Tardigrades Trapped In Amber Reveal Origins

They can survive deadly radiation, arctic temperatures, the vacuum of space, and even bullet impacts. You can find them anywhere there’s water—in the ocean, freshwater, and plants and soil. But the evolutionary roots of these microscopic, mostly aquatic creatures called tardigrades have been mysterious because of their sparse fossil record.

Now, a reanalysis of two tardigrade fossils embedded in the same lentil-size chunk of Cretaceous amber has illuminated the tiny creatures’ family tree. The findings, which researchers report this week in Communications Biology, also suggest that by 180 million years ago, the animals had evolved their superpower: the ability to enter a state of suspended animation impervious to outside conditions.

Tardigrade fossils “are very, very few, and very rare,” says zoologist Roberto Guidetti of the University of Modena and Reggio Emilia, who was not involved with the study. In fact, there have only been three widely accepted fossils in total, all trapped in amber. Guidetti says the new results on two of them reveal how little the animals have changed over millions of years.

Tardigrades, colloquially known as “water bears,” are a diverse group of eight-legged invertebrates that can be up to half a millimeter in length. “They look like tiny sausages with legs,” says Javier Ortega Hernández, a paleobiologist at Harvard University’s Museum of Comparative Zoology. Tardigrades have captivated the popular imagination in part because they can nearly halt their metabolism for long periods—a feat known as cryptobiosis—to endure hellish conditions.

Although researchers have studied living water bears for decades, in the 1960s, biologist Kenneth Cooper used a light microscope to describe and name the first-ever fossilized tardigrade, which was embedded in an amber pebble from Canada dating to about 80 million years ago during the Cretaceous period. Cooper named the critter Beorn leggi, after a fantasy character invented by J. R. R. Tolkien that could assume a bearlike form. Cooper concluded B. leggi belonged to the eutardigrades—a branch of tardigrades that mostly lives in freshwater and terrestrial environments—but couldn’t see the fossil clearly enough to describe it in detail. The same piece of amber held a second tardigrade that was too “curled on itself and shriveled” for Cooper to describe; it remained unnamed. Until now.

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Ortega Hernández and graduate student Marc Mapalo reexamined the amber using laser confocal microscopy, a method that allows researchers to study the 3D structure of tiny structures in stunning detail using a narrow beam of light at a specific wavelength. When hit by a laser, the chitin that makes up the protective cuticle of tardigrades fluoresces naturally. By measuring the fluorescence the researchers reconstructed 3D images of the tardigrades, including details of their fishhooklike claws, which scientists use to differentiate species.

The images have by far the greatest resolution ever obtained for tardigrade fossils, Ortega Hernández says. The images of B. leggi—which measures 310 microns, or about double the width of a human hair—show seven well-preserved claws. The claws that curve toward the body are smaller than those curving away from it, a pattern found in the modern-day Hypsibiidae family of tardigrades. That confirms its place in the eutardigrade branch, the team concludes.

The second, previously unidentified specimen, which measured about 100 microns, had claws of similar length on each of its first three pairs of legs, but longer outer claws on its fourth set of legs. This pattern is also characteristic of the modern Hypsibioidea superfamily. The team named it Aerobius dactylus, from “aero” meaning relating to air, because the fossil appears to be floating on air in the amber, and “dactylo,” or finger, after its long claw.

“[The amber piece] is a tiny little thing and has two species—half the tardigrade fossil record,” Ortega Hernández says. It’s a “two-for-one, like Black Friday for tardigrades.” That suggests tardigrades were already diverse 80 million years ago, he says, adding that “these animals were definitely hanging out with dinosaurs.”

Even though the two species belong to the same family within eutardigrades, their age helps scientists readjust the timing of when various tardigrade groups evolved key traits and diverged over time. For example, it suggests the split between the terrestrial eutardigrades and the more diverse and marine heterotardigrades, the other major group, happened later than previously thought, about 500 million years ago.

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Researchers had previously suggested cryptobiosis evolved independently in the two tardigrade lineages, but couldn’t figure out when this happened. With the recalibrated family tree, Ortega Hernández and his team estimate, for the first time, that water bears’ superpowers had certainly evolved by 180 million years ago and may have appeared as early as 420 million years ago. They add that these powers may have helped tardigrades survive several waves of extinction.

Guidetti says the revised evolutionary timelines are “quite substantial, and sound quite reliable.” He hopes researchers scour amber for more specimens, which could offer answers to other questions such as why tardigrades gradually shrank in size over time.

Ortega Hernández agrees. “We have to be very diligent in looking at the right places,” he says. “They’re really small and cute, [but] … inconsequential as they may seem, they are one of the great stories of animal survival in the planet.”

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