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Michael Wall, Ph.D.
Director of the Biodiversity Research Center of the Californias

Osborn, 1936. Reproduced with permission of the American Museum of Natural History.
Osborn, 1936. Reproduced with permission of the American Museum of Natural History.

Tell-tale Teeth

By Lynett Gillette

Paleontologists obsess over teeth for good reason. No other body part reveals as much information about an animal’s choice of food, its taxonomic place in the animal kingdom, its age, its social life, even such seemingly far-removed attributes as lifetime migration routes, or a choice of home range.

Traditionally, paleontologists note tooth size and shape, number, position in the jaw, and replacement patterns. Henry Fairfield Osborn knew the skull purchased by the American Museum of Natural History in 1922 from Rochester, Indiana, belonged to a young mastodon. Mastodon because of the unique cone-shaped cusps of the molars. Young because of the presence of unfused sutures in skull bones and in addition, small and unworn baby teeth in the front of the jaws. To the rear, the next larger molars waited to push forward. Had the young mastodon lived, six successive molars on each side of the upper and lower jaws would have totaled 24 teeth during a lifespan of around 60 years. Similar patterns are seen in mammoths and modern elephants.

In the San Diego Natural History Museum’s new exhibition Fossil Mysteries, a cast of Osborn’s skull atop a composite cast mastodon skeleton reminds us of the unrivaled size and power of those magnificent elephant relatives.

Worn molar of a mastodon, Mammut americanum, SDNHM 86541
Worn molar of a mastodon (Mammut americanum),
SDNHM 86541

Now a new generation of tooth studies—spearheaded by paleontologists Daniel Fisher at the University of Michigan’s Museum of Paleontology and David Fox at the University of Minnesota—is providing modern researchers nuanced data only dreamed of in Osborn’s day.

Slices of Life
Take tusks from extinct mastodons and mammoths. By making a cross-sectional cut in one of these unusual upper incisors, removing a thin sliver, and observing the angles between bands of dentin, a paleontologist can distinguish between the mammoth and its distant relative, the mastodon. This technique proves useful in identifying isolated tusk fragments.

Annual seasonal growth in mammoth and mastodon tusks, expressed in alternating light and dark bands, allows an independent estimate of its owner’s age. Unlike bone, which constantly realigns its internal structure according to current loads it bears, teeth don’t remodel over a lifetime. Each layer of new dentin remains as a permanent record.

Besides age, other cyclical patterns can be marked on the accreting bands in the tusks. Sheer thickness varies according to seasonal availability of food; a thin band may indicate stress. Unusual patterns of stress in females may indicate gestation and nursing of a calf. The number of births can be counted, the season of death pinpointed.

Fixed in the Chemistry
In lockstep with these cycles of tooth deposition, isotope studies on the same tusks reveal precise data regarding diet and environment. Different weight isotopes of carbon in teeth have made possible measurements of the amount of trees, shrubs, and herbs (plants with three-carbon photosynthesis) in animals’ diets compared to the amount of grasses (plants using four-carbon photosynthesis). Mammoths seem to have preferred the grasses whereas the mastodons utilized more mixed diets of shrubs, herbs, and trees.

Isotopes of strontium reflect the soils in an animal’s home range. Seasonal fluctuations in the strontium levels suggest migrations into areas with different bedrock, such as if the animal moved from a sandstone terrain to one of granite.

Oxygen isotopes found in teeth give details of local water intake, since most oxygen in body fluids comes from drinking water. Yearly fluctuations in this isotope suggest possible wet/dry climatic regimes. By comparing isotopes in teeth from mastodons living at different times in the same place, even climate change can be inferred.

New tooth discoveries
San Diego Natural History Museum paleontologists had the good fortune to find both mammoth and mastodon molars, partial tusks, and bones at the same site near Oceanside, California, in 2002.

Intriguing questions arose. Mastodon and mammoth’s very different tooth shapes suggest that their diets differed. Were these two large herbivores specializing on different food items in the same place? Or, did they occupy different habitats? If so, how did they happen to be buried in the same place? Did the mastodon make seasonal treks to other regions and the mammoth remain close to home? That’s a partitioning of the environment suggested by recent mastodon tooth studies in Florida.

Isotopes of carbon, oxygen, and strontium in the Oceanside fossil teeth may have the answer to some of these outstanding fossil mysteries. Osborn certainly would have been amazed at the sophistication of these new studies. In turn, we shouldn’t forget that in the future, ever more information may be extracted from our priceless fossil treasures.