But their tiny size makes finding fossilized tardigrades is hard.
A new study describes a tardigrade fossil scientists found in amber from the Dominican Republic.
Tardigrades have a reputation as some of the hardiest critters in the animal kingdom.
It's well-earned: The microscopic creatures can survive in the vacuum of space, inside a volcano, and in an Antarctic lake more than half-a-mile underground. They have even returned to normal functioning after being frozen for three decades.
The organisms have been on Earth for at least 540 million years, but only two fossilized tardigrades had ever been found - until this week, when researchers announced a third.
According to a study published Wednesday, scientists discovered this new tardigrade fossil in amber found in the Dominican Republic.
"The discovery of a fossil tardigrade is truly a once-in-a-generation event," Phil Barden, a biologist at the New Jersey Institute of Technology who coauthored the new study, said in a press release. "What is so remarkable is that tardigrades are a ubiquitous ancient lineage that has seen it all on Earth, from the fall of the dinosaurs to the rise of terrestrial colonization of plants. Yet, they are like a ghost lineage for paleontologists, with almost no fossil record."
Barden's group determined that the fossil is 16 million years old and is a previously undiscovered type of tardigrade called Paradoryphoribius chronocaribbeus.
The fossil was so tiny, scientists didn't spot it for months
Tardigrades are also known as water bears or moss piglets - apt nicknames for 0.05-inch-long organisms that, under a microscope, look like eight-legged potatoes with scrunched-up faces and tiny paws.
The critters can withstand temperatures between minus 458 degrees Fahrenheit (minus 272 degrees Celsius) and 304 degrees Fahrenheit (151 degrees Celsius), and pressure up to six times that of the deepest part of Earth's oceans.
They're able to survive temperatures and conditions that would be lethal to humans, because water bears - like their namesake mammal - can enter a state of hibernation during which they go without water and oxygen.
But although tardigrades are nearly indestructible in life, they lack the hard tissues that would help their bodies persist after death as fossils in rock. So an amber tomb is one of the only ways tardigrades can be preserved over millennia.
That, coupled with their tiny size, means the chances that scientists find ancient tardigrades are slim.
In fact, Barden said, his group didn't spot this new fossil for months in their New Jersey lab, even as they closely studied the three ants, beetle, and flower that were fossilized in the same piece of amber.
The ancient tardigrade's body was about six-tenths of a millimeter long - less than the width of a credit card.
Using tardigrade guts to classify a new species
All three of the fossilized tardigrades scientists have discovered were found in amber. One species was found near Cedar Lake in Manitoba, Canada, in 1964, and was about 78 million years old. The other, which popped out of an amber deposit in New Jersey two decades ago, dates back 92 million years.
But this new addition to the tardigrade fossil record is especially valuable because it was close enough to the amber's surface for scientists to study its body parts and organs.
Using a special type of microscope, Barden's group was able to study the ancient tardigrade's mouth and claws, as well as reconstruct what its throat and guts looked like.
That helped the group figure out that the specimen was a new kind of tardigrade, belonging to a separate genus and species.
"For the first time, we've visualized the internal anatomy of the foregut in a tardigrade fossil and found combinations of characters in this specimen that we don't see in living organisms now," Marc Mapalo, an evolutionary biologist at Harvard University who led the study, said in the release.
About 1,300 tardigrade species are alive today. Fossils like this one, Mapalo said, help paleontologists better understand how these creatures evolved and survived all five major mass extinctions on Earth.
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