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Tens of millions of years ago, tiger sharks hunted in the waters off the Antarctic Peninsula, gliding above a thriving marine ecosystem on the seabed below.
All that’s left of them today are their sharp teeth, but those teeth tell a story.
They help solve the mystery of why the Earth, about 50 million years ago, began to go from a “greenhouse” climate that was warmer than it is today to conditions of ” cooler ”cooler.
Many theories on this climate change focus on Antarctica. There is geological evidence that the Drake Passage, which is the water between South America and the Antarctic Peninsula, and the Tasman Gateway, between Australia and East Antarctica, have widened and deepened during this time as the Earth’s tectonic plates shifted.
(Copernicus Marine Services / European Union)
Above: A map of ocean surface temperature as measured by satellites shows the Antarctic Circumpolar Current, marked by dark lines. The Antarctic sea ice appears in light blue.
The wider and deeper passages would have been necessary for the waters of the major oceans to come together and for the Antarctic Circumpolar Current to form. This current, which circulates around Antarctica today, traps the cold waters of the Southern Ocean, keeping Antarctica cold and frozen.
The now extinct species of sand tiger shark Striatolamia macrota was once a constant in the waters around the Antarctic Peninsula, and it left perfectly preserved fossil teeth on what is now Seymour Island, near the tip of the peninsula.
By studying the chemistry preserved in these shark teeth, my colleagues and I found evidence of the opening of the Drake Passage, which allowed the waters of the Pacific and Atlantic Oceans to mix, and the feel of water. at the time.
The temperatures recorded in shark teeth are among the warmest in Antarctic waters and verify climate simulations with high concentrations of carbon dioxide in the atmosphere.
Oxygen captured in very sharp teeth
Sand Tiger Sharks have sharp teeth that protrude from their jaws to grab their prey. A single shark has hundreds of teeth in several rows. Over the course of his life, he loses thousands of teeth as new ones grow.
Important environmental information is encoded in the chemistry of every tooth and stored there for millions of years.
Illustrations of sand tiger shark teeth used by scientists. (Christina Spence Morgan)
For example, the outer layer of a shark tooth is made up of an enameloid hydroxyapatite, similar to the enamel of human teeth. It contains oxygen atoms from the water in which the shark lived. By analyzing oxygen, we can determine the temperature and salinity of the surrounding water during the life of the shark.
Teeth from Seymour Island show that Antarctic waters – at least where the sharks lived – stayed warmer for longer than scientists had estimated.
Another clue comes from the element neodymium, which adsorbs and replaces other elements in the outer emeloid of the tooth at the onset of fossilization.
Each ocean basin has a separate ratio of two different neodymium isotopes depending on the age of its rocks. Looking at the report in the shark’s teeth allows us to detect the water sources where the shark died.
If conditions are stable, the composition of neodymium would not change. However, if the composition of neodymium changes in fossil teeth over time, this indicates changes in oceanography.
Big sharks, warm water
We studied 400 teeth from Seymour Island, of all shark ages, juvenile to adult, from individuals living between 45 and 37 million years old. The combination of tooth size and chemistry has given surprising clues to 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 to about 10 feet long.
(Jlencion / Wikimedia Commons / CC BY-SA)
Above: A modern sand tiger shark, also known as the gray nurse shark, shows its rows of teeth.
Additionally, the water temperatures in which the sharks lived were higher than suggested by previous studies involving Antarctic clam shells. It’s possible that the difference is between waters closer to the surface and deeper at the bottom of the sea, or the sharks whose teeth we found may have spent part of their lives in South America.
Today’s sand tiger sharks stalk the warm waters. They spend the summer and early fall between the coasts of Massachusetts and Delaware, but when the waters cool, they migrate to the coasts of North Carolina and Florida. Because their teeth continuously form and advance almost like a treadmill, some teeth in the jaw represent a different habitat than where a shark lives.
It is possible that the ancient sand tiger sharks also migrated, and when the waters of Antarctica cooled, they moved north to warmer waters at lower latitudes.
The teeth suggested that the sharks’ water temperature then was similar to the water temperature where modern sand tiger sharks can be found today. Carbon dioxide concentrations were also three to six times higher than today, so scientists would expect temperatures to amplify in the regions.
Finally, the neodymium present in the fossil teeth of the sand tiger shark provides the first chemical evidence of water flow in the Drake Passage that matches the tectonic evidence. The early timing of the opening of the Drake Passage, but the delayed cooling effect, indicates that there are complex interactions between Earth systems that affect climate change.
What about their northern cousins?
Sand tiger sharks have been found all over the world during the Eocene, suggesting they survived in a wide range of environments. In the Arctic Ocean, for example, they lived in brackish waters less salty than the open ocean 53 to 38 million years ago and were much smaller than their southern cousins off Antarctica.
Differences in the salinity of tiger shark habitat and shark sizes also appear in the Gulf of Mexico during this time. This range of environmental tolerance bodes well for the survival of modern sand tiger sharks as the planet warms again.
Unfortunately, the rate of warming is now faster and may exceed the adaptive capacity of the sand tiger shark.
Sora Kim, assistant professor of paleoecology, University of California, Merced.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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