For the roughly 300 known octopus species dwelling in the world's oceans, having eight arms is a defining characteristic. But that is not the way it started.
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Scientists said a fossil unearthed in central Montana of a species named Syllipsimopodi bideni represents the oldest-known relative of today's octopuses and boasts 10 arms, with two being twice as long as the other eight.
The fossil, so well preserved that it reveals two parallel rows of suckers up and down each arm, dates to about 328 million years ago.
Syllipsimopodi, about 4-3/4 inches (12 cm) long, had a torpedo-shaped body and squid-like appearance though it was not closely related to squids, which appeared much later.
It also is the oldest-known creature with suckers, which enable the arms to better grasp prey and other objects.
Syllipsimopodi pushes back by 82 million years the origins of a group called vampyropods that includes today's octopuses.
Syllipsimopodi represents the only member of the octopus lineage with 10 arms, meaning two were lost in later evolution. There are numerous similar examples in the history of life on Earth – such as the reduction in the number of digits seen in meat-eating dinosaurs or horses.
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