Sometime during the Cretaceous Era, roughly 100 million years ago, a tiny crab marched out of its home in the sea, climbed a tree, and got itself trapped in a dollop of tree trunk goop. But little did this young crustacean know that the amber would immortalise it, and it would weather harsh conditions for 100 million years to serve as a "missing link" in crab evolution.

This crab dates all the way back to the age of dinosaurs and is believed to be the oldest modern-looking crab and a distant cousin of the present-day red crabs. The group this crab belongs to would migrate to the ocean for breeding.

As per researchers who studied the ancient specimen, the crab's body, including its claws, shell, bulging eyes, and gills, are perfectly preserved in amber—making this an incredibly rare find. Researchers think this might be the oldest evidence of crab intrusions into non-marine habitats!

Hiding in plain sight

The tale of the crab's discovery began in 2015, when researchers stumbled upon a piece of amber jewellery with a two-millimetre-long crab lodged in it in a market in Tengchong, China.

"When I saw it for the first time, I could not believe my eyes. This spectacular crab looks so modern, like something you may find in B.C. flipping rocks, but it is actually quite old and different from anything seen before, fossil or alive," said Javier Luque, a research associate at Harvard University. She is a former post-doctoral researcher at Yale University who began looking at the encased crab as a PhD student at the University of Alberta.

The newly discovered species has been named Cretapsara athanata, which translates to the "immortal spirit of the clouds and waters."

This find is even more remarkable because the most well-known amber fossils are usually terrestrial arthropods, primarily insects, with aquatic animals being very unusual. Land-dwelling animals, primarily insects, are the most common fossils found in amber, created from resin dripped from tree bark.

While it's common for crabs to climb trees today, scientists have previously suggested that these crustaceans began doing so only 50 to 75 million years ago. So this raises the question: how on Earth did the 100 million-year-old Cretapsara athanata climb a tree?

Filling the gaps in crab evolution

Studying the specimen revealed that the crab's gills are well-developed, indicating that it was an aquatic or semi-aquatic creature. But the lack of sand in the fossil and the way the sap poured over the crab suggest that the animal lived in fresh or brackish water and was not a beach dweller.

The crab's evolutionary shift away from ocean water must've been a big step backwards. It meant the animal had to change how it managed water, breathed, and kept from drying up to adapt to living in brackish or freshwater.

Crabs have mastered living on land, in brackish and fresh water at least twelve times since the "Cretaceous crab revolution", as per Luque. Therefore, their gills have evolved to have lung-like tissue that allows them to breathe in and out of the water. C. athanata lacked lung tissue but possessed well-developed gills, indicating that it did not live entirely on land.

The molecular record, which is built by comparing similarities and differences in DNA and RNA, predicts that non-marine crabs, such as the Christmas Island red crabs that live in mountains or freshwater crabs found in rivers all over the world, split from their marine counterparts more than 125 million years ago.

However, marine crabs invaded land and freshwater much later, between 75 and 50 million years ago, according to the fossil record of non-marine crabs, which consists of just minute bits and pieces of claws. This means there's been a 50-million-year gap between the expected molecular timing of non-marine crab split and the known fossil record, explained Luque. The discovery of Cretapsara athanata helps bridge this gap to a large extent.

“There is still a lot of work to do and fossils to discover to keep filling the puzzle of crab evolution, and Cretapsara athanata is bringing us closer to it one crab at a time," said Luque.

The study was published in Science Advances last week and can be accessed here.