Tiny crab encased in amber reveals evolutionary march out of the ocean

One of the first crabs to migrate inland could help scientists understand how animals transition from the oceans to fresh water or land.
 An ancient crustacean found in amber, little more than a fossilized speck, may reveal a critical point in the evolutionary history of one of Earth’s most versatile animals: crabs. This 100-million-year-old fossil, discovered in Myanmar, is helping researchers resolve a prehistoric puzzle about when crabs started to move away from the seas.

The tiny crab’s preservation is “spectacular,” says Yale University paleontologist Javier Luque, lead author of a new study describing the specimen in the journal Science Advances. Luque and colleagues were able to see details of the animal’s jointed legs, claws, compound eyes, and even its gills through the amber.

Paleontologists are unsure whether the new fossil represents an adult crab or a juvenile, but the crab is so well preserved that Luque and colleagues were able to determine that the creature is a new species, named Cretapsara athanata, belonging to a still-living group of crabs called Eubrachyura.

Amber is fossilized tree resin, making it all the more surprising to find a crustacean encased inside. “Finding a crab in amber is like finding a needle in a haystack,” says Florida International University biologist Heather Bracken-Grissom, who was not involved in the new study.

Some crabs can live in fresh water or on land, while others can climb trees—such as the mangrove tree crabs pictured here—perhaps explaining how a crab was trapped in tree resin 100 million years ago.

The study team proposes that Cretapsara could represent the oldest known nonmarine crab, holding clues about how crabs made the evolutionary jump from the sea to inland environments. “The new fossil crab in amber spectacularly bridges the gap,” Luque says.

The scientifically valuable fossil also highlights an ethical discussion surrounding the collection, purchase, study, and publication of amber fossils from Myanmar. Highly prized amber specimens are often smuggled to markets in China, where some paleontologists compete with private dealers to purchase the fossils—a trade that can fund the Tatmadaw military forces that have committed violent human rights violations in Myanmar.

Earlier this year, following a Tatmadaw coup to seize power, the Society of Vertebrate Paleontology called for a moratorium on the publication of research about Myanmar amber fossils collected after 2017, when the country’s military forces began to seize the amber mines. The piece of amber containing Cretapsara was collected in 2015, the researchers report, and sold to a vendor in Myitkyina, Myanmar, before being purchased by the Longyin Amber Museum in Yunnan Province, China.

Luque hopes that publishing research about a fossil collected prior to the moratorium will help raise awareness of the conflict in northern Myanmar’s Kachin State, where amber mines produce fossils that capture the imaginations of professionals and fossil enthusiasts alike.

Crabs march ashore

The condition of the amber and other clues are just as revealing as the little arthropod inside. The crab is intact, rather than an empty shell left over from molting, suggesting the animal truly did live in the habitat where it became preserved. The lack of sand grains in the amber, and the way the sap flowed over the crab, also hint that this fossil came from an environment away from the beach—likely a brackish or freshwater environment.

Moving away from the ocean was a big step for crabs. Becoming adapted to life in brackish or fresh water isn’t as simple as throwing a switch. The animals had to change the way they breathed, regulated water, and kept from drying out, Luque says.

Xiao Jia, center, the curator of the Longyin Amber Museum, shows the crab in amber to a student.

PHOTOGRAPH BY XIAO JIA, LONGYIN AMBER MUSEUM

“The largest barrier has to be changes associated with osmoregulation,” or how an organism manages water and electrolytes like salt in its body, Bracken-Grissom says—not to mention new predators ready to snack on novel morsels.

Nevertheless, crabs have moved inland from the seas over and over again. Modern crabs not only live on the beach, among coral reefs, and in the ocean depths, but can also be found in estuaries, rivers, and lakes. Some crabs—such as the purple land crab of the Caribbean—spend most of their time on land. Others have taken truly unique paths, such as the coconut crab, an immense arthropod that can weigh up to nine pounds and climbs the trees of island habitats in the Indian and Pacific Oceans.

Researchers who specialize in arranging family trees based on biomolecules, such as genes, estimate that nonmarine crabs first evolved about 130 million years ago, during the earliest part of the Cretaceous period. The oldest known nonmarine crab fossils were only about 70 million years old until this new find from Myanmar brought the fossil record more in line with the genetics estimate.

Windows to the past

While the amber fossil may be the oldest known nonmarine crab, it likely wasn’t the first—or the last—crab to venture away from the seas. “We suggest that true crabs have become adapted to a primarily freshwater mode of life at least six times, and to habitats including land and brackish water at least 12 times,” Luque says.

And crabs aren’t the only organisms to have undergone striking transformations as they left the oceans. The steelhead trout of Lake Michigan, for example, descended from saltwater ancestors, adapting to fresh water in less than 120 years. Multiple species of whale and dolphin have also made themselves at home in freshwater habitats, such as the Amazon river dolphin.

No standard set of adaptations allows an animal to make the switch from salt water to fresh, making this repeated evolutionary tale all the more remarkable. Now, with this Cretaspara crab seemingly caught in the midst of this transition, scientists have a new window into this mysterious process.

But even as ancient amber fossils offer new windows into the past, scientists are grappling with the ethics of looking through them. In addition to the quandary over the amber trade, the fossil’s current home in the Longyin Amber Museum is far from Myanmar, and paleontologists are becoming increasingly concerned about the repatriation of fossils as part of a country’s natural history heritage.

Researchers have pointed out that Myanmar’s laws regarding the export of fossils in amber are in conflict with each other. In a June letter to the journal Nature Ecology and Evolution, Zin-Maung-Maugn-Thein of the University of Mandalay and Khin Zaw of the University of Tasmania recommend that paleontologists report significant finds in amber to government or scientific authorities in Myanmar to prevent important fossils from being scattered around the world.

“By doing so,” the duo write, “not only will scientific research standards improve within the country, but Myanmar’s people will gain a better understanding of the importance and scientific value of their own natural heritage rather than being robbed of it.”