protein matrix. Second, rings are not examined by cross sectioning the tusk, and the oldest layer is not at the center. Tusks do not grow radially like a tree. Instead, they grow outwards from a conical pulp structure incased by dentin-producing odontoblasts. Each layer of tusk is essentially a hollow dentin cone in the shape of the conical pulp cavity which is displaced outwards when a newer layer is formed underneath. Consequently, the first layers produced at the beginning of the mammoths life will be at the very tip of the tusk. The final layers produced are at the very base of the tusk (Rountrey, 2007). The lower edges of these nested dentin cones are what form the rings, making the age-determining process very much like counting the number of Solo cups in a stack by looking at the rims. If a researcher had the exceptional luck to find a pristine preserved mammoth tusk, they could in theory determine the exact amount of time the animal lived, almost to the day.
Model and explanation of tusk growth, from Rountrey, 2007.
This principle typically holds true for the tusks of other proboscideans as well. Tusks can provide remarkable opportunities for analyzing climate conditions in very high resolution, and for unraveling the specific diet and ecology of the creatures that grew them. Entire tusks are difficult to find, so age cannot always be determined—but the unique patterning is still useful for determining seasonal changes when sampling for isotopes, a feature put to use in one particular paper to provide evidence that climate change may not be the cause of Eurasian mammoth extinction (Fox, 2007).
Molars lack the life-long, precisely measurable growth patterns of tusks, but are still completely viable for isotope analyses, and offer up a tool that tusks do not. Microwear of the tooth enamel indicates traits of the animal’s diet. Italian researchers used this and isotope analysis to determine that Pleistocene elephant molars taken from two separate sites in Southern Italy were indicative of two markedly different climates, one a closed canopy forest and the other an arid grassland (Palombo, 2005). A high markedness of pitting and coarse, crossed scratches on molar samples correlated with a high-cellulose diet, like grass. The other sample had more fine scratches and less pitting, suggesting the elephant primarily browsed leaves. This was corroborated by carbon isotope levels in the enamel, and climate conditions indicated oxygen-18 levels. Strontium isotope levels further clarified the geographic areas the elephants spent most of their lives.
Example photomicrographs of enamel microwear taken at 35x. The scale bars are 0.4mm. From Rivals, 2011.
The enamel scars observed in microwear analysis are usually microscopic. Originally scanning electron microscopes were used, but new techniques have made analysis possible using light-microscopy (Rivals, 2011). Microwear techniques are applicable to fossils and subfossils far more varied then those of proboscideans. For example, microwear analysis is used to show that diversification of early Bovini coincides with global vegetation changes and increased consumption of grasses (Bibi, 2006), as well as the diets of the toothed ancestors of modern sloths and anteaters, and the ecology of prehistoric horse species (Heuser, 2010).
I haven’t even scratched the enamel on the amount of work being done with isotope and microwear analysis of subfossil teeth. The Rivals 2011 paper aims to determine and differentiate the ecology of three different extinct proboscideans, and gives very comprehensive background on the processes. The webpage I cite below is an overview of several papers relevant to the topic from the University of Bonn, and fortunately most of it is in english.
References
Bibi, Faysal (2006) Origin, Paleoecology, and Paleobiogeography of Early Bovini. Paleogeography, Paleoclimatology, Palaeoecology. 248: 60-72.
Fox, David L., Daniel C. Fisher, Sergey Vartanyan, Alexei N. Tikhonov, Dick Mol, Bernard Buigues (2007) Paleoclimatic Implications of Oxygen Isotopic Variation in Late Pleistocene and Holocene Tusks of Mammathus primigenius from Northern Eurasia. Quaternary International. 169-170: 154-165.
Heuser, Alexander. “Weitere Forschungsprojekte (Other Research Projects).” University of Bonn, 23/02/2010. Accessed 26/03/2014. <http://www.steinmann.uni-bonn.de/arbeitsgruppen/knochengeochemie/forschung-1/weitere_forschungsprojekte#section-5>
Palombo, M.R., M.L. Filippi, P. Iacumin, A. Longinelli, M. Barbieri, A. Maras (2005) Coupling Tooth Microwear and Stable Isotope Analyses for Palaeodiet Reconstruction: the Case Study of Late Middle Pleistocene Elephas (Palaeoloxodon) antiquus teeth from Central Italy (Rome Area). Quaternary International. 126-128: 153-170.
Rivals, Florent, Cina Semprebon, Adrian Lister (2011) An Examination of Dietary Diversity Patterns in Plestocene Proboscideans from Europe and North America as Revealed by Dental Microwear. Quaternary International. 255: 188-195.
Rountrey, Adam N., Daniel C. Fisher, Sergey Vartanyan, and David L. Fox (2007) Carbon and Nitrogen Isotope Analyses of a Juvenile Woolly Mammoth Tusk: Evidence of Weaning. Quaternary International. 169-170: 166-173.
Ecology by Proxy 