Sharks are often called "living fossils," and for good reason: The first sharks appeared in the fossil record roughly 450 million years ago and have lived through all five mass extinctions, including the one that wiped out the nonavian dinosaurs. In addition to being long-lived as a group, individual sharks have long life spans. So just how long can sharks live, and what's the oldest shark on record?
On the lower end of the longevity scale, the great hammerhead shark (Sphyrna mokarran) lives about 44 years, although one individual caught by a fisher in Florida was estimated to be as old as 50. The great white shark (Carcharodon carcharias) can live up to 70 years, according to a 2014 study in the journal PLOS One.
But of all the shark species, one stands out for its mind-boggling longevity.
Related: The longest-living animals on Earth
"The Greenland shark is the longest-lived shark species by far," Brynn Devine, a marine biologist at the University of Windsor in Ontario and an expert in Greenland shark conservation, told Live Science.
Greenland sharks (Somniosus microcephalus) scour the ocean floor in the North Atlantic and Arctic. These huge sharks, some the size of great whites, are truly remarkable.
"Greenland sharks are not just the oldest sharks," Devine said, "but they are possibly the oldest animals with backbones, which is crazy to think about."
In a 2016 study in the journal Science, researchers determined that the average age of a group of 28 Greenland sharks in their sample was 272 years old. The oldest in the group was estimated to be 392 years old, plus or minus about 120 years. That led to a widely held — but now debunked — misconception that the oldest shark was 512 years old.
To date an ancient shark
Typical aging methods didn't work for that 2016 study. "Aging is typically done by counting growth rings in calcified structures, like vertebrae or spines," which are laid down seasonally, like tree rings, Devine said. But those methods can be inaccurate if the shark doesn't grow at predictable rates throughout its life, so researchers usually check dating from this method against other evidence. Age estimates can be verified with mark-recapture studies, which involve tracking a population of animals over time.
"For Greenland sharks, conventional dating methods simply don't work," Devine said. Most deep-sea-shark skeletons don't calcify enough to produce usable growth rings, and these sharks lack the spines used for dating in other sharks. Even if scientists could determine ages from growth rings or spines, manually verifying the sharks' ages with annual mark-recapture studies is out of the question, given that Greenland sharks in any study will easily outlive a researcher's great-grandchildren.
Instead, researchers relied on radiocarbon dating to estimate the sharks' ages. For this, the researchers sampled tissue from the eye lenses, which form at birth, and determined each animal's age based on the ratios of carbon isotopes, or versions of the element, in their eyes. Because radiocarbon dating is rather imprecise, and the exact physics of how carbon behaves in a Greenland shark's body are not known, there is a large margin of error in the final estimates.
"Unfortunately, there aren't a lot of alternative techniques, so this is a major challenge in aging deep-sea sharks," Devine said.
Because researchers know so little about the Greenland shark's metabolism, it is unclear what enables them to live so long. One thing we do know, according to Devine, is that female Greenland sharks might not reach sexual maturity until 134 years old. To put that into perspective, the person widely considered to be the oldest human to have ever lived, Jeanne Calment, died at the age of 122, having outlived her own grandchildren. A Greenland shark born on the same day as Calment would still have been awaiting shark puberty when she died.
"They are truly remarkable deep-sea animals," Devine said.
Originally published on Live Science.