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The discovery in Australia of a 106-million-year-old crayfish fossil—and even older "trace fossils" of the animal's streamside burrows—help fill in a puzzling gap in the history of the small crustaceans.

The finding supports a theory that the evolution of crayfish has been strongly shaped by the drift of Earth's continents, researchers say.

Crayfish are the freshwater cousins of marine lobsters. Hundreds of known species are divided into two distinct groups, one in the Northern Hemisphere and the other in the Southern Hemisphere.

"It's been a mystery how and when they split into these groups," said Anthony Martin of Emory University in Atlanta, Georgia, who led the fossil discovery team.

Contributing to the mystery was an absence of any very old crayfish fossils from the Southern Hemisphere.

Fossils have shown that crayfish were present in the Northern Hemisphere at least 150 million years ago. But for over a century biologists have been puzzled by the lack of any comparably old fossils from the southern continents.