MILLIONS OF YEARS AGO, SHARK TEETH LOST IN ANTARCTICA RECORDED THE EARTH'S CLIMATE HISTORY.

 Sand tiger sharks hunted in the waters off the Antarctic Peninsula tens of millions of years ago, gliding over a vibrant marine environment on the bottom below.

They're assisting in the investigation of why, 50 million years ago, the Earth began to move from a warmer "greenhouse" environment to cooler "icehouse" circumstances.

      Striatolamia macrota, a now-extinct sand tiger shark, was once a common sight in the waters off the Antarctic Peninsula, and it left behind superbly preserved fossil teeth.

Many of the climate change ideas centre on Antarctica. As Earth's tectonic plates moved, the Drake Passage, which runs between South America and the Antarctic Peninsula, and the Tasman Gateway, which runs between Australia and East Antarctica, widened and deepened, according to geologic data. For the major oceans' waters to come together and form the Antarctic Circumpolar Current, broader, deeper channels would have been required. Today, that current runs around Antarctica, trapping cold waters in the Southern Ocean and keeping Antarctica cold and frozen.

The now-extinct sand tiger shark species Striatolamia macrota was formerly a common sight in the waters off the Antarctic Peninsula, leaving superbly preserved fossil teeth on Seymour Island.

Oxygen is trapped in a set of razor-sharp teeth.

To catch prey, sand tiger sharks have sharp teeth that protrude from their jaw. Hundreds of teeth are arranged in many rows on a single shark. It sheds thousands of teeth throughout the course of a lifetime as new ones come in.

Important environmental data is encoded and preserved in the chemistry of each tooth over millions of years.

The outer layer of a shark's tooth, for example, is made up of enameloid hydroxyapatite, which is comparable to enamel in human teeth. It's made up of oxygen atoms from the sea where the shark resided. We can determine the temperature and salinity of the surrounding water during the shark's life by examining the oxygen.

The teeth from Seymour Island reveal that the Antarctic seas stayed warmer for longer than scientists had predicted, at least where the sharks lived.

The element neodymium, which adsorbs and substitutes other elements in the outer enameloid of the teeth during early fossilisation, provides another hint. Based on the age of its rocks, each ocean basin has a unique ratio of two separate neodymium isotopes. The shark teeth ratio can be used to determine the source of the water where the shark died.

If conditions are stable, the neodymium content would not vary. If the composition of neodymium in ancient teeth changes through time, it could imply changes in oceanography.

Warm water, big sharks

The size of some of the teeth indicated that these ancient Antarctic sand tigers were larger than today's sand tiger shark, Carcharias taurus, which may grow to be around 10 feet long.

Furthermore, the water temperatures in which the sharks resided were warmer than prior research using Antarctic clam shells had shown. It's possible that the discrepancy was due to the change in water depth between the surface and the sea floor, or that the sharks whose teeth we discovered spent part of their life in South America. Sand tiger sharks today are drawn to warm seas. Coastal Massachusetts and Delaware are where they spend the summer and early fall, but as the waters calm down, they travel to coastal North Carolina and Florida. There are certain teeth within the jaw that reflect a different habitat than where a shark is living since their teeth continuously grow and migrate ahead almost like a conveyor belt. It's probable that ancient sand tiger sharks moved as well.

The sharks' teeth suggested that the water temperature back then was similar to what current sand tiger sharks encounter now. Carbon dioxide levels were also three to six times higher than they are now, implying that scientists can predict increased temperatures in the affected areas.

Finally, the neodymium found in the teeth of fossil sand tiger sharks gives the earliest chemical evidence of water flowing through the Drake Passage, which is consistent with tectonic findings. The early opening of the Drake Passage, but the delayed cooling effect, illustrates that climate change is influenced by intricate interactions across Earth's systems.

What about their cousins in the north?

Sand tiger sharks were discovered all over the planet throughout the Eocene, indicating that they were able to live in a variety of settings. They lived in brackish waters that were less salty than the open ocean in the Arctic Ocean, for example, 53 million to 38 million years ago, and were much smaller than their southern cousins off Antarctica.

During this time, differences in the salinity of the tiger sharks' habitat and the sharks' size appear in the Gulf of Mexico. As the earth warms again, the modern sand tiger sharks' wide spectrum of environmental tolerance bodes well for their survival. Unfortunately, today's rate of warming is faster, and the sand tiger shark's ability to adapt may be limited.