Rare Fossils Give Clues to How Tardigrades Survived Mass Extinctions by Hitting the Snooze Button

Tardigrades are known for being some of the most resilient animals to exist, capable of surviving extreme temperatures, pressure, radiation and starvation—they can even withstand exposure to outer space. A new study of the fossils of early tardigrades is shedding light on their evolution, including when and why the eight-legged microscopic animals became so indestructible.

Only four known fossils of tardigrades have been found to date, and all of them are preserved in amber, or ancient tree resin. One of these fossils, a pebble-sized piece of amber discovered in Canada in the 1940s, contains two Cretaceous tardigrades from between 72 million and 83 million years ago. Scientists in 1963 identified one as a species called Beorn leggi, the first fossilized tardigrade ever discovered. The other remained a mystery for decades.

“Lots of tardigrade folks have pondered these fossils over the last 60 years, but there was a hard limit to how much could be gleaned because the tardigrades were really small and a bit obscured by the amber,” New Jersey Institute of Technology biologist Phil Barden, who was not involved in the study, tells Riley Black of National Geographic.

Now, researchers have used cutting-edge technology to re-examine the specimens. In a study published in Communications Biology earlier this month, scientists captured high-definition images of the preserved tardigrades using a technique called confocal fluorescence microscopy.

The results revealed previously unknown information about both specimens’ claws, “which are very important taxonomic characteristics in tardigrades,” lead author Marc Mapalo, a “tardigradologist” at Harvard University, tells Scientific American’s Mindy Weisberger. For scale, the animals’ claws are about one-tenth the width of a human hair.

Because tardigrade anatomy has stayed largely consistent for millions of years, the new images of the claws contained valuable information for placing the specimens on the tardigrade family tree, as Jasmine Nirody, an organismal biologist at the University of Chicago who was not involved in the research, says to Scientific American.

Mapalo’s team determined the second specimen represented a new species, naming it Aerobius dactylus. Their work also revealed that B. leggi and A. dactylus—both extinct today—belong to the same one of the two major tardigrade lineages. This knowledge—along with comparisons to two other fossils from New Jersey and living tardigrades—allowed the scientists to calculate when the two lineages of tardigrades diverged. It also provided clues to when the animals acquired what is perhaps their most powerful survival mechanism: cryptobiosis.

Cryptobiosis is the secret to tardigrades’ incredible resilience. In survival situations, these hardy creatures will expel the water from their bodies and suspend their metabolism almost completely. Tardigrades even produce a special protein that preserves their DNA as they hibernate for years—defying their natural lifespans of just a few months. In this so-called “tun” state, they lie dormant, aiming to outlast the unfavorable conditions of their environment.

Tardigrades did not always possess this ability, however. Based on their research, the authors propose that at least two separate tardigrade lineages evolved cryptobiosis independently, one between 175 and 430 million years ago and another between 175 and 382 million years ago.

While broad, these time spans are notable because they include a number of mass extinction events, including the Permian extinction, when Earth lost 96 percent of marine life and 70 percent of terrestrial life. The researchers suggest cryptobiosis “could be one of the factors that have helped [tardigrades] evade extinction,” enduring dramatic shifts in climate, drops in oxygen and other extraordinary environmental conditions that characterized those periods, according to the paper.

“Knowing when cryptobiosis evolved in tardigrades can help us contextualize how and why they gained this mechanism,” Mapalo tells National Geographic. He adds that cryptobiosis may have helped tardigrades withstand changes in salinity as they expanded from their marine origins to freshwater habitats.

A more extensive collection of tardigrade fossils would equip Mapalo and other scientists with the additional data they need to further explore the hypothesis that cryptobiosis evolved to help tardigrades escape extinction.

“Hopefully, by sharing this result, we will entice other people to be aware that fossil tardigrades exist and there are still more to be found,” Mapalo tells Scientific American.

Tara Wu | | READ MORE

Tara Wu is a freelance writer based in the northeastern U.S. and a former editorial intern with Smithsonian magazine. She covers science, society, culture and their intersection.