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To understand how Callichimaera used her eyes, Ms. Jenkins and Dr. Luque used the abundance of available Callichimaera specimens to construct a growth sequence. They compared this to 14 living species from the crab family tree. They were surprised to find that unlike other crab species, Callichimaera retained its large eyes into adulthood.
In fact, their calculations showed that Callichimaera’s compound eyes grew faster than those of the modern crabs the team sampled. At their final size, their eyes took up about 16 percent of their body; it was the equivalent of a person walking around with eyes the size of a soccer ball.
Ms. Jenkins said animals with compound eyes basically have a pixelated view of the world, with each facet of the eye giving off a separate pixel. The higher the pixel count, the sharper the view. The team’s analysis of Callichimaera’s eyes shows it has unusually sharp vision for a crab—closer to efficient, clear-eyed predators like dragonflies and praying mantis shrimp.
Dr. “Whatever this animal was doing, it must have actively used large eyes,” said Luque. ‘They’re a big drag in the water and they’re defenseless. Whatever disadvantages there may be for such large eyes, there must be nothing left besides the advantages.”
Dr. Combined with the paddle legs and streamlined body, these fast-growing, powerful eyes suggest adult Callichimaera preyed on smaller creatures, Luque said. And they did so by retaining their predatory larval form into adulthood, rather than making the final transformation into the flat, rubbing shape favored by other crabs.
Callichimaera is also the youngest known arthropod fossil with two eyes and preserved nervous tissue. Most arthropods with fossilized brains come from sites half a billion years old where it’s rare to get a good look at an animal’s visual processing system. “Often you can find elements of the brain, but the eyes are gone or vice versa. But Callichimaera has both,” said Dr. Luque.
The site that produced the Callichimaera is likely to have many more secrets to share.
Dr. “There’s a huge gap in the fossil record because we don’t collect enough fossils or do a lot of fieldwork in the tropics,” Luque said. “Often these places are covered with vegetation and rocks are weathered very quickly. Finding such excellent preservation opens up new avenues to study the fossil record with fresh eyes. No puns are intended.”
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