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The discovery of a 69 million-year-old crab fossil in Mexico, with an oversized right claw, a feature previously thought to appear more than 20 million years later, has lead scientists to rewrite the evolutionary history of crabs and the shelled mollusks upon which they preyed.

Washington, April 23 : The discovery of a 69 million-year-old crab fossil in Mexico, with an oversized right claw, a feature previously thought to appear more than 20 million years later, has lead scientists to rewrite the evolutionary history of crabs and the shelled mollusks upon which they preyed.

Cornell paleontologist Greg Dietl was the one who recognized the 67- to 69-million-year-old fossil from the late Cretaceous period of a big crab with an oversized right claw, which was kept in a museum display case in the state of Chiapas, Mexico.

Such crabs with claws of different sizes were not known to exist until the early Cenozoic era, about 20 million years later.

Aside from being larger than most known late Cretaceous crabs and having asymmetrical claws, this ancient crab also sported a curved tooth on the movable finger of the larger right claw. This was another specialized adaptation that paleontologists thought developed millions of years later for peeling snail shells open.

"The fossil re-opens the question of the role crabs played in the well-documented restructuring of marine communities that occurred during the Mesozoic era 251 million years ago to 65 million years ago," said Dietl.

The museum's staff showed Dietl and colleagues another fossil of the same species in a back room.

Both specimens, found near the town of Ocozocoautla in southeastern Mexico, are the oldest fossils with these unique features on record and represent a new species, Megaxantho zogue.

The large right "crusher" claw generated a great deal of force to break shells, while the smaller left cutter claw moved faster and could manipulate prey into position. Also, the curved tooth increased the power of the claw.

According to Dietl, the discovery will spur other researchers to search for similar examples of these curved tooth structures from the late Cretaceous period, just prior to the extinction of the dinosaurs.

Although Megaxantho crabs became extinct around 65 million years ago, these features evolved again in other crab species throughout the Cenozoic era, leading to present-day crabs, according to the study.

"The repeated evolution suggests that such power-enhancing adaptations may evolve during times and places where resources are abundant and accessible," said Dietl.

The study may be relevant to the current stresses of habitat loss, overfishing, climate change and other human-influenced activities that are reducing the productive capacity of the environment.

ANI