Telling Time with Teeth: Shark Dental Remains Reveal the Earth’s Past

By Dawn Mangine

If sharks had a tooth fairy, she would go broke. Unlike humans, who only have two sets of teeth, sharks grow and shed thousands of teeth over the course of their lifetimes. Although shark ancestors came into existence around 400 million years ago—200 million years before dinosaurs—not many shark fossils exist because their skeletons consist of cartilage, which rots away after sharks die. However, fossilized shark teeth are abundant, and can tell us a lot, not just about these amazing ocean predators, but about the geological history of land.

Recently, scientists discovered that fossil shark teeth can corroborate theories of mammalian evolution, according to an article in The Conversation titled “Fossil shark teeth are abundant and can date the past in a unique way.” Paleontologists (scientists who study fossils) at the Florida Museum of Natural History measured variations in ocean strontium to determine the age of shark teeth. They then used this information to more accurately date two Neogene-period fossil sites in Florida.

Fossils Fuel Science

Fossils give us insight into many facts about the Earth, helping us understand evolution, climate change, and geological events. Most fossils are discovered on land, but fossils can also be found in ocean beds and along coastlines. For paleontologists, getting the most accurate dates of fossils is vital, because they tell us about important times in geologic history.

What Shark Teeth Can Tell Us

Strontium is a chemical element found in rock, soil, and water. Strontium in the ocean primarily comes from two sources: land erosion that washes out to the sea and marine animals that produce and release strontium when they build their skeletons. While strontium dating of regular fossils has limitations—any chemical alteration to the fossil can throw off the dates—shark teeth are resistant to these changes because of their enamel-like surface.

The Neogene period, from 23 to 2.6 millon years ago, was a time of significant change in biodiversity related to changing climates. The paleontologists in Florida wanted to find out more about two important fossil sites in that state, Montbrook and Palmetto Fauna Bone Valley. Most scientists thought these sites were the same age, but by analyzing strontium levels in shark teeth from coastal areas near these sites, they have differentiated the age of the sites by 600,000 years.

This more accurate information gives new insights into the Great American Biotic Interchange, the migration of land mammals between North and South America across the Isthmus of Panama 4 to 5 million years ago. It can help scientists determine when these species migrated—not all at once, but over thousands of years.