杏吧原创

A tiny crab is the first to be found trapped in amber from the dinosaur era. It lived in a forest area of South-East Asia 99 million years ago.

Remarkably similar to modern crabs, the 5-millimetre-long crustacean is fully preserved, making it 鈥渢he most complete crab [fossil] ever discovered鈥�, says at Harvard University.

鈥淲e are talking about pristine preservation, as in, not missing a single hair,鈥� Luque says. 鈥淎nd even though it鈥檚 so small, we were able to see so many details so exquisitely, including the gills. That鈥檚 just mind-boggling.鈥�

杏吧原创s have already studied a few crabs in 15-million-year-old amber from Mexico, but this new specimen, which came from amber mined recently in Myanmar, fills important knowledge gaps about how crabs 鈥� including those that can walk on land or live in freshwater 鈥� evolved.

Although molecular estimates set the origins of non-marine crabs at approximately 130 million years ago, we hadn鈥檛 found any evidence of such crabs beyond 75 million years ago. Because the fossil appears to be a freshwater crab, it potentially extends the record of the group back almost 25 million years.

The 鈥渆xciting鈥� discovery represents a new genus that the team has named Cretapsara: 鈥淐reta鈥� for the Cretaceous period and 鈥淎psara鈥� meaning the cloud and water spirit in South-East Asian mythology, to honour the local culture and heritage. The species itself is C. athanata.

Luque and his colleagues analysed the specimen under a standard microscope and X-ray micro-CT scanner. They clearly identified the animal鈥檚 eyes, antennae, pincers, mouthparts, fine hairs and all eight legs 鈥� including one that had separated from the body, probably as the crab struggled to free itself from the tree resin that engulfed it 鈥渓ike a time capsule鈥�, says Luque.

Despite its small size 鈥� its body measures a mere 2 millimetres wide 鈥� the ancient crab, which might be a juvenile, shares many common features with today鈥檚 crabs. 鈥淵ou have this roundish carapace [upper shell] and the very well-developed walking legs, the big eyes, the small tail tucked under the body,鈥� he says. 鈥淎ll these features are modern-like.鈥�

Yet it also has distinct differences that connect it to its primitive origins, says Luque. In particular, it has deep grooves on its carapace, unlike the smooth tops of current crabs. And its chest is much narrower, more like that of a shrimp or lobster than a modern-day crab鈥檚 broad chest. 鈥淚t鈥檚 not a missing link, but more like a distant cousin to modern crabs,鈥� he says.

The crab鈥檚 gills suggest it mostly lived in water, unlike current land-dwelling crabs which have lung-like air pockets sharing the body space for the gills. 鈥淭his was a really puzzling and exciting point,鈥� Luque says. 鈥淗ow does an aquatic animal get into the sap of a tree in a forest? It gives some food for thought.鈥�

One plausible explanation is that the crab was taking a brief land-based journey between two bodies of water when it got trapped, says Luque. Its misfortune, however, has led to an 鈥渋nvaluable鈥� scientific treasure: resin consolidates quickly in water 鈥� 鈥渓ike super glue鈥�, he says 鈥� so it almost never creates fossils. 鈥淚t was sad for the animal, but it was basically the only opportunity we have to know that it existed.鈥�

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