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WASHINGTON (Reuters) - They developed their chomp before they developed their stomp.
Even before fully emerging as land dwellers, Earth's first amphibious four-legged animals attained a biting feeding style that distinguished them from their fishy ancestors and prepared them for a terrestrial existence, scientists said on Monday.
Thus, these primordial amphibians were better at eating like a land beast than walking like one, according to research by Harvard University's Molly Markey and Charles Marshall. They looked closely at the early amphibian Acanthostega, which lived 370 million years ago.
"They don't capture prey like fish. They don't suck it into their mouths. Instead, they captured prey more like what you'd think of a crocodile doing or a lizard," Markey said in a telephone interview.
That meant biting down with a big chomp on their meal, the researchers wrote in the Proceedings of the National Academy of Sciences.
The study shed light on a key evolutionary change during a crucial time in the history of life, as vertebrates were first leaving the water and colonizing dry land. This transition set the stage for the appearance of the later major groups of land vertebrates like reptiles, birds and mammals.
The researchers first examined the skull structure of the living African freshwater Bichir fish, a primitive species with some similarities to fish alive in those ancient times.
They found that the shape of certain features on the skull roof indicated whether a fish caught its prey by sucking it into the mouth, much like a goldfish, or by biting on it directly like a crocodile.
They then examined similar features on the fossilized skulls of three extinct animals.
Acanthostega, which likely lived almost exclusively in the water, caught prey more like later land animals, with a big bite, the researchers said. It was one of the first tetrapods -- four-legged animals -- to appear in the fossil record.
"This was unexpected because even though Acanthostega has legs and is an early tetrapod, it still has internal gills (like a fish). So people had thought that this meant that it would spend most of its time in the water," Markey said.
"You would expect if that's the case then it might capture it's prey more like a fish. But it didn't."
The fish Eusthenopteron, which also lived 370 million years ago and had fins that may have enabled it to crawl onto land for short spells, caught its prey by sucking it into its mouth, they found. Fish like Eusthenopteron are seen as precursors to amphibians, which lived both in the water and on land.
Not surprisingly, the later amphibian Phonerpeton, dating from about 275 million years ago, also chomped its prey, they found. This was a more advanced amphibian that had robust limbs and was fully land-dwelling, they said.