Canada: missing link fish-amphibian found?
Date: 4/4/06 at 9:55PM
Mood: Looking Playing: Smoke on the waters, by Deep Purple
From LiveScience:
Fishy Land Beast Bridges Evolutionary Gap
By Bjorn Carey
LiveScience Staff Writer
05 April 2006
A newfound beast with swim fins and a head like a crocodile fills an evolutionary gap between fish and four-legged land animals.
The creature had gills, a fish-like jaw, and scales.
But its mobile neck let it lift its head above water, and it could support its own weight on thick ribs and wrist-like bones.
It may have even trudged across mudflats in the manner of four-legged animals, called tetrapods.
“I would say it’s a key transitional form in the fish-to-land animal transition,” said study co-author Neil Shubin of the University of Chicago.
“It has features of both fish and tetrapods—we call it a ‘fishapod.'”
Fish push-ups
The animal, Tiktaalik roseae, lived about 375 million years ago, spanning the period between a fish called Panderichthys (385 million years ago) and tetrapods known as Acanthostega and Ichthyostega (365 million years ago).
The team discovered several well-preserved fossil specimens ranging from 4 to 9 feet long from nose to the estimated tail.
“If you look at joints of limbs, you can tell that the thing could perform a fish version of a push-up,” Shubin told LiveScience.
“It could bend its elbow and flex its wrist.
It was certainly capable of supporting its body underwater, in the shallows, or in mudflats with its limbs.”
The somewhat awkward animal probably didn’t walk, since it likely couldn’t rotate its shoulders.
Instead it might have dragged itself along on land.
“It could either push itself straight up and down or pull itself forward,” Shubin said.
“It more likely flopped around like a seal rather than walked like a horse.”
The longer snout suggests Tiktaalik bit at prey like a croc, rather than sucking on it like a fish.
Where it was
The fossils were discovered in layered rock in Ellesmere Island in Canada.
Although the site is 600 miles north of the Arctic Circle today, 375 million years ago this land straddled the equator.
The landmass had a subtropical climate and Tiktaalik likely lived in a series of shallow streams similar to the Mississippi River Delta.
“Outside, it’s a classic Arctic scene,” Shubin said. “But inside those rocks is a tropical world.”
When a fossil like this fills a big gap, two smaller gaps are created, Shubin said, leaving the possibility that better transitional species have yet to be discovered.
The study is detailed in the April 6 issue of the journal Nature.
Tiktaalik update 2008: here.
Biting in fish to amphibian evolution: here.
Dear Kitty. Some blog 
Saturday 18th April, 2009
Fossils suggest earlier land-water transition of tetrapod
ANI Saturday 18th April, 2009
Washington, April 18 : New evidence has emerged that suggests an earlier land-water transition of a four-limbed animal with backbones, which was known to have moved from fish to landlubber.
A Duke graduate student, in his research work, came across the evidence from CT scans of fossils of Ichthyostega and Acanthostega, locked inside rocks.
Both extinct species, known as Ichthyostega and Acanthostega, lived an estimated 360-370 million years ago in what is now Greenland.
Acanthostega was thought to have been the most primitive tetrapod, that is, the first vertebrate animal to possess limbs with digits rather than fish fins.
But, the latest evidence from Viviane Callier, a Duke graduate student, indicates that Ichthyostega may have been closer to the first tetrapod.
“In fact, Acanthostega may have had a terrestrial ancestor and then returned full time to the water,” said Callier.
“If there is one take-home message, it is that the evolutionary relationship between these early tetrapods is not well resolved,” she added.
Co-author Jennifer Clack of the University Museum of Zoology in Cambridge, England, where she supervised Callier’s work for a master’s degree, found the fossils embedded in rocks collected from East Greenland.
Rather than trying to remove them, the researchers studied the fossils inside the stone with computed tomography (CT) scanning.
Callier “reconstructed” the animals using imaging software (Amira and Mimics) to analyze the CT scans, focusing on the shapes of the two species’ upper arm bones, or humeri.
The CT slices revealed that Clack had found the first juvenile forms of Ichthyostega. Previously known fossils of Ichthyostega had come from adults.
Anatomies can morph as animals move towards adulthood, and such shifts can help scientists deduce when in development the animal acquired the terrestrial habit.
The fossils suggest that Ichthyostega juveniles were aquatically adapted, and that the terrestrial habit was acquired relatively late in development.
The fossils bore evidence that the muscle arrangement in adults was better suited to weight-bearing, terrestrial locomotion than the juvenile morphology.
It is possible that Ichthyostega came out of the water only as a fully mature adult.
In contrast, in Acanthostega, “there is less change from the juvenile to the adult. Although Acanthostega appears to be aquatically adapted throughout the recorded developmental span, its humerus exhibits subtle traits that make it more similar to the later, fully terrestrial tetrapods,” Callier said.
According to Callier, “If Ichthyostega is actually more primitive than Acanthostega, then maybe animals evolved towards a terrestrial existence a lot earlier than originally believed.”
“Maybe Acanthostega was actually derived from a terrestrial ancestor, and then, went back to an aquatic lifestyle,” she said.
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