'Water bear' fossils show how they survived several mass extinction events

Tardigrades, endearingly referred to as “water bears,” boast a reputation for their astounding resilience. These microscopic marvels can outlast anything — from lethal radiation and arctic temperatures to the vacuum of space.

Despite being ubiquitous across our planet, wherever there’s water, the evolutionary history of these eight-legged mini creatures has remained a cryptic puzzle due to a limited fossil record.

Why tardigrades are known as “water bears”

Tardigrades, also known as water bears, are tiny creatures that measure about 0.3 to 0.5 millimeters long. They have a segmented body with eight legs, each ending in tiny claws or suction disks.

Their bodies are covered in a flexible cuticle that they shed as they grow. These little guys have a simple structure with no respiratory or circulatory system, relying on diffusion to exchange gases.

You’ll find tardigrades in a variety of colors, from transparent to shades of red, orange, and green. They munch on plant cells, algae, and small invertebrates by piercing them with their stylet, which is a needle-like mouthpart.

Despite being so small, tardigrades are incredibly resilient and can survive extreme environments that would wipe out most other life forms.

Water bears can handle temperatures from near absolute zero to over 300 degrees Fahrenheit. They can also endure radiation levels that would be deadly to most.

Tardigrades can even survive in the vacuum of space and can go for years without food or water, springing back to life once they’re rehydrated.

Researchers have found tardigrades in diverse environments, from deep ocean trenches to the highest mountains.

They possess unique proteins that protect their cells from damage, allowing them to enter a state called cryptobiosis, which we will now discuss in greater detail.

Tardigrade fossils and advanced microscopy

An enlightening study has emerged from the Department of Organismic and Evolutionary Biology at Harvard University. Associate Professor Javier Ortega-Hernández and PhD candidate Marc Mapalo took a fresh look at an old mystery.

They revisited a piece of amber discovered in the 1960s from Canada, which housed the known fossil tardigrade, Beorn leggi, and another assumed specimen of a tardigrade that was inadequately described previously.

For their investigation, the scientists used confocal laser microscopy, an innovative technique traditionally used for cell biology studies, allowing them to explore the minute structures of these fossil tardigrades in spectacular detail.

Amber fossils reveal tardigrade secrets

The team’s study elucidated not only a definitive classification of B. leggi within the tardigrade lineage but also unveiled a previously unidentified species.

As Ortega-Hernández explained, “Both of them are found in the same piece of amber that dates to the Cretaceous Period, which means that these water bears lived alongside dinosaurs.”

The microscopic fossil images of B. leggi depict seven well-preserved claws, the ones curving towards the body being smaller than those extending outwards — a pattern resonating with modern-day tardigrades.

The second specimen, now named Aerobius dactylus, sported claws of roughly equal length on its first three pairs of legs but longer claws on the fourth pair.

Modern tech used for ancient insights

Emboldened by a photograph featured in the 2019 book, “Water Bears: The Biology of Tardigrades,” Mapalo found inspiration to apply confocal laser microscopy on his current fossil under study.

This fresh approach led to the discovery of another new tardigrade species, enclosed in a piece of Dominican amber, which was documented in a 2021 paper alongside Ortega-Hernández and researchers from the New Jersey Institute of Technology.

Apart from expanding the tardigrade fossil record, the discoveries offer critical calibration points for molecular clock analysis.

This tool helps scientists estimate the timing of pivotal evolutionary events.

Cryptobiosis and historical extinctions

The latest findings postulate that modern tardigrades likely diverged during the Cambrian Period around 500 million years ago.

Furthermore, the research shines a spotlight on cryptobiosis, the technical term for the remarkable survival mechanism that enables tardigrades to endure extreme conditions by entering a state of stasis.

Ortega-Hernández elaborates, “The study estimates that this survival mechanism likely evolved during the mid to late Paleozoic, which may have played a crucial role in helping tardigrades endure the end-Permian mass extinction, one of the most severe extinction events in Earth’s history.”

The research by Ortega-Hernández and Mapalo represents a significant stride in paleontology, their endeavors have deepened our understanding of one of Earth’s most enduring life forms.

As a humble paean to their journey, Mapalo shared, “Before I started my PhD, there were only three known fossil tardigrades, and now there’s four.”

Most, if not all, fossil tardigrades were discovered by chance. However, this serendipitous research has made one thing clear: new discoveries are waiting to be found, offering hope for the uncovering of more water bear mysteries hidden in amber.

Broader implications of tardigrade research

Tardigrades’ extraordinary resilience raises intriguing questions regarding the possibilities of life beyond Earth, as their survival strategies might inform the search for extraterrestrial life in similarly harsh environments.

Moreover, examining how these creatures endure extreme conditions offers insights into the potential impacts of climate change on biodiversity.

By unlocking the secrets of their genetics, researchers can also investigate applications in biotechnology, particularly in fields like cryobiology, where understanding the mechanics of their stasis can lead to advancements in preserving biological materials.

Thus, the ongoing study of tardigrades not only elucidates the past but could also shape the future of various scientific inquiries.

The full study was published in the journal Communications Biology.

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