Researchers describe new tardigrade fossil found in 16 million year old Domincan amber

Paradoryphoribius chronocaribbeus Gen. et. Sp. nov. in the mosses. Credit: Holly Sullivan created the original art
The charismatic group of microscopic invertebrates known as water bears are known as the tardigrades. They are known for their remarkable ability to survive in extreme environments. One famous example is a 2007 space trip where tardigrades were exposed in space to harmful ionizing radiation and space vacuum. They survived and reproduced after their return to Earth. Tardigrades can be found on all continents and in different environments, including freshwater and terrestrial.

Although the five Phanerozoic Great Mass Extinction events have been survived by tardigrades, the oldest known modern-looking tardigrades date back to the Cretaceous (80 million years ago). The tardigrade fossil record of their global distribution and long evolutionary history is extremely sparse. Because of their small size and non-biomineralizing bodies, there is little chance that tardigrades will become fossilized.

Researchers from the Royal Society published a paper on October 6th in Proceedings of the Royal Society. They describe a modern-looking, tardigrade fossil. It is a new genus. Confocal laser microscopy was used to obtain better resolution images of key anatomical features that were useful in phylogenetic analyses and to determine the fossil's taxonomic position.

Only the third tardigrade amber fossil, Paradoryphoribius Chronocaribbeus, has been fully described and officially named. Milnesium SWOLENSKI and Beorn LEGG are the other modern-looking tardigrade fossils. Both were found in North American Cretaceous-age amber. Paradoryphoribius, the first fossil found in Miocene (approximately 16,000,000 years ago), Dominican amber, and the first representative of the tardigrade superfamily Isohypsibioidea.

Phillip Barden (New Jersey Institute of Technology), was the co-author and presented the fossil to Marc A. Mapalo Ph.D. Both Professor Javier Ortega Hernndez (candidate) and the senior author, are both from Harvard University's Department of Organismic and Evolutionary Biology. Barden's laboratory discovered the fossil, and worked with Ortega-Hernndez (Mapalo) to analyze it in detail. Mapalo, a specialist in tardigrades took the lead in analysing the fossil with confocal microscopes at the Harvard Center for Biological Imaging.

Mapalo stated that the difficulty in working with this amber specimen was that it is too small to be dissected by microscopes. He needed a special microscope to see the fossil. The light from dissecting microscopes is able to reveal the morphology and size of larger inclusions like spiders and insects in amber. Paradoryphoribius has a body length of 559 micrometers, which is slightly less than half a millimeter. A microscope cannot reveal the internal morphology of this fossil at such a small scale.

Left: Lateral view of Paradoryphoribius Chronocaribbeus Gen. et Sp. nov. Viewed with transmitted light under the streomicroscope (top), and with autofluorescence using confocal laser microscopy (bottom). Right: Ventral view Paradoryphoribius Chronocaribbeus Gen. et Sp. nov. Viewed with transmitted light under the streomicroscope, (top), and with autofluorescence with confocal laser microscope (bottom). Credit: Marc A. Mapalo

Tardigrade's cuticle, which is composed of chitin, is made from fibrous glucose substance chitin. It is also a primary component in cell walls of fungi and exoskeletons arthropods. Confocal laser microscopy allows you to see the tardigrade fossil in its entirety because chitin is fluorescent. Confocal laser microscopy was used instead of transmitting light to examine the fossil. This allowed for higher levels of fluorescence, which allows for a clearer view of the internal morphology. Mapalo was able see the two most important features of the fossil: the claws, and the buccal apparatus. This also allows Mapalo to view the foregut, which is also made from cuticle.

Mapalo stated that even though it appeared to be a modern tardigrade from the outside, confocal microscopy revealed its unique foregut organization. This allowed us to create a new genus within the extant tardigrade superfamilies. "Paradoryphoribius, the only genus with this unique character arrangement within the superfamily Isohypsibioidea," said Mapalo.

"Tardigrade fossils are rare," said Ortega-Hernndez. The new study has only revealed four specimens. Of these, only three have been formally named and described, which leaves only Paradoryphoribius. This paper covers a third the tardigrade fossil record that has been documented. Paradoryphoribius also provides the only information on a tardigrade buccal device in their entire fossil record.

Due to their habitat preferences and small size, there is a strong preservation bias in amber for tardigrade fossils. Even though amber deposits are the most reliable source of new tardigrade fossils it doesn't mean that finding them is easy. A tardigrade fossil was discovered in Dominican Amber. This suggests that other sites such as Baltic and Burmese amber deposits could also contain tardigrade fossils. There has been a tendency to include larger amounts of amber in the past. Tintigrades, which are small and difficult to see, require specialist knowledge as well as excellent observational skills.

Scientists have determined that tardigrades are closely related to arthropods and that their origins date back to the Cambrian Explosion. This is a very lonely phylum that has only three fossils. Ortega-Hernndez stated that most fossils in this phylum can be found in amber, but they may not be easy to see because they are so small.

Mapalo also agreed that "if you look at the exterior morphology of tardigrades you might assume there have not been any changes within their bodies." We were able to see characters in the fossils that we could not observe in extent species using confocal microscopy. However, this was possible because of the confocal laser microscope. This allows us to understand how the body has changed over millions of years. This suggests that, even though tardigrades are the same externally as they are internally, there are some internal changes.

Mapalo and Ortega-Hernndez use confocal laser microscope technology to study tardigrades in amber. They hope to expand the tardigrade fossil record.