Ancient Tardigrades in Amber Reveal the Origins of Cryptobiosis

Tardigrades - microscopic, eight-legged animals regarded as some of the hardiest organisms on Earth - have existed for an extraordinarily long time. Their molecular clock indicates that these highly adaptable creatures appeared before the Cambrian period, about 541 million years ago, and have been inching their way across the planet ever since.

Today, tardigrades inhabit almost every environment on Earth. They have managed to survive and flourish everywhere from icy tundra and dry deserts to the ocean floor. Yet despite their widespread success, remarkably few examples appear in the fossil record.

This scarcity is unsurprising. Tardigrades are tiny and comparatively soft-bodied; after death, they decompose rapidly, and their bodies are poorly suited to the demands of fossilisation. Even so, a small number of ancient tardigrades have endured for millions of years through the remarkable preservative properties of amber.

Tardigrades preserved in ancient amber

Only four tardigrade specimens caught in sticky tree resin, which later solidified into amber, have been recovered by humans. Nearly 150 million years old, these fossils are especially valuable because they may reveal details of tardigrade evolution and, perhaps, explain their extraordinary ability to survive.

Studying animals encased in amber is not straightforward, however, and researchers have struggled to place these specimens precisely within the tardigrade family tree. Amber may be cloudy and dark, while tardigrades themselves are exceptionally small. Three amber-preserved tardigrades had previously been examined and named, but the fourth was too minute for its features to be clearly identified.

A group of zoologists led by Marc Mapalo of Harvard University has now overcome that obstacle. The team used confocal fluorescence microscopy, a method involving a pinhole that captures substantially more detailed images of microscopic subjects than widefield microscopy can produce.

The scientists examined two tardigrade fossils enclosed in the same piece of Canadian amber from the Cretaceous period, dating from between 72 and 83 million years ago, in the final era of non-avian dinosaurs. The results of confocal microscopy are clear: it delivered considerably sharper images of both tardigrades than earlier efforts had achieved.

Beorn leggi and Aerobius dactylus

The first fossil is Beorn leggi, which was formally named and described in 1964. More thorough imaging of B. leggi allowed the researchers to identify anatomical details missed in earlier work, including the form of its tiny claws and the absence of protrusions on its wrinkled body.

For the first time, the second tardigrade in the amber has also been viewed in detail. Previously considered too small and insufficiently preserved to reveal much, this minute specimen has now received the formal name Aerobius dactylus and been allotted its own branch on the vast, complex family tree.

Like B. leggi, A. dactylus has a barrel-shaped body without protrusions and distinctive claws at the ends of its eight legs. Both species have claws resembling one another and those of Hypsibioidea, a tardigrade superfamily. In each case, the claws that curve towards the body are shorter than those that curve away from it, indicating that both species belong within this group.

Although B. leggi and A. dactylus have long been extinct, other Hypsibioidea species still live today. At last, B. leggi and A. dactylus have been reunited with their tardigrade relatives.

There is an intriguing difference, though: the claws on the rearmost leg pair of A. dactylus are notably longer. These claws resemble those of the tardigrade genus Isohypsibius. This unusual characteristic has also been documented in living tardigrade species, implying that the fourth pair of tardigrade legs may have followed a different evolutionary path from the other three pairs on the same animal.

Cryptobiosis and tardigrade evolution

The analysis also enabled the researchers to draw conclusions about tardigrade evolutionary history. Tardigrades fall into two principal lineages: heterotardigrades, which frequently live in marine environments, and eutardigrades, which are mostly found in freshwater.

While B. leggi and A. dactylus are both eutardigrades, their age indicates that these lineages separated roughly 500 million years ago - slightly more recently than scientists had previously believed.

By comparing the fossils with modern tardigrades, the researchers also established a timeframe for the emergence of the tardigrade superpower known as cryptobiosis: the capacity to dry out almost entirely and remain in suspended animation for unlimited periods. This ability arose no later than 180 million years ago and may have originated as far back as 420 million years ago.

That span includes several of Earth's mass extinctions and could offer clues to the astonishing longevity of these exceptional animals.

"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 write.

Their findings were published in Communications Biology.