Harvard Study on Fossil Tardigrades reveals new species and evolutionary milestones

Discovery of Planet’s Most Resilient Life Form: They may be tiny, but tardigrades, or “water bears,” have a larger-than-life reputation. These microscopic, eight-legged creatures are virtually indestructible, surviving extreme conditions that would kill most other forms of life — from freezing Arctic temperatures to the intense radiation of space. Despite their resilience, the evolutionary history of tardigrades remains a mystery, mainly due to a very sparse fossil record.

However, a recent study published in Communications Biology has added a new chapter to the story of these ancient micro-animals. A team of researchers led by Javier Ortega-Hernández, Associate Professor of Organismic and Evolutionary Biology at Harvard, and PhD candidate Marc Mapalo has uncovered new insights into the evolutionary history of tardigrades by re-examining an old fossil. Their work represents a significant step forward in understanding one of the planet’s most resilient creatures, with findings that may also shed light on how tardigrades developed their unique survival abilities.

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A Fortuitous Discovery

Tardigrades are tiny and extremely difficult to fossilize due to their small size and soft bodies. This means that fossilized tardigrades are incredibly rare. In fact, before this new study, only three fossil tardigrades were known to exist. One of those fossils, Beorn leggi, was discovered in a piece of amber found in Canada in the 1960s. This amber fossil contained not just B. leggi but also a second presumed tardigrade that, at the time, couldn’t be fully identified.

More than half a century later, Ortega-Hernández and Mapalo decided to take a closer look at these ancient specimens using advanced technology. They employed confocal laser microscopy, a method commonly used in cell biology to create high-resolution images of tiny structures. This technique allowed the researchers to study the minute details of the fossil tardigrades in ways that weren’t possible before.

“We were able to re-examine the fossil of Beorn leggi and identify a previously unknown species of tardigrade,” Ortega-Hernández explained. “This discovery not only expands the fossil record of tardigrades but also gives us valuable insights into the evolution of these fascinating organisms.”

Tardigrades and the Age of Dinosaurs

The fossils the team analyzed came from amber dating back to the Cretaceous period, meaning these ancient tardigrades were alive at the same time as the dinosaurs. The fossil of Beorn leggi shows seven well-preserved claws, similar to the claw structure found in modern tardigrades. The new species, Aerobius dactylus, has a distinctive claw structure, with claws of equal length on its first three pairs of legs but longer outer claws on its fourth pair.

The name Aerobius dactylus has a special meaning: “Aero” refers to air, as the fossil appears floating in the amber, and “dactylus” means finger, a nod to its long, finger-like claw.

The fact that these two species were found together in the same piece of amber provides palaeontologists with a unique opportunity to learn more about the biodiversity and evolutionary history of tardigrades during the Cretaceous period.

Molecular Clock Analysis and Evolutionary Insights

The discovery of these two fossilized tardigrades adds to the fossil record and helps scientists understand the evolutionary timeline of tardigrades. Ortega-Hernández and Mapalo used molecular clock analysis — a method that estimates the timing of evolutionary events based on genetic information — to study these fossils.

According to their findings, modern tardigrades likely diverged from other life forms more than 500 million years ago, during the Cambrian period. This places their origins far earlier than previously thought, offering new clues about how they survived in a world vastly different from today’s Earth.

One of the most remarkable abilities of tardigrades is cryptobiosis, a state in which they essentially shut down their metabolism to survive extreme conditions. The research suggests this survival mechanism may have evolved during the mid to late Paleozoic era. This could have been a critical adaptation that helped tardigrades endure one of Earth's most severe extinction events. This end-Permian mass extinction wiped out a significant percentage of life.

Chance Plays a Key Role in Fossil Discoveries

Mapalo, who describes himself as a “paleo-tardigradologist,” was inspired to re-examine these fossils after reading the 2019 book Water Bears: The Biology of Tardigrades. In the book, he saw images of fossil tardigrades that had been visualized using confocal laser microscopy, which sparked the idea of applying the same technique to the fossil he studied.

“In one of the chapters, there was a photo of the oldest fossil tardigrade,” Mapalo explained. “That gave me the idea to use confocal laser microscopy on the fossil I was working with. The technology allowed us to see the tiny details of the claws and body structure that had never been visible before.”

This isn’t the first time a tardigrade fossil has been discovered by chance. Most fossilized tardigrades were found accidentally while researchers were looking for other organisms. The fossil of Beorn leggi was discovered while scientists were studying amber fossils in Canada. Similarly, a fossilized tardigrade found in Dominican amber was unearthed while researchers searched for fossil ants.

“Before I started my PhD, there were only three known fossil tardigrades, and now there’s four,” Mapalo said. “Most of these fossils were discovered by happenstance, so I always encourage researchers working with amber fossils to check for tardigrades. You never know what you might find.”

A Glimpse Into the Past

The discovery of Aerobius dactyls and the re-examination of Beorn leggi represent a significant advancement in palaeontology. These tiny creatures, preserved in amber for millions of years, are helping scientists unlock new information about the evolution and survival of tardigrades.

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As scientists continue to study these ancient fossils, they understand how life has adapted to survive in some of the harshest environments on Earth. The discovery of a new tardigrade species is a reminder that even the most minor and most overlooked organisms can provide invaluable insights into the history of life on our planet. Keep up with the news that matters—stay connected to Education Post News.