It comes as no surprise that microscopic tardigrades, otherwise known as “water bears,” have fascinated scientists for centuries due to their near indestructibility. A new look at ancient fossils encased in amber reveals a new narrative of how these resilient creatures might have outlived some of the most catastrophic events on Earth.
“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 Phil Barden, a biologist at New Jersey Institute of Technology who wasn’t involved in the new study. For example, the tiny claws on their feet are only about one-tenth the width of a human hair, so a detailed study is not easy.
While amber is unusually good at preserving tardigrades in exquisite detail, the rarity of such fossils is not due solely to their minuscule size. Few paleontologists specialize in fossil tardigrades, and many are shocked to learn that no such fossils exist at all. But modern imaging techniques are now allowing scientists to extract new information from amber samples collected years ago.
A graduate student from the Museum of Comparative Zoology at Harvard University, Marc Mapalo and his team, established high-resolution images using confocal fluorescence microscopy. Their results showed that these two fossil tardigrade species inside this piece of amber no longer exist today but do indeed represent families that have persisted until now. They did this by comparing the fossils’ age with molecular data from living species, thereby estimating when tardigrades evolved and when they evolved one of their most remarkable abilities: cryptobiosis.
Cryptobiosis is a state of suspended animation in which tardigrades expel most of the water from their bodies and ball up, thereby shutting off metabolic processes. It means that, due to this feature and a covering protein that shields DNA from damage, tardigrades can survive extreme conditions from the vacuum of space, which may also help them in surviving apocalyptic future events.
Mapalo and his colleagues have suggested that at least two major tardigrade groups evolved cryptobiosis independently, one between 430 and 175 million years ago, the other between 382 and 175 million years ago. These are dates that include several mass extinctions. A tardigrade that could just go into stasis, waiting for things to get better, would have had a survival advantage during these global disasters.
The researchers also analyzed two tardigrade fossils from Canada, which dated from 84 to 71 million years ago, found in the 1940s. Back in 1963, one was described as Beorn leggi, while a second was too small to identify. High-contrast microscopy exposed details that were not seen before in the specimens’ claws, critical for taxonomic classification.
The smaller tardigrade represented a new genus and species, Aerobius dactylus, while B. leggie’s description and classification were amended. Both were classified in the same superfamily, Hypsibioidea, with B. leggi being moved into the family Hypsibiidae. This reclassification let the researchers estimate when the two major tardigrade lines diverged, thus suggesting that cryptobiosis first arose during the Carboniferous period, long before the Permian mass extinction, sometimes called the “Great Dying”.
It makes the evolutionary study of cryptobiosis difficult due to the rarity of tardigrade fossils. However, he hopes that his findings will be able to open doors for other people to accept that there are fossil tardigrades and allow them to accommodate further discoveries. “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,” he said.
This study on ancient tardigrade fossils not only gives us a view of the past but also sheds light on the great tenacity of these little animals. Their resistance to extreme conditions by cryptobiosis could be one of the reasons why they can bear the most devastating mass extinctions on Earth.