Tardigrades might be the toughest little creatures around. These minuscule eight-legged vertebrates can survive in outer space, and have been revived after decades spent in the mosses of Antarctica. These microscopic animals are hard to kill. They're also hard to find as fossils. Until now, there's been only two tardigrade fossils in existence, and now, there are three. Reporting in the Proceedings of the Royal Society B, scientists have announced the discovery of a third tardigrade fossil, which represents a new genus and species, called Paradoryphoribius chronocaribbeus gen. et sp. nov. (Pdo. chronocaribbeus). This incredibly tiny fossil was found in a 16-million-year-old sample of amber from what is now the Dominican.
The researchers described this fossil as the best one ever to be imaged, and is the first tardigrade from the Cenozoic era that's ever been found. The Cenozoic era began 66 million years ago. The tardigrade is only about half a millimeter long, and is a relative of a modern tardigrade superfamily known as Isohypsibioidea.
"The discovery of a fossil tardigrade is truly a once-in-a-generation event," said senior study author Phil Barden, an assistant professor of biology at the New Jersey Institute of Technology. "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." This is an exciting moment for researchers, he added.
The external appearance of this fossil seems very similar to modern tardigrades, but the researchers revealed the internal anatomy of the tardigrade fossil's foregut, and found things that aren't seen in tardigrades that live now, said lead study author Marc A. Mapalo, a graduate student at Harvard University. "Not only does this allow us to place this tardigrade in a new genus, but we can now explore evolutionary changes this group of organisms experienced over millions of years."
This fossil and other finds like it could show scientists how tardigrades evolved into the roughly 1,300 species that can be found on Earth today. But since they are so small, it will remain difficult to find them.
"It's a faint speck in amber," said Barden. "In fact, Pdo. chronocaribbeus was originally an inclusion hidden in the corner of an amber piece with three different ant species that our lab had been studying, and it wasn't spotted for months."
The tiny tardigrade body is well-suited to preservation in amber that comes from plant resin, added Barden. But this process seems to have only started with arthropods about 230 million years ago, which is under half the time tardigrades have been around.
"We are just scratching the surface when it comes to understanding living tardigrade communities, especially in places like the Caribbean where they've not been surveyed," said Barden. "This study provides a reminder that, for as little as we may have in the way of tardigrade fossils, we also know very little about the living species on our planet today."