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Scientists studying a fossil fish that lived 375 million years ago are finding striking evidence of the intermediate steps by which some marine vertebrates evolved into animals that walked on land.
John Noble Wilford writes in the New York Times about the study report, which has been published in the journal Nature.
According to the report, research exposed delicate details of the creature’s head and neck, confirming and elaborating on its evolutionary position as “an important stage in the origin of terrestrial vertebrates.”
The new research was done on the head skeleton of Tiktaalik and was conducted at the Academy of Natural Sciences in Philadelphia and the University of Chicago.
There was more to the complex transition than fins evolving into sturdy limbs. The head and braincase changed and a mobile neck was emerging. A bone associated with underwater feeding and gill respiration was diminishing in size, a beginning of the bone’s adaptation for an eventual role in hearing for land animals.
The fish, a predator up to nine feet long, was a predecessor of amphibians, reptiles and dinosaurs. Named Tiktaalik roseae, the fossil is nicknamed “fishapod” because its fishlike features combined with limbs similar to those of tetrapods, four-legged land animals.
“The braincase, palate and gill arch skeleton of Tiktaalik have been revealed in great detail,” says Jason Downs, a research fellow at the academy and lead author of the report.
“By revealing new details of the pattern of change in this part of the skeleton, we see that cranial features once associated with land-living animals were adaptations for life in shallow water.”
Several skeletons were excavated four years ago on Ellesmere Island, in the Nunavut Territory of Canada, 700 miles above the Arctic Circle, by a team led by Drs Neil Shubin, an evolutionary biologist at the University of Chicago and the Field Museum, and Ted Daeschler of the Academy of Natural Sciences.
Devonian Age rocks containing the fossils indicate that the fishapod lived in shallow waters of a warm climate. It may have made brief forays on to land.
“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,” says Dr Daeschler.
According to Shubin, Tiktaalik was “still on the fish end of things, but it neatly fills a morphological gap and helps to resolve the relative timing of this complex transition.”
We all know that fish have no neck. But “we see a mobile neck developing for the first time in Tiktaalik,” says Shubin. “When feeding, fish orient themselves by swimming, which is fine in deep water, but not for an animal whose body is relatively fixed, as on the bottom of shallow water or on land. Then a flexible neck is important.”
An intriguing finding, say the researchers, is the reduction in size of a bony element that, in fish, links the braincase, palate and gills and is associated with underwater feeding and respiration. The bony part is what is called the hyomandibula; in more primitive fish, it is large and shaped like a boomerang. But in this fossil species, the bone was greatly reduced, no bigger than a human thumb.
“This could indicate that these animals, in shallow water settings, were already beginning to rely less on gill respiration,” says Downs. The specimen lost rigid gill-covering bones, which apparently allowed for increased neck mobility.
The researchers say that in the transition from water to land, the hyomandibula gradually lost its original functions and, in time, gained a role in hearing. In humans, as in other mammals, the hyomandibula, or stapes, is one of the bones of the middle ear.
Says Dr Daeschler, “The new study reminds us that the gradual transition from aquatic to terrestrial lifestyles required much more than the evolution of limbs.”
sole source: John Noble Wilford’s article on 10/16/08 in the New York Times. www.nytimes.com
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