Philip Anderson at the University of Massachusetts Amherst and others said they have confirmed what scientists have long suspected about the evolutionary lag -- that early land vertebrates kept their fish-like jaws for millions of years after developing legs.
"This pattern had been hypothesized previously, but not really tested," Anderson said. "Now we've done that."
The researchers examined images of 89 fossils of early tetrapods and their fish-like forebears ranging in age from about 400 million to 300 million years old.
The mechanical properties of tetrapod jaws did not show significant adaptation to land-based feeding until about 40 million to 80 million years after the four-legged creatures initially came out of the water, the researchers said.
The finding suggests tetrapods may have shown a limited variety of feeding strategies in the early phases of their evolution on land, Anderson said.
"What it took to really initiate evolutionary changes in the jaw system was for these animals to start eating plants," he said.
Simply moving into a new environment is not always enough to trigger functional adaptations, the researcher said.
"They stayed essentially fish-like for a long time," Anderson said.