(THE CONVERSATION) Tens of millions of years ago, sand tiger sharks hunted in the waters off the Antarctic Peninsula and glided across a thriving marine ecosystem on the ocean floor below.
All that remains of them today are their sharp, pointy teeth, but these teeth tell a story. They help solve the mystery of why the earth began moving from a warmer “greenhouse” climate to cooler “ice house” conditions about 50 million years ago.
Many theories about this climate change focus on Antarctica. There is geological evidence that both the Drake Passage, which is the water between South America and the Antarctic Peninsula, and the Tasman Gateway between Australia and East Antarctica, expanded and deepened during this time as the Earth’s tectonic plates moved . The wider, deeper passages would have been necessary so that the waters of the great oceans could come together and the Antarctic Circumpolar Current could form. This current, which now flows around Antarctica, traps cold water in the Southern Ocean and keeps Antarctica cold and frozen.
The now extinct sand tiger shark species Striatolamia macrota was once a constant in the waters around the Antarctic Peninsula and left exquisitely preserved fossil teeth on what is now Seymour Island near the tip of the peninsula.
By studying the chemistry in these shark teeth, my colleagues and I found clues as to when the Drake Passage opened, which mixed the Pacific and Atlantic waters, and what the water felt like at that time. The temperatures recorded in shark teeth are among the warmest for Antarctic waters and confirm climate simulations with high atmospheric carbon dioxide concentrations.This story is part of Oceans 21 Our series on the global ocean began with five depth profiles. Be on the lookout for new articles on the state of our oceans in the run-up to the next UN climate change conference COP26. The series is brought to you by The Conversation’s international network.
Oxygen trapped in very sharp teeth
Sand tiger sharks have sharp teeth that protrude from their jaws to grab prey. A single shark has hundreds of teeth in multiple rows. Over the course of life, it loses thousands of teeth when new ones grow back.
Important environmental information is encoded in the chemistry of every tooth and has been preserved there for millions of years.
For example, the outer layer of a shark’s tooth is made of an enamel-melting hydroxyapatite, similar to the enamel of human teeth. It contains oxygen atoms from the water the shark lived in. By analyzing the oxygen, we can determine the temperature and salinity of the surrounding water during the shark’s life.
Seymour Island’s teeth show that Antarctic waters – at least where the sharks lived – stayed warmer longer than scientists had estimated.
Another clue is the element neodymium, which absorbs and replaces other elements in the external tooth enamel during early fossilization. Each ocean basin has a certain ratio of two different neodymium isotopes based on the age of its rocks. If we look at the ratio in the shark’s teeth, we can see the water sources where the shark died.
Under stable conditions, the neodymium composition would not change. However, when the neodymium composition in fossil teeth changes over time, it suggests changes in oceanography.
Big sharks, warm water
We examined 400 teeth from Seymour Island, from sharks of all ages, from young to adults, from individuals who lived 45 million to 37 million years ago. The combination of tooth size and chemistry provided surprising clues about the past.
Some of the teeth were extremely large, suggesting that these ancient Antarctic sand tigers were larger than today’s sand tiger shark, Carcharias taurus, which can grow up to 10 feet (3 meters) long.
Additionally, the water temperatures the sharks lived in were warmer than previous studies of Antarctic clamshells suggested. It is possible that the difference was between waters closer to the surface and deeper on the ocean floor, or the sharks whose teeth we found may have lived part of their lives in South America. Today’s sand tiger sharks chase warm water. They spend summer and early fall between the coasts of Massachusetts and Delaware, but when the waters cool, they move to the coasts of North Carolina and Florida. Since their teeth are constantly forming and moving forward almost like an assembly line, there are some teeth in the jaw that represent a different habitat than where a shark lives. It is possible that the ancient sand tiger sharks migrated as well, and as the Antarctic waters cooled they moved north to warmer waters at lower latitudes.
The teeth indicated that the water temperature of the sharks at that time was similar to the water temperatures at which modern sand tiger sharks can be found today. The carbon dioxide concentrations were also three to six times higher than today, so that scientists in the regions would expect increased temperatures.
Finally, the neodymium in the fossil sand tiger shark teeth provides the earliest chemical evidence of water flowing through the Drake Passage, consistent with tectonic evidence. The early opening of the Drake Passage, but the delayed cooling effect, suggests that there are complex interactions between Earth’s systems that affect climate change.
What about her northern cousins?
Sand tiger sharks were found around the world during the Eocene, suggesting that they survived in a wide variety of environments. In the Arctic Ocean, for example, they lived in brackish water, which is less salty than the open sea 53 to 38 million years ago, and were much smaller than their southern Antarctic cousins.
Differences in the salinity of the tiger shark habitat and the size of the sharks can also be seen in the Gulf of Mexico during this period. This range of environmental sustainability bodes well for the survival of modern sand tiger sharks as the planet warms up again. Unfortunately, the warming is faster today and may be beyond the adaptability of the sand tiger shark.