A tiny crab is the first to be found trapped in amber from the Mesozoic era. It lived in a forest area of South-East Asia 99 million years ago, during the Cretaceous period.
Remarkably similar to modern crabs, the 5-millimetre-long crustacean is fully preserved, making it “the most complete crab [fossil] ever discovered”, says Javier Luque at Harvard University.
Scientists 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 – including those that can walk on land or live in freshwater – evolved.
Although molecular estimates set the origins of non-marine crabs at approximately 130 million years ago, direct evidence of such crabs beyond 75 million years ago has not previously been found. Because the fossil, named Cretapsara athanata, appears to be a freshwater crab, this finding potentially extends the record of the group back almost 25 million years.
Luque and his colleagues analysed the specimen under a standard microscope and X-ray micro-CT scanner. They clearly identified the animal’s eyes, antennae, pincers, mouthparts, fine hairs and all eight legs – including one that had separated from the body, probably as the crab struggled to free itself from the tree resin that engulfed it.
Despite its small size – its body measures only 2 millimetres wide – the ancient crab, which might be a juvenile, shares many common features with today’s crabs. “You have this roundish carapace [upper shell] and the very well-developed walking legs, the big eyes, the small tail tucked under the body,” he says. “All these features are modern-like.”
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’s broad chest. “It’s not a missing link, but more like a distant cousin to modern crabs,” he says.
The crab’s 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. It is not clear how an aquatic animal like this could become caught in the sap of a tree in a forest.
One possibility 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 invaluable scientific treasure: resin consolidates quickly in water, so it almost never creates fossils. “It was sad for the animal, but it was basically the only opportunity we have to know that it existed.”