In the Mesozoic, crabs were active predators and had excellent eyesight.

(ORDO NEWS) — Paleontologists have studied in detail the fossilized remains of the Callichimaera perplexa crab, which lived on Earth almost 100 million years ago. It turns out that the eyes of the callichimera were extremely complex, and she herself was adapted to the lifestyle of a predator-interceptor. The result changes our views on this group of crustaceans and raises the question of what is generally meant by crab.

Every year our knowledge of the history of the biosphere becomes more complete and detailed. Paleontologists find new fossils, clarify the systematic affiliation of the former ones, and adopt more modern methods. As a result, our knowledge of the global tree of evolution is also improving.

And crabs do not stay away from this. From shrimp, crayfish and even craboids, this distinctive group of crustaceans is distinguished by a wide shell shield (carapace) and a reduced bent belly. Modern crabs do not look like a “thunderstorm of the seas” at all: they crawl along the bottom (mainly sideways), being content with carrion there, and prey on trifles.

However, this was not always the case. Recently, scientists have been actively studying the fossil remains of the Callichimaera perplexa crab found in Colombia . This species inhabited the local seas during the Cretaceous period of the Mesozoic era, 95 million years ago.

More than a hundred samples have been recovered from the rock, which allowed us to learn a lot about the structure of Callichimaera . It turned out, despite its very small size (only two centimeters), the callichimera actively swam thanks to its rowing limbs, pursued ancient shrimp and, most surprisingly, had huge and very complex eyes. They account for no less than 16% of her body.

In the Mesozoic crabs were active predators and had excellent eyesight 2

Good preservation, an abundance of finds and microscopes allowed scientists to distinguish the finest details of the Mesozoic crab’s eye structure. Moreover, comparison of animals at different stages of life gave insight into the development of the organ of vision Callichimaera perplexa , as well as the crab itself as a whole.

In a previous post about the callichimera, the reconstructions featured expressive, even cartoonish, eyes. Now it turned out that they were still faceted , that is, complex, consisting of many connected simple eyes – like in the most vigilant of modern insects.

A good way to judge visual acuity in such cases is to compare the viewing angle of each eye. The authors of a new article in iScience turned to the example of callichimera relatives, now living crabs. According to this parameter, all of them hopelessly lagged behind it , and only modern dragonflies bypassed the callichimera – another predator that intercepts the prey “on the fly”. True, already in a different habitat.

Paleontologists have collected 14 Callichimaera perplexa footprints of different ages into a sequence illustrating developmental stages. It turns out that most of the samples really belong to a sexually mature animal, and not its larval stage.

Callichimaera perplexa has adopted an evolutionary strategy called neoteny , or pedogony. So she “refused to grow up” and turn into an armored, walking crab. Instead, Callichimaera perplexa learned to reproduce in a form resembling the larvae of other crabs, and through this became a small but spectacular Mesozoic predator.

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