Tardigrades are survivors. For more than 500 million years, the microscopic “water bears” have spread all over the planet and endured some of the harshest conditions Earth has to offer. Now a new analysis of ancient tardigrades in a piece of Cretaceous amber has not only clarified the timeline of tardigrade evolution, but hints how the tiny animals have been able to survive disasters that drove other forms of life to extinction.
The tiny critters were trapped in tree sap in prehistoric Canada between 83 and 72 million years ago, when giant tyrannosaurs and horned dinosaurs roamed the same conifer forests. One of the tardigrades is a species paleontologists have seen before. Named Beorn leggi, the tardigrade was the first fossil species ever discovered by paleontologists. But Harvard University paleontologist Marc Mapalo and his colleagues also found a second, never-before-seen species, Aerobius dactylus. The researchers named the new species and used it and the handful of other ancient species known to science to analyze the evolutionary history of tardigrades in Communications Biology earlier this month.
Preserved in fossilized tree sap, the ancient tardigrade Beorn leggi was the first discovered by scientists.
Photograph By Marc Mapalo
Fossilized within the ancient tree resin that forms today’s amber, the two tardigrades had been waiting decades for a good look. Paleontologists could barely make out the B. leggi fossil in the Canadian specimen when they first described it 1964. Now, thanks to enhanced imaging technology, Mapalo and colleagues were able to get a much more detailed look.
“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,” says New Jersey Institute of Technology biologist Phil Barden, who was not involved in the new study. The animals are so small, he notes, that the tiny claws on their feet are about one tenth the width of a human hair.
Only amber can preserve tardigrades in such minute detail. The rarity of tardigrade fossils, however, is not just attributable to their tiny size. Just a few paleontologists study fossil tardigrades, Mapalo says, noting that some colleagues react with surprise that any fossil tardigrades are known at all. Modern imaging techniques can help experts to squeeze new information out of previously collected amber samples.
A closer look
Mapalo and his coauthors turned to a technique called confocal fluorescence microscopy to create high-resolution images of the tiny creatures. The experts found that the two fossil tardigrade species in the amber sample aren’t alive today, but both belong to tardigrade families that are still around. By comparing the Canadian fossils and two others found in New Jersey to molecular data from living species, Mapalo and his colleagues were able to estimate when tardigrades evolved and when they gained one of their most remarkable abilities.
Many tardigrades are capable of cryptobiosis, a temporary and almost complete slowdown of their bodies’ processes. In this state of suspended animation, the creatures shed their water and curl into balls. Along with carrying a protein that protects their DNA from damage, being able to shut down and wait for better conditions helped tardigrades to survive in extreme environments, even the vacuum of space, and could help them withstand a future apocalypse.
Mapalo and colleagues propose that at least two major tardigrade groups evolved their cryptobiotic abilities independently, one gaining cryptobiosis between 430 and 175 million years ago and another doing so between 382 and 175 million years ago. More fossils could help refine the exact timing, but the researchers note that this span of prehistoric time is significant because it includes several mass extinctions. Tardigrades that were able to go into a form of stasis until conditions recovered would have been better able to survive the oxygen drops, climate shifts, and other pressures associated with these global disasters.
“Knowing when cryptobiosis evolved in tardigrades can help us contextualize how and why they gained this mechanism,” Mapalo says. Tardigrades likely evolved in the seas before spreading onto land, he notes. Cryptobiotic abilities would have helped tardigrades survive changes in salt levels when they moved from the marine realm to habitats full of mosses and lichens that relied on freshwater.
How exactly cryptobiosis played into the survival and evolutionary history of water bears will need more research to confirm. “It’s fun to imagine cryptobiosis as a kind of magic trick that tardigrades used to evade extinction,” Barden says, but notes that other invertebrate groups were able to survive the disaster without the ability.
More fossil tardigrades will help test the hypothesis that cryptobiosis helped tardigrades survive mass extinctions, and new species are almost certainly awaiting discovery. “Whenever I get the chance, I always tell people who work with amber to check if their samples have tardigrades in it,” Mapalo says. Barden agrees. “There are almost certainly other fossilized tardigrades awaiting discovery in museum collections,” he says, “probably entombed in amber next to a comparatively gargantuan fly or beetle.”
