By Sora Kim for The Dialog
Tens of million years in the past, sand tiger sharks hunted within the waters off the Antarctic Peninsula, gliding over a thriving marine ecosystem on the seafloor under.
All that is still of them at the moment is their sharp pointed enamel, however these enamel inform a narrative.
They’re serving to remedy the thriller of why the Earth, some 50 million years in the past, started shifting from a “greenhouse” local weather that was hotter than at the moment towards cooler “icehouse” situations.
Many theories about this local weather shift deal with Antarctica. There’s geologic proof that each the Drake Passage, which is the water between South America and the Antarctic Peninsula, and the Tasman Gateway, between Australia and East Antarctica, widened and deepened throughout this time as Earth’s tectonic plates moved.
The broader, deeper passages would have been vital for the waters of the key oceans to return collectively and the Antarctic Circumpolar Present to type. That present, which flows round Antarctica at the moment, traps chilly waters within the Southern Ocean, conserving Antarctica chilly and frozen.
The now-extinct sand tiger shark species Striatolamia macrota was as soon as a continuing within the waters across the Antarctic Peninsula, and it left exquisitely preserved fossil enamel on what’s now Seymour Island close to the tip of the peninsula.
By finding out the chemistry preserved in these shark enamel, my colleagues and I discovered proof of when the Drake Passage opened, which allowed the waters of the Pacific and Atlantic oceans to combine, and what the water felt like on the time. The temperatures recorded in shark enamel are among the warmest for Antarctic waters and confirm local weather simulations with excessive atmospheric carbon dioxide concentrations.
Oxygen captured in very sharp enamel
Sand tiger sharks have sharp enamel that protrude from their jaw to know prey. A single shark has a whole lot of enamel in a number of rows. Over a lifetime, it sheds 1000’s of enamel as new ones develop.
Necessary environmental data is encoded inside the chemistry of every tooth and preserved there over tens of millions of years.
For instance, the outer layer of a shark’s tooth consists of an enameloid hydroxyapatite, much like enamel in human enamel. It accommodates oxygen atoms from the water the shark lived in. By analyzing the oxygen, we are able to decide the temperature and salinity of the encompassing water throughout the shark’s life.
The enamel from Seymour Island present that the Antarctic waters – at the least the place the sharks lived – stayed hotter longer than scientists had estimated.
One other clue comes from the component neodymium, which adsorbs and replaces different parts within the outer enameloid of the tooth throughout early fossilization. Every ocean basin has a definite ratio of two completely different neodymium isotopes based mostly on the age of its rocks. Wanting on the ratio within the shark enamel permits us to detect the sources of the water the place the shark died.
If situations are steady, the neodymium composition wouldn’t change. Nonetheless, if neodymium composition does change in fossil enamel over time, that signifies modifications in oceanography.
Huge sharks, heat water
We studied 400 enamel from Seymour Island, from all ages of shark, juvenile to grownup, from people dwelling between 45 million to 37 million years in the past. The mixture of tooth dimension and chemistry yielded some stunning clues to the previous.
Among the enamel had been extraordinarily massive, suggesting these historical Antarctic sand tigers had been bigger than at the moment’s sand tiger shark, Carcharias taurus, which might develop to about 10 ft lengthy.
As well as, water temperatures the sharks lived in had been hotter than earlier research involving Antarctic clam shells urged. It’s attainable the distinction was between waters nearer to the floor and deeper on the ocean ground, or the sharks whose enamel we discovered could have spent a part of their lives in South America.
In the present day’s sand tiger sharks observe heat waters. They spend summer season and early fall between coastal Massachusetts and Delaware, however when waters cool off, they migrate to coastal North Carolina and Florida. As a result of their enamel repeatedly type and transfer ahead nearly like a conveyor belt, there are some enamel inside the jaw that characterize a unique habitat than the place a shark resides. It’s attainable that the traditional sand tiger sharks additionally migrated, and when Antarctic waters cooled off, they headed north to hotter waters at decrease latitudes.
The enamel urged that the sharks’ water temperature then was much like the water temperatures the place trendy sand tiger sharks may be discovered at the moment. Carbon dioxide concentrations had been additionally three to 6 instances larger than at the moment, so scientists would count on amplified temperatures within the areas.
Lastly, the neodymium within the fossil sand tiger shark enamel gives the earliest chemical proof of water flowing via the Drake Passage that aligns with tectonic proof. The early timing of the Drake Passage opening, however the delayed cooling impact, signifies there are advanced interactions between Earth’s methods that have an effect on local weather change.
What about their northern cousins?
Sand tiger sharks had been discovered around the globe throughout the Eocene, suggesting they survived in a variety of environments. Within the Arctic Ocean, for instance, they lived in brackish waters which can be much less salty than the open ocean 53 million to 38 million years in the past and had been a lot smaller than their southern cousins off Antarctica.
Variations within the saltiness of the tiger sharks’ habitat and dimension of the sharks additionally present up within the Gulf of Mexico throughout this time. That vary of environmental tolerance bodes properly for the fashionable sand tiger sharks’ survival because the planet warms as soon as once more. Sadly, the tempo of warming at the moment is quicker and could also be past the sand tiger shark’s potential to adapt.
(The writer is an Assistant Professor on the College of California)