First land animals kept fish-like jaws for millions of years

Land animals developed legs millions of years before their feeding systems changed enough to let them eat a strictly land-based diet, US paleontologists say.
For the first time, fossil jaw measurements by Philip Anderson at the University of Massachusetts Amherst and others, who studied how early land vertebrates evolved from fishes, have tested and statistically confirmed this lag.
"This pattern had been hypothesised previously, but not really tested. Now we've done that," Anderson said.
Researchers found that the mechanical properties of tetrapod jaws did not show significant adaptations to land-based feeding until some 40 to 80 million years after the four-legged creatures initially came out of the water.

Until then, tetrapod jaws were still very fish-like, even though their owners had weight-bearing limbs and the ability to walk on land. Anderson says this finding suggests tetrapods may have shown a limited variety of feeding strategies in the early phases of their evolution on land.
"What it took to really initiate evolutionary changes in the jaw system was for these animals to start eating plants," he said.

For the study, published in the journal Integrative and Comparative Biology, Anderson and researchers Matt Friedman of the University of Oxford and Marcello Ruta of the University of Lincoln, UK, examined images of 89 fossils of early tetrapods and their fish-like forebears.

The fossils ranged in age from about 400 to 300 million years old. Anderson and his team were interested in how the jaws of these fossilised animals differed through time.
They used 10 biomechanical metrics to describe jaw differences. One of these, called mechanical advantage, measured how much force an animal can transfer to its bite.
The researchers compared jaw features from the fossil record and calculated the rates at which jaws evolved.
"The basic result was that it took a while for these animals to adapt their jaws for a land-based diet. They stayed essentially fish-like for a long time," Anderson said.

It turns out that just moving into a new environment is not always enough to trigger functional adaptations.