Written by: RILEY BLACK
This 100 million-year-old crustacean is well preserved, revealing the adventures of crabs out of the ocean.
The first crabs to migrate inland could help scientists understand how animals transition from the ocean to freshwater or land. Photo by LIDA XING
Scientists have found an ancient crustacean in amber that is not much larger than the impurity plaques in fossils and may represent a pivotal moment in the evolutionary history of crabs. The 100-million-year-old fossil is helping researchers solve a prehistoric puzzle: When crabs began to leave the ocean.
A new study in the journal Science Advances describes the fossil specimen in detail, and lead author Javier Luque, a paleontologist at Yale University, said the baby crab was surprisingly well preserved. Luque and his colleagues were able to see several details of the animal through amber, including appendages, claws, compound eyes, and even gills.
Paleontologists aren't sure whether the new fossil represents adult or juvenile crabs, but it's so well preserved that Luque and colleagues were able to determine that the creature was a new species called Cretapara athanata, a group of crabs that are still alive, called Eubrachyura.
Amber is a resin fossil, so people were shocked when they found it wrapped in crustaceans. Heather Bracken-Grissom, a biologist at Florida International University, said: "Finding a crab in amber is like finding a needle in a haystack. She was not involved in the new study.
Some crabs can live on freshwater or on land, while others can even climb trees — such as the mangrove tree crab shown in the picture — which may explain why crabs were trapped in resin 100 million years ago. Photo by JAVIER LUQUE, HARVARD UNIVERSITY
The team suggests that Cretapsara may be the oldest known non-marine crab species, which provides clues as to how crabs evolved from the ocean to their inland environment. "The crabs in the amber fossil fill that gap," Luque said.
Luque hopes that studies around fossils collected before the moratorium will be published as scheduled, which will help raise awareness of the conflict in Myanmar's northern Kachin state. Fossils from the amber mine in Kachin State have attracted the attention of professionals and fossil enthusiasts alike.
Crabs come ashore
The amber fossil is in good condition, providing researchers with several clues. For example, the crab is intact, rather than the empty shell left behind after molting, suggesting that the animal does live in its preserved habitat. From the way the sap flows through the crabs and the lack of sand grains in the amber, it is speculated that the fossil came from an environment far from the beach – perhaps saltwater or freshwater.
Judging from the evolutionary history of crabs, leaving the ocean is a very significant step. Adapting to life in saltwater or fresh water is not as simple as pressing a switch. They have to change the way they breathe, regulate moisture, and avoid dryness, Luque said.
Xiao Jia (center), director of the Long yin Amber Museum, shows a student the crabs in amber. Photograph: Xiao Jia, Long Yin Amber Museum
"The biggest hurdles are changes related to osmotic regulation, or how organisms manage water and electrolytes (such as salt) in their bodies," Bracken-Grissom said, not to mention how to avoid the attention of new predators.
Despite this, crabs migrate inland from the ocean again and again. Modern crabs not only live on beaches, reefs and deep in the ocean, they also make their home in estuaries, rivers and lakes. Some crabs spend most of their time on land, such as the purple land crab in the Caribbean. Other crabs have gone their own way in evolution, such as the coconut crab, which lives in indian and Pacific island habitats, a giant arthropod that weighs up to 4.5 kilograms and can even climb trees.
Researchers who specialize in using biomolecules, such as genes, to arrange the family tree estimate that non-marine crabs first evolved about 130 million years ago, in the early Cretaceous period. But the oldest known non-marine crab fossil is only about 70 million years old, and it wasn't until this new fossil in Myanmar was discovered that the fossil record was unified with genetic estimates.
A window into the past
While this amber fossil may be the oldest known non-marine crab, it is likely not the first or last crab to venture out of the ocean. "We believe that crabs have successfully adapted to a freshwater-dominated lifestyle at least 6 times and conquered habitats including land and saltwater at least 12 times," Luque said.
Crabs aren't the only creatures that have undergone amazing changes since leaving the ocean. For example, the Hardhead Trout of Lake Michigan, whose ancestors originally lived in saltwater, have adapted to fresh water in less than 120 years. A variety of whales and dolphins also make their home in freshwater habitats, such as the Amazon pufferfish.
There is no standard adaptation method for animals to switch from salt water to fresh water, and this repetitive evolutionary story is therefore more noteworthy. Now, the crab in the amber fossil appears to be in the midst of this transformation, and scientists have a new understanding of the mysterious process.
But even as ancient amber fossils offer a new window into the past, scientists are grappling with the ethics of fossil research. In addition to the dilemma of the amber trade, the fossils are currently located far from Myanmar at the Long yin Amber Museum, which is part of Myanmar's national natural history heritage, and paleontologists are increasingly concerned about the return of fossils.
The researchers note that Myanmar's laws regarding the export of fossil amber are conflicting. In June, Zin-Maung-Maugn-Thein of the University of Mandalay and Khin Zaw of the University of Tasmania suggested in a letter to the journal Natural Ecology and Evolution that paleontologists report major discoveries in amber to the Myanmar government or scientific authorities to prevent important fossils from scattering around the world.
(Translator: Strange Flowers Blossom)