Inside the head of the fishapod

The braincase, palate and gill skeleton of 375 million-year-old fossil specimens named Tiktaalik roseae have revealed more details about fish to land-based vertebrate evolution.

In a new study published in Nature today, US researchers describe new features of the fossils, filling in some of the intermediate stages in the fossil record between primitive fish and tetrapods.

Tiktaalik roseae was found in 2004 in Devonian-age rock on Ellesmere Island in Canada. It caused a stir when a paper was first published in 2006, as the body plan and the nature of the deposits where it was found suggested that while it lived in shallow water, it may have taken a few forays onto land.

Its skull, neck and ribs share features with tetrapods but it also has scales and fin rays. The new findings concentrate on a bone called the hyomandibula, which in fish links the braincase, palate and gill skeletons and co-ordinates their relative motions during feeding and respiration.

In humans and other mammals the hyomandibula is one of the tiny bones in the middle ear.

Today’s study reveals that this feature in Tiktaalik is much smaller than in other primitive fish. “This could indicate that these animals, in shallow-water settings, were already beginning to rely less on gill respiration,” lead author Dr Jason Downes, of the Academy of Natural Sciences of Philadelphia, said.

It also seems to have made their heads and necks more mobile, important for animals that live in shallow water or on land but less important for fish, who can orient their bodies more flexibly.

“The new study reminds us that the gradual transition from aquatic to terrestrial lifestyles required much more than the evolution of limbs,” co-author Dr Ted Daeschler said.

“Our work demonstrates that, across this transition, the head of these animals was becoming more solidly constructed and, at the same time, more mobile with respect to the body.

“The cranial endoskeleton of Tiktaalik roseae” by Downs et al [doi:10.1038/nature07189] is published in Nature.