Fossil helps understand evolution from fish to land animals
Thursday, 16 Oct 2008 00:01
The fossil is helping shed light on the transition between fish and land animals
A 375 million-year-old fossil discovered in Canada is helping shed light on the evolutionary transition between fish and land animals.
In a new study published in the journal Nature today, academics claim the transition from aquatic to terrestrial lifestyles involved complex changes not only of the appendages but also to the internal head skeleton.
The Tiktaalik rosease was first discovered in 2004 more than 700 miles above the Artic Circle on Ellesmere Island.
The creature was described as a large aquatic predator and experts believe it represents a "textbook example of a transitional fossil".
Tests carried out on the head skeleton demonstrated the intermediary of the Tiktaalik roseae.
"The braincase, palate and gill arches of Tiktaalik help reveal the pattern of evolutionary change in this part of the skeleton," said Dr Jason Downs, from the Academy of Natural Sciences.
"We see that cranial features once associated with land-living animals were first adaptations for life in shallow water."
Dr Ted Daeschler added: "The new study reminds us that the gradual evolutionary transition from fish to tetrapod and the transition from aquatic to terrestrial lifestyles required much more than the evolution of limbs," said Daeschler.
"Our work demonstrates that the head of these animals was becoming more solidly constructed and, at the same time, more mobile with respect to the body across this transition."
Along the lineage of lobe-finned fish that leads to tetrapods, trends in head shape include a flattening of the skull and a lengthening of the snout, the authors describe.
During the transition, the interactions among the different parts of the head skeleton also were changing.
For example, the hyomandibula bone gradually becomes smaller. In fish it links the braincase, palate and gill skeletons and coordinates their relative motions during underwater feeding and respiration.
In the transition to life on land, the bone gradually loses these functions and is eventually used in enabling an animal to hear and in humans, the hyomandibula, or stapes, is one of the tiny bones in the middle ear.