for National Geographic News
New fossils of ancient crayfish and their branching burrows provide the oldest evidence of crayfish in the Southern Hemisphere, experts say.
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.
"There was a 60- to 70-million-year gap in the fossil record," Martin said. "Now we're a lot closer to the origin point of the Southern Hemisphere crayfish."
The study by Martin's team is set to appear in a forthcoming edition of the journal Gondwana Research.
Evolution and Continental Drift
Scientists have long debated whether today's northern and southern crayfish all descend from a common ancestor that was already adapted to fresh water, or if the two groups evolved the freshwater adaptation independently of one another.
A shared freshwater ancestor would require that crayfish first evolved very early, when all of Earth's continents were united in a single landmass called Pangaea.
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