Thursday, October 11, 2007

Ecological adaptations and tracking climatic changes through time using dental wear analysis

Two studies recent published conduct us to two important conclusions:

- Dental wear reveals to be a better tool to investigate adaptations than tooth morphology, especially for the analysis of samples from Pleistocene localities. Dental wear analysis reveals changes that dental morphology does not detect.

- These studies revealed that fossils are of value in developing an understanding of the dietary breadth and ecological versatility of species that, in recent times, are rare, endangered, and occupy only a small remnant of their former ranges.

Preparation of high-resolution molds in silicone for dental wear analysis

Recreating paleodiets from fossils has often included considerable speculation. Even rigorous studies can sometimes misjudge diet due to phylogenetic constraints in the morphology. In spite of the difficulties, diet aids in interpretation of the habitat and ecological interactions. The indirect evidence from herbivores is sometimes the only indication of plant types in the area, because the fluctuation of herbivore abundance yields insight to the climatic transitions of an area.
The diet of fossil ungulates provides valuable information about the food resources in a given habitat and thus is a useful tool in reconstructing paleohabitats. The dietary interpretation of mammalian teeth has traditionally involved either direct (actualistic) comparison with living animals, the application of general functional principles, or - increasingly during the latest few decades - the study of the wear patterns left on teeth by food.

Using dental wear analysis (mesowear and light stereomicroscopy microwear), various species are used to reconstruct ecological adaptations and to track climatic changes through time.

Groups with a long evolutive history are interesting to analyze changes through time. The project realized on the Antilocapridae (a group endemic to North America) result from a collaboration with Gina M. Semprebon (Professor of Biology and Chair of the Science and Mathematics Department at Bay Path College, USA). All specimens available in the Frick Collection from the American Museum of Natural History (New York) were sampled. Species analyzed spanned from the early Miocene to the late Pleistocene. Results are concordant with well known trends toward increasing aridity and shifts in vegetational structure in the late Miocene–early Pliocene of North America.

Results indicate a shift toward more abrasive diets beginning in the late Miocene and Pliocene, and then a return to a less abrasive dietary regime for the duration of the Pleistocene and into the Recent. More interesting is that the antilocaprines (the more derived group, more hypsodont) apparently depended more on grass than the less advanced merycodontines, but even the earliest of the merycodontines seem to have relied more on grass as a dietary staple than the modern pronghorn. Seasonal grit encroachment on food items encountered by fossil antilocaprids coupled with a heavier reliance on grasses may provide a possible explanation for the extreme hypsodonty present in the modern pronghorn despite its mainly browsing dietary behavior.

From: Semprebon G. M., Rivals F. 2007. Was grass more prevalent in the pronghorn past? An assessment of the dietary adaptations of Miocene to recent Antilocapridae (Mammalia: Artiodactyla). Palaeogeography, Palaeoclimatology, Palaeoecology 253: 332-347.

Populations of caribou

A second study was undertaken on a large sample of modern caribou (Rangifer tarandus) from the Kaminuriak population of eastern Canada. The project is part of a collaboration with Nikos Solounias (Professor at New York College of Osteopathic Medicine and Research Associate at the American Museum of Natural History in New York). The modern sample was compared to fossil specimens from Pleistocene localities in North America (Alaska) and Western Europe (Caune de l’Arago in France and Salzgitter in Germany).

Microwear features on a bovid tooth (magnification x35)

The results show that the extant samples from eastern Canada have seasonal variation in microwear and presumably in diet. Those differences in microwear may reflect a cyclic migration of the population within a year. Rangifer samples from the three fossil localities show that diet of a brachyodont taxon can be highly abrasive, comparable to the diet of modern zebras or plains bison. More over their diet may vary across most of the dietary morphospace of ungulates, indicating an important adaptability of the individuals.

The three Pleistocene samples exhibit microwear that is different from the extant population in question. This observation implies that the recent diet of Rangifer has changed from the typical caribou diet in the past. This indicates dietary change within a species. This is important because it represents dietary evolution without changes in tooth morphology. These results are significant due to climatic deterioration and can be used to detect changes in time and trends in the future of some species as humans alter their habitats.

From: Rivals F., Solounias N. 2007. Differences in tooth microwear of populations of caribou (Rangifer tarandus, Ruminantia, Mammalia) and implications to ecology, migration, glaciations and dental evolution. Journal of Mammalian Evolution 14: 182-192.

For further information: Florent Rivals,