Callichimaera perplexa, recently discovered, is a strange and strange crab

An international team led by paleontologists from Yale University announced the discovery of a treasure chest containing hundreds of well-preserved crab specimens. Fossils include species of true shrimp, comma shrimp and, most excitingly, a new branch of the crab evolutionary tree. The specimens were recovered from Colombia and the United States and date back to the Middle Cretaceous (about 90 to 95 million years ago).

Googly Eyed by design

"Callichimaera perplexa is so unique and strange that it can be considered the platypus of the crab world," said Javier Luque, who led the research.

"This suggests that new forms are evolving and becoming so disparate over time. Usually, we think that crabs are large animals with large shells, strong claws, small eyes in long locks, and a small tail folded under the body. Well, Callichimaera defies all these "crabby" characteristics and forces us to rethink our definition of what makes a crab a crab. "

Reconstruction of Callichimaera perplexa.
Image credits J. Luque et al., 2019, Science Advances.

The team writes that Callichimaera perplexa is one of the oldest species of paddle-fin hopping anthropodes we've ever discovered, just behind the group of sea scorpions (which lived more than 250 million years ago). He was named for the chimera, a mythological creature whose body included features of several animals. His full name translates as "perplexed beautiful chimera".

The specimen, which Luque describes as "unusual and cute", is definitely perplexed. The old crab was quite small, about the size of a quarter. There was also a pair of large, striking compound eyes – but without orbit. Curved claws, leg-shaped mouthparts and a long body ending in an exposed tail completed her face.

The team writes that these latter characteristics are typical of pelagic crab larvae to date. This suggests an evolutionary link linking them to ancient species. The team believes that these traits have been preserved and amplified in "miniaturized adults" through changes in timing and rate of development. This process, called "heterochrony", is one of the mechanisms by which species develop new body parts.

"It's very exciting that today we found in the tree of life completely new branches, especially from regions like the tropics, which, despite their great diversity, are places we know least about. of the diversity of the past, "said Luque.

The paper "Outstanding preservation of Middle Cretaceous marine arthropods and evolution of new forms via heterochrony" was published in the journal Progress of science.