Tardigrades the tiny, resilient eight-legged creatures known for their unparalleled survival abilities, have been around for a remarkably long time. These microscopic marvels, which first appeared over 541 million years ago, have managed to thrive in nearly every environment on Earth, from the frozen tundras to scorching deserts and even the ocean depths. Despite their success and widespread presence, the fossil record has very few examples of these nearly indestructible creatures, primarily due to their small size and soft bodies that don’t fossilize well.
However, some tardigrades have been preserved in amber, offering rare glimpses into their ancient past. Only four such specimens have been discovered, trapped in tree resin that later hardened into amber, dating back nearly 150 million years. These specimens are highly valued as they provide crucial insights into tardigrade evolution and their extraordinary survival skills.
Studying these ancient specimens has been challenging, though. Amber can be dark and cloudy, making it difficult to observe tiny tardigrades in detail. Of the four known tardigrades in amber, three had been studied and classified, but the fourth remained a mystery due to its small size and poor visibility.
This challenge was recently overcome by a team of zoologists led by Marc Mapalo of Harvard University, who used a technique called confocal fluorescence microscopy. This advanced imaging method allowed the team to capture highly detailed images of two tardigrades embedded in the same piece of Canadian amber, dating back to the Cretaceous period, between 72 and 83 million years ago.
The first tardigrade, named Beorn leggi, was initially described in 1964. The new imaging has revealed previously unseen physical characteristics, such as the shape of its claws and the lack of protuberances on its body. The second tardigrade, previously too small to study in detail, has now been formally named Aerobius dactylus and placed on its own branch of the tardigrade family tree.
Both B. leggi and A. dactylus share similarities with a group of tardigrades called Hypsibioidea, particularly in the shape of their claws. However, A. dactylus has longer claws on its rearmost legs, a trait observed in other modern tardigrade species, suggesting that different pairs of legs in tardigrades may have distinct evolutionary histories.
The study also sheds light on the evolutionary timeline of tardigrades. The researchers suggest that the two main lineages of tardigrades heterotardigrades, often found in oceans, and mostly freshwater eutardigrades diverged around 500 million years ago, slightly later than previously thought. The study also provides clues about when tardigrades developed their unique survival ability known as cryptobiosis, where they can dry out and enter a state of suspended animation. This ability likely emerged at least 180 million years ago, and possibly as far back as 420 million years, helping them survive multiple mass extinction events.
“The acquisition of cryptobiotic abilities of these tardigrades around this time could be one of the factors that have helped them evade extinction,” the researchers concluded.
The findings of this study have been published in Communications Biology, offering a deeper understanding of the remarkable resilience and longevity of tardigrades.
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