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Paleontologists working in Patagonia have found the tiny fossil jaw that may be the first evidence of early mammals in South America.

The fossil, which measures less than a quarter-inch long, is believed to be from the middle or late Jurassic Period, perhaps 170 million years old. Researchers said it suggests that mammals developed independently in the Southern Hemisphere.

The fossil, named Asfaltomylos patagonicus, was discovered in a mudstone formation in the province of Chubut, 950 miles south of Buenos Aires. The now-arid region also has yielded some remarkable dinosaur fossils from the same era in a vast ancient boneyard covering hundreds of square miles.

Researchers from Germany and Argentina reported in the current issue of the journal Nature that Asfaltomylos belongs to a branch of southern egg-laying mammals that are extinct today except for the platypus and echidna of Australia. It is distantly related to Jurassic mammals found in Madagascar and Tanzania.

Researchers identified it by tiny characteristics of its molars, which they said are similar to those of mammals that evolved 25 million years later in the Northern Hemisphere.

Most early mammal species are identified by peculiar cusps, bumps or points on their teeth.

The Jurassic was a 70-million-year-long period ending 140 million years ago. The land that would become North America was splitting apart from Europe, creating what is now the Atlantic Ocean. Similar changes were occurring in the Southern Hemisphere.

Dinosaurs were the dominant land animal. Mammals were small and inconspicuous, resembling modern shrews and weasels. They hunted insects under the cover of dense tropical foliage.

Nearly all modern mammals are believed to have evolved from Jurassic ancestors in the Northern Hemisphere.