First described in 2019, the fossilized remains of an extinct species of brachyuran crab called Callichimaera perplexase were found in Boyacá, Colombia, and Wyoming, United States. The fossil, specifically found in a Cretaceous rock layer in the Colombian Andes, showed extraordinary preservation of both the external elements of the eye and the internal optic neural tissue. It is not usual (rather the opposite), since areas as delicate as the eyes do not usually keep well over time.
‘To see you better’
This 95-million-year-old creature featured large, socketless compound eyes, bent claws, paw-like mouthparts, an exposed tail, and a long body. Now, new research has elucidated that Callichimaera perplexa’s visual system was impressive: its eyes made up about 16% of its total body size. An unusual and striking feature, especially considering that living crabs often have small compound eyes located at the end of a long stalk with a socket to cover and protect themselves.
It was a very visual predator that inhabited well-lit environments, according to this new research led by paleontologists from Yale University (USA) and published in the journal iScience.
“The specimens we have of the unusual crab from the Cretaceous preserve some very delicate eye tissue that is not normally preserved,” says Kelsey Jenkins, a graduate student at Yale University. “This includes things like facets and internal optic tissues. This excellent preservation is rare.”
The researchers analyzed almost 1,000 living and fossil crabs, including crabs in different stages of development , for a total of 15 different species of crab. They compared the size of the crustaceans’ eyes and how fast they grew. Callichimaera perplexase topped the list in both categories. His eyes were about 16% the size of his body. “If anything has eyes that big, they are definitely very visual animals,” says Jenkins. As a comparison, it would be as if our eyes were about 23-25 centimeters.
And it is that the eyes are crucial in the environment . They serve to find food, mate, avoid predators, locate potential prey… and also help the body’s internal clock to differentiate day from night.
The visual acuity of this prehistoric crab was similar to that of dragonflies, which are among the top predators of the insect world, and a mantis shrimp.
“Crabs whose eyes grow very fast have a higher visual inclination, they are probably very good predators that use their eyes when they hunt , while slow-growing eyes tend to be found in scavenger crabs that are less dependent on sight,” he says. Derek EG Briggs, a researcher at Yale University and co-author of the study.
Referencia: Kelsey M. Jenkins et al, The remarkable visual system of a Cretaceous crab, iScience (2021). Volume 25, Issue 1, 21 January 2022, 103579 DOI: 10.1016/j.isci.2021.103579
