An exceptional find of a 100-million-year-old crab trapped in amber

Today there are marine, freshwater and land crabs. When did the transition from marine to other environments occur during the evolution of this group? An exceptional fossil provides some answers to this question.

During the evolution of polychaetes, some groups changed their environment from aquatic to terrestrial. to How easy was the transition? Of course, it is difficult to quantify this today, as this transition is ancient and has taken place on timescales much larger than a human lifetime.

Study of adaptations and ecologyecology living organisms in the environment, terrestrial and aquatic (marine or limneticlimnetic), however, make it possible to assess the difficulties for a species to permanently change its habitat. The transition from a marine environment to a freshwater environment is also rare in animals, because life in marine and freshwater requires a special physiology, in particular in casecase osmoregulation. Changing habitats also mean that organisms will compete with species occupying the same ecological niche and that they will be exposed to new predators.

There are terrestrial and freshwater crabs belonging to the Eubrachyura group. They have – according to molecular phylogenymolecular phylogeny (as opposed to fossil dating) — diverged from their marine relatives no later than chalkychalky below, about 125 million years ago. The problem with this molecular data is that it is difficult to correlate it with the fossil record, since most fossil remains of these crabs are shells and claws dating from the Upper Cretaceous to quaternaryquaternarythat is, between 73 million years ago andHoloceneHolocene. Due to the lack of complete fossils, it is difficult to understand how the crabs left their marine life and how many times they did so during the evolution of the group.

A species that was an amphibian about 100 million years ago

An international group of researchers also published an exceptional discovery in the journal Achievements of science on the evolution of Eubrachyura. The researchers describe a new species of crab belonging to this group, which they named Cretapsara athanata from a sample preserved in amber. The specimen was found in Myanmar (Southeast Asia) and dates back to 99 million years ago (Cretaceous period).

This crab belongs to one of the oldest groups, Eubrachyura, and represents a combination of ancestral and derived features of this group. Its exceptional preservation allows observation with the help of microtomography X-raysX-raysit is big eyeseyes compounds, its antennae and even gills. The posture and appearance of the carapace suggest that the animal was immersed in amber during life.

Amber also includes fibers woodwood and fecesinsectsinsectsindicating that the amber came from a forest environment and that the crab lived in fresh water or brackishbrackish close. Finally, the authors believe C. athanata was probably amphibious rather than terrestrial, but Eubrachyura had already broken free from the marine environment nearly 100 million years ago.

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