Tardigrades, also known as water bears, are a highly diverse group of microscopic invertebrates, famous among other things for their ability to survive in extreme conditions. One example was a space trip in 2007 in which tardigrades were exposed to the space vacuum and harmful ionizing solar radiation, yet still managed to survive and reproduce after returning to Earth. Tardigrades are found on every continent in the world and in different environments, including marine, freshwater, and terrestrial.
Tardigrades have survived the five great mass extinction events of the Phanerozoic, however the first modern-looking tardigrades are only known from the Cretaceous, approximately 80 million years ago. Despite its long evolutionary history and global distribution, the tardigrade fossil record is extremely rare. Due to their microscopic size and non-biomineralizing body, the possibility of tardigrades becoming fossilized is small.
Now, an article published in the journal PNAS describes a new modern-looking tardigrade fossil that represents a new genus and a new species. The study used confocal laser microscopy to obtain higher resolution images of important anatomical features that aid phylogenetic analyzes to establish the taxonomic location of the fossil.
Baptized as Paradoryphoribius chronocaribbeus , it is the first fossil found embedded in Dominican amber from the Miocene ( approximately 16 million years ago ) and the first representative fossil of the Isohypsibioidea superfamily of tardigrades.
Problems for your analysis
“The difficulty in working with this amber specimen is that it is too small for a dissecting microscope, we needed a special microscope to fully see the fossil,” explains Marc A. Mapalo, a researcher at Harvard University and author of the study. 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. At such a small scale, a dissecting microscope can only reveal the external morphology of the fossil.
Fortunately, the cuticle of tardigrades is made of chitin, a fibrous glucose substance that is a major component of the cell walls of fungi and arthropod exoskeletons. Chitin is fluorescent and easily excited with lasers, allowing the tardigrade fossil to be fully visualized by confocal laser microscopy . The use of confocal laser microscopy instead of transmitted light to study the fossil created degrees of fluorescence that allowed a clearer view of the internal morphology. With this method Mapalo was able to fully visualize two very important parts of the fossil, the claws and the oral apparatus, or the animal’s foregut, which is also made of cuticle.
The authors note that there is a strong preservation bias for tardigrade fossils in amber due to their small size and habitat preferences – in fact, only three species of tardigrade fossils are known. Therefore, amber deposits provide the most reliable source for finding new tardigrade fossils, although that does not mean that it is an easy task. The discovery of a tardigrade fossil in Dominican amber suggests that other frequently sampled sites, such as the Baltic and Burmese amber deposits, could also host tardigrade fossils.
