New species of tardigrade (water bear) found in 16-million-year-old Dominican amber

Artistic reconstruction of Paradoryphoribius chronocaribbeus gen. and sp. nov. in mosses. Credit: Original art created by Holly Sullivan

Tardigrades, also known as water bears, are a diverse group of charismatic microscopic invertebrates that are best known for their ability to survive extreme conditions. A famous example is a space trip in 2007 where the tardigrades were exposed to space vacuum and harmful ionizing solar radiation, and still managed to survive and reproduce after returning to Earth. Tardigrades are found on all continents of the world and in different environments including marine, freshwater, and land.

Tardigrades survived the five Phanerozoic mass extinction events, but the earliest modern-looking tardigrades are only known to the Cretaceous, about 80 million years ago. Despite their long evolutionary history and worldwide distribution, tardigrade fossil records are extremely rare. Due to their microscopic size and non-biomineralizing body, the risk of fossilization of tardigrades is low.

In an article to be published on October 6, 2021 in Proceedings of the Royal Society B researchers describe a new, modern-looking tardigrade fossil that represents a new genus and species. The study used confocal laser microscopy to obtain higher resolution images of important anatomical features that facilitate phylogenetic analyzes to establish taxonomic placement of the fossil.

Lateral ventral views Paradoryphoribius chronocaribbeus

Left) Lateral view of Paradoryphoribius chronocaribbeus gen. and sp. nov. seen in transmitted light under stereomicroscope (top) and in autofluorescence under confocal laser microscope (bottom). Right) Ventral view of Paradoryphoribius chronocaribbeus gen. and sp. nov. seen in transmitted light under stereomicroscope (top) and in autofluorescence under confocal laser microscope (bottom). Credit: Images by Marc A. Mapalo

The new fossil Paradoryphoribius chronocaribbeus is only the third tardigrade amber fossil to be fully described and officially named to date. The other two fully described late modern-looking fossils are Milnesium swolenskyi and Leggi from Beorn, both known from Cretaceous amber in North America. Paradoryphoribius is the first fossil to be found embedded in Dominican amber from the Miocene (about 16 million years ago) and the first representative fossil of the tardigrade superfamily Isohypsibioidea.

Co-author Phillip Barden, New Jersey Institute of Technology, presented the fossil to lead author Marc A. Mapalo, Ph.D. Candidate and lead author, Professor Javier Ortega-Hernández, both in the Department of Organic and Evolutionary Biology at Harvard University. Barden’s lab discovered the fossil and partnered with Ortega-Hernández and Mapalo to analyze the fossil in detail. Mapalo, which specializes in tardigrades, took the lead in analyzing the fossil using confocal microscopes located at the Harvard Center for Biological Imaging.

“The difficulty in working with this amber specimen is that it is way too small for dissecting microscopes, we needed a special microscope to fully see the fossil,” Mapalo said. Generally, the light transmitted by dissecting microscopes works well to reveal the morphology of larger inclusions such as insects and spiders in amber. Paradoryphoribius, however, has a total body length of just 559 micrometers, or just over half a millimeter. On such a small scale, a dissecting microscope can only reveal the external morphology of the fossil.

Fortunately, Tardigrade’s cuticle is made up of chitin, a fibrous substance of glucose that is a major component of the cell walls of fungi and arthropod exoskeletons. Chitin is fluorescent and easily excited by lasers allowing full visualization of the tardigrade fossil using confocal laser microscopy. The use of confocal laser microscopy instead of transmitted light to study the fossil created degrees of fluorescence allowing a clearer view of internal morphology. With this method, Mapalo was able to fully visualize two very important characters of the fossil, the claws and the oral apparatus, or the fore intestine of the animal which also consists of the cuticle.

“Even though on the outside he looked like a modern tardigrade, with confocal laser microscopy we could see that he had this unique organization of the foregut that justified us erecting a new genus within this group. existing tardigrade superfamilies, ”Mapalo said. “Paradoryphoribius is the only genus that has this specific uniquely character arrangement in the Isohypsibioidea superfamily.

“Tardigrade fossils are rare,” said Ortega-Hernández. “With our new study, the full tally includes only four specimens, of which only three are officially described and named, including Paradoryphoribius. This article essentially covers a third of the late fossil records known to date. Furthermore, Paradoryphoribius offers the only data on a tardigrade oral appliance in their entire fossil record.

The authors note that there is a strong conservation bias for tardigrade fossils in amber due to their small size and habitat preferences. So, amber deposits are the most reliable source for finding new tardigrade fossils, although that doesn’t mean finding them is an easy task. The discovery of a tardigrade fossil in Dominican amber suggests that other frequently sampled sites, such as the Burmese and Baltic amber deposits, may also harbor tardigrade fossils. Historically, there is a bias in favor of larger inclusions in amber, as inclusions as small as tardigrade are difficult to see and require very good observing skills, as well as specialist knowledge.

“Scientists know where the tardigrades are in the tree of life, that they are related to arthropods, and that they have a deep origin during the Cambrian explosion. The problem is that we have this extremely solitary phylum with only three named fossils Most of the fossils of this phylum are found in amber but, since they are small, even if they are preserved, it can be very difficult to see them, ”said Ortega-Hernández.

Mapalo agreed, “If you look at the external morphology of the tardigrades, you can assume that no changes have occurred in the body of the tardigrades. However, using confocal laser microscopy to visualize internal morphology, we have seen characters that are not seen in extended species but are seen in fossils. It helps us understand what changes in the body have taken place over millions of years. Additionally, it suggests that while tardigrades may be the same on the outside, some changes are occurring internally. “

Mapalo and Ortega-Hernández continue to use confocal laser microscopy technology to study other tardigrades in amber in hopes of expanding the tardigrade fossil record.

Reference: “A tardigrade in Dominican amber” by Marc A. Mapalo, Ninon Robin, Brendon E. Boudinot, Javier Ortega-Hernández and Phillip Barden, October 6, 2021, Proceedings of the Royal Society B.
DOI: 10.1098 / rspb.2021.1760

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