Tardigrade “once in a generation” fossil record



image: Dominican amber containing Paradoryphoribius chronocaribbeus gen. and. sp. nov. (in box), dime image digitally added for size comparison. Amber also contains three ants, a beetle and a flower.
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Credit: Phillip Barden (Harvard / NJIT)

They survived the vacuum of space and even came back to life after being frozen for decades in the moss of Antarctica. But as hard as it is to kill the bizarre microscopic animal, the tardigrade, it’s harder to find a fossilized one. In fact, only two have been discovered and officially named – so far.

In in the Proceedings of the Royal Society B, principal researchers at the New Jersey Institute of Technology and Harvard University described only the third tardigrade fossil ever recorded, a new genus and a new species Paradoryphoribius chronocaribbeus gen. and sp. Nov. (Pdo. Chronocaribbeus), which is entirely preserved in Dominican amber dating from the Miocene, 16 million years old.

Measured at just over half a millimeter, the specimen has been identified as a relative of the modern tardigrade superfamily, Isohypsibioidea, and represents the first tardigrade fossil recovered from the Cenozoic, the current geological era beginning 66 million ago. years.

Researchers say the pristine specimen is the best-represented fossil tardigrade to date – capturing micron-level detail of the mouthparts of the eight-legged invertebrate and needle-like claws 20 to 30 times thinner than ‘a human hair. The new fossil is deposited in the Invertebrate Division of Zoology of the American Museum of Natural History.

“The discovery of a fossil tardigrade is truly a once-in-a-generation event,†said Phil Barden, lead author of the study and assistant professor of biology at the New Jersey Institute of Technology. “What is so remarkable is that the tardigrades are an ancient ubiquitous lineage that has seen everything on Earth from the fall of dinosaurs to the rise of terrestrial colonization of plants. Yet they are like a ghost lineage to them. paleontologists with almost no fossil record. Finding the remains of tardigrade fossils is an exciting time where we can empirically see their progression through Earth’s history. “

“At first glance, this fossil appears similar to modern tardigrades due to its relatively simple external morphology,†said Marc A. Mapalo, lead author of the study and graduate student in the Department of Organic and Evolutionary Biology at Harvard. “However, for the first time, we 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. Not only does this allow us to place this tardigrade in a new genus, but we can now explore the evolutionary changes this group of organisms has undergone over millions of years.

Tardigrades, or water bears, are renowned for their unusual appearance and self-sustaining abilities – some species are known to survive extreme conditions by curling up in a dehydrated ball and entering a state of animation. suspended where their metabolism is virtually paused, known as cryptobiosis.

Rare finds of late fossils such as PDO. chronocaribeus, suggests the team, could provide new molecular estimates that offer new insight into the major evolutionary events that have shaped the more than 1,300 species on the planet today, such as the miniaturization of their bodily plane into one of The smallest known animals on Earth with legs.

Perhaps the biggest challenge in finding tardigrade fossils is their size.

“It’s a slight dot in the amber,†Barden said. “In reality, PDO. chronocaribeus was originally an inclusion hidden in the corner of a piece of amber with three different ant species our lab had studied, and it hasn’t been spotted for months.

Barden says the non-biomineralized microscopic bodies of tardigrades are also particularly suited for preservation in amber derived from plant resin, which is able to safely envelop and preserve organisms as tiny as water bears and even individual bacteria.

“This particular mode of fossilization helps explain the uneven fossil record,†Barden explained. “Fossil amber with arthropods trapped inside is only known from 230 million years ago to the present day… it is less than half of the history of tardigrades.”

Placement PDO. chronocaribeus on the tardigrade tree

While tardigrades are believed to have diverged from other pre-Cambrian panarthropod lineages 540 million years ago, only two definitive tardigrade fossils have been formally described, both from Cretaceous fossil deposits in America. North.

To explore PDO. chronocaribeus and Out of its place on the tardigrade ancestral tree, Mapalo used high-power laser confocal fluorescence microscopy to finely image the specimen. The team then compared it to a range of morphological features associated with the main tardigrade groups living today, including key identifiers such as body surface, claws, oral system, and egg morphology. .

“The fact that we have had to rely on imaging techniques usually reserved for cell and molecular biology shows how difficult it is to study fossil tardigrades,” said Javier Ortega-Hernandez, assistant professor of biological biology and scalable at Harvard. “We hope this work will encourage our colleagues to take a closer look at their amber samples with similar techniques to better understand these cryptic organisms.”

The team’s places of analysis PDO. chronocaribeus in one of the three basic classes of tardigrade, Eutardigadra, and makes them the first definitive fossil member of the superfamily called Isohypsibioidea – a diverse species that today inhabit aquatic and terrestrial environments and are generally characterized by their distinct claws which vary in size from leg to leg.

The discovery also places a minimum age on the Isohypsibioidea family.

“We are only scratching the surface when it comes to understanding living Lateigrade communities, especially in places like the Caribbean where they have not been studied,†Barden said. “This study is a reminder that as little as we can get any tardigrade fossils, we also know very little about the living species on our planet today.”

Pictures illustrating the work are available at Google drive.

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Mapalo MA, Robin N, Boudinot BE, Ortega-Hernández J, Barden P. 2021, A tardigrade in Dominican amber. Proc. R. Soc. B 20211760. https://doi.org/10.1098/rspb.2021.1760

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