Tardigrades. Water bears. Moss piglets. Whatever their name, the microscopic invertebrates are notoriously hardy, and now a team of researchers has studied the ancestors of tardigrades to understand how the animal evolved.
The research team studied lobopodians, a group of extinct soft-bodied worms from the Cambrian period, and modern tardigrades to determine which characteristics of the latter group were inherited by the former. The team’s findings were published this week in the Proceedings of the National Academy of Sciences.
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Before our work, our understanding of tardigrade evolution was limited, said Ji-Hoon Kihm, a paleobiologist at the Korea Polar Research Institute and lead author of the study, in an email to Gizmodo. We revealed the ancestral and derived morphological traits in tardigrades and established a close relationship between tardigrades and luolishaniid lobopodians.
The luolishaniid lobopodians were a group of Cambrian creatures with five or six pairs of shaggy appendages at their rear ends. Modern tardigrades have four pairs of claws per limb, rather than fingers.
Two key differences between tardigrades and luolishaniids are the relative length of the legs and the number of trunk segments: tardigrades have relatively shorter legs and fewer trunk segments than luolishaniids, possibly due to loss of developmental genes, Kihm added.
Although creatures like the lobopod ancestors of tardigrades had soft bodies, their remains are perfectly preserved thanks to their seafloor habitats. The Cambrian Period encompassed a 53 million year period over half a billion years ago (the Cambrian Explosion) that saw an incredible explosion of biological diversity on Earth, most of it on the sea floor. Filter feeders have gone crazy.
Incredible fossils of these creatures are found in places like the Qingjiang Biota in China and the Burgess Shale in Canada, now a mountain range, which lay on the sea floor 500 million years ago.
The hypothesis that luolishaniid lobopodians are stem-group tardigrades is new and deserves serious consideration, said Joe Moysiuk, a paleobiologist at the Royal Ontario Museum in Toronto who was not affiliated with the new study, in an email to Gizmodo. Speculations about developmental processes that may have contributed to the origin of the tardigrade body plan are also fascinating and insightful.
Moysiuk reported several new species from the Burgess Shale, including the zeppelin form Titanokorys gainesi AND Sickled Cambroraster, named after Han Solo’s Millennium Falcon. He noted that some of the features the researchers linked between luolishaniids and tardigrades also appear in other lobopodian creatures, such as Hallucigeniaa decidedly stunning animal from the Cambrian.
Given the interpretative challenges when working with half-billion-year-old fossils, I doubt this will be the final word on luolishaniid relationships, Moysiuk added.
Tardigrades walk like creatures 500,000 times their size, possibly a relic of their macroscopic ancestors. At some point in the evolutionary timeline, the animals began to shrink, giving way to the creature by about 1 mm (0.04 in).
Kihm noted that previous studies have reported the loss of developmental genes associated with the belly button and legs of tardigrades. In our study, we confirm the findings suggested by these studies, using Cambrian fossils, she said. The loss of these genes could be linked to the miniaturization process observed in tardigrades.
There are many secrets that tardigrades still keep close to their microscopic chests. Invertebrates are notoriously hardy, they are extremophiles, meaning they like conditions inhospitable to most other organisms. When tardigrades encounter environmental stress, they undergo cryptobiosis, a process that allows them to suspend all metabolic activity.
The hardiness of tardigrades also makes them fodder for scientists’ more quixotic experiments. Tardigrades were hit by ultraviolet radiation in 2020 and shot from handguns in 2021 to test a theory about aliens. Last year, a team of physicists claimed to have made a tardigrade part of a quantum system.
A few tardigrade fossils have been discovered, including a 16-million-year-old tardigrade encased in amber, but it will take a multidisciplinary combination of paleozoology, genetic analysis and more to figure out exactly how this unique animal came to be.
Other: Tardigrades are still a complete evolutionary mystery
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