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 From the New York Times, October 16 th

This report presents further details for the evidence, from a fossil,  
for the evolution of life forms from marine vertebrates into animals  
that walked on land.

Martin


October 16, 2008
Fish Fossil Yields Anatomical Clues on How Animals of the Sea Made It  
to Land
By JOHN NOBLE WILFORD
In a new study of a fossil fish that lived 375 million years ago,  
scientists are finding striking evidence of the intermediate steps by  
which some marine vertebrates evolved into animals that walked on land.

There was much more to the complex transition than fins evolving into  
sturdy limbs. The head and braincase were changing, a mobile neck was  
emerging and 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 anatomy of this early transformation in life from water to land  
had never before been observed with such clarity, paleontologists and  
biologists said Wednesday in announcing the research.

The scientists said in a report being published Thursday in the  
journal Nature that the 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.”

In that case, the fish, a predator up to nine feet long, was a  
predecessor of amphibians, reptiles and dinosaurs, mammals and  
eventually humans. The fossil species was named Tiktaalik roseae,  
nicknamed “fishapod” for its fishlike features combined with limbs  
similar to those of tetrapods, four-legged land animals.

The new research on the head skeleton of Tiktaalik (pronounced tic-TAH- 
lick) was conducted at the Academy of Natural Sciences in Philadelphia  
and the University of Chicago.

“The braincase, palate and gill arch skeleton of Tiktaalik have been  
revealed in great detail,” said 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 first  
adaptations for life in shallow water.”

Several skeletons of the fish 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 Neil H. Shubin, an evolutionary  
biologist at the University of Chicago and the Field Museum, and Ted  
Daeschler of the Academy of Natural Sciences. The Devonian Age rocks  
containing the fossils indicated that the fishapod lived in shallow  
waters of a warm climate. It may have made brief forays on land.

Since the discovery was reported in 2006, Dr. Downs and two specimen  
preparators, C. Frederick Mullison of the academy and Bob Masek at  
Chicago, spent more than a year prying deeply into the skulls of  
several fishapod skeletons. The results were also analyzed by Dr.  
Shubin and two other co-authors of the report, Dr. Daeschler and  
Farish Jenkins Jr., an evolutionary biologist at Harvard.

“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,” Dr. Daeschler said.

Dr. Shubin said 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.”

For example, fish have no neck but “we see a mobile neck developing  
for the first time in Tiktaalik,” Dr. Shubin said.

“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,” he added. “Then a flexible  
neck is important.”

One of the most intriguing findings, scientists said, was 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. In more primitive fish, the bony part of what is  
called the hyomandibula is large and shaped like a boomerang. 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,” Dr. Downs  
said, noting the specimen’s loss of rigid gill-covering bones, which  
apparently allowed for increased neck mobility.

In the transition from water to land, the researchers said, 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 tiny bones in the middle ear.

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

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