Enigmatic swimming crab fossil offers lessons on evolution’s versatility

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Fourteen years ago, Javier Luque was a geology student wrapping up a day of field work in his native Colombia when he swung his hammer at a section of rock and opened a window into the past. The layer where the rock had split yielded a trove of small, well-preserved marine fossils dating back some 90 million years. Among them was something he couldn’t recognize.

“It looked like a spider – a really weird spider – with flattened legs and big eyes,” he said. “Then I noticed it had claws.”

Fast-forward to 2019 and Dr. Luque, who recently earned his PhD at the University of Alberta based on his study of the fossil, is finally able to say what it is he found. The unusual creature is a type of crab – but one that apparently abandoned the crab’s classic body shape and scuttling walk to evolve into a full-time swimmer that propelled itself through the water by using its flattened legs as paddles and employed its big eyes to hunt down tiny prey.

“We’re dealing with something completely new that no one has seen before, dead or alive,” said Dr. Luque, who is now a postdoctoral researcher at Yale University.

His detailed study of the creature, published Wednesday in the journal Science Advances, is based on 70 individual specimens and is co-authored by experts in four countries. The result sheds new light on how the machinery of evolution works in response to ecological opportunities.

Today, crabs are among the most successful marine invertebrates in the world, with approximately 7,000 species known, including some that live in freshwater environments or range onto land. Many have a strikingly similar appearance, despite being only distantly related – a sign that evolution has repeatedly pushed crabs toward the same type of body plan based on their role in ecosystems.

It was a different story in the mid-Cretaceous period, when dinosaurs were the dominant life forms on land. At that time, crabs underwent a frenzy of diversification, possibly driven by the emergence of coral reefs that were proliferating in the warm, shallow seas of the period.

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Yet even relative to that time, Dr. Luque’s find is a mysterious outlier in the fossil record, as reflected in its scientific name, Callichimaera perplexa, which means “perplexing beautiful chimera."

In classical mythology, a chimera is a creature that looks like parts of different animals stuck together. The chimera crab is also a biological mix – not just of different crab features but of different stages in crab development. Its adult form includes many features that typically are only found in crab larvae.

“It’s like an adult in the body of a juvenile,” Dr. Luque said. He added that the fossils displayed several clues, including penises on some of the male specimens, that showed they were full grown. “That was the smoking gun that told us these things are sexually mature.”

The analysis began several years ago, when Dr. Luque was a master’s student at the University of Montreal and his supervisor, Christopher Cameron, encouraged him to try to place the fossil within the crab family tree, based on details in the appearance of other crabs fossils through time, and on genetic clues from existing crab species. This proved to be a considerable challenge, as different ways of approaching the question kept turning up different answers about where Dr. Luque’s specimen fit in.

“The crab was jumping all over the place,” Dr. Cameron said. “We just couldn’t make any sense of this thing.”

When Dr. Luque moved on to the University of Alberta he incorporated far more data on the relationships between different types of crabs, and finally arrived at an answer that worked. It showed that the chimera crab branched off from the main crab lineage early, carrying with it a revealing blend of characteristics. The species’s larva-like features may have arisen from a genetic change that affected the timing of developmental steps, Dr. Luque said.

“It helps us understand the sequence of events in [crab] evolution,” said Joanna Wolfe, a Canadian evolutionary biologist based at Harvard University who was not involved with Dr. Luque’s find but who is now collaborating with him.

In a separate study, also published Wednesday, in the Proceedings of the Royal Society B, Dr. Wolfe relied on more than 400 genes to trace the larger relationships between crabs and a wider group of crustaceans that includes lobsters and shrimp. Because of their segmented, modular bodies and abundance in the fossil record, scientists have long considered such creatures an ideal test bed for studying the general principles of evolution in action.