Scientists have long known that fish evolved into the first creatures on land
with four legs and backbones more than 365 million years ago, but they've had
precious little fossil evidence to document how it happened.
The new find of several specimens looks more like a land-dweller than the
few other fossil fish known from the transitional period, and researchers speculate
that it may have taken brief excursions out of the water.
"It sort of blurs the distinction between fish and land-living animals," said
one of its discoverers, paleontologist Neil Shubin of the University of Chicago.
Experts said the discovery, with its unusually well-preserved and complete
skeletons, reveals significant new information about how the water-to-land
evolution took place. "It's an important new contribution to (understanding)
a very, very important transition in the history of life," said Robert Carroll
of McGill University in Montreal.
The new find includes specimens, 4 to 9 feet long, found on Ellesmere Island,
which lies north of the Arctic Circle in Canada. It is reported in Thursday's
issue of the journal Nature by Shubin, Ted Daeschler of the Academy of Natural
Sciences in Philadelphia and Farish A. Jenkins Jr. of Harvard.
Some 375 million years ago, the creature looked like a cross between a fish
and a crocodile. It swam in shallow, gently meandering streams in what was then
a subtropical climate, researchers say. A meat-eater, it lived mostly in water.
Yet, its front fins had bones that correspond to a shoulder, upper arm, elbow,
forearm and a primitive version of a wrist, Shubin said. From the shoulder to
the wrist area, "it basically looks like a scale-covered arm," he said.
"Here's a creature that has a fin that can do push-ups," he said. "This is
clearly an animal that is able to support itself on the ground," probably both
in very shallow water and for brief excursions on dry land. On land, it apparently
moved like a seal, he said. It might have pulled itself onto stream banks,
perhaps moving from one wet area to another, and even crawled across logs in
swamps, said Daeschler.
The researchers have not yet dug up any remains from the hind end of the creature's
body, so they don't know exactly what the hind fins and tail might have looked
like. The creature was dubbed Tiktaalik (pronounced "tic-TAH-lick") roseae,
and also had the crocodile-shaped head of early amphibians, with eyes on the
top rather than the side. Unlike other fish, it could move its head independently
of its shoulders like a land animal. The back of its head also had features
like those of land-dwellers. It probably had lungs as well as gills, and it
had overlapping ribs that could be used to support the body against gravity,
Shubin said.
Yet, the creature's jaws and snout were still very fishlike, showing that "evolution
proceeds slowly; it proceeds in a mosaic pattern with some elements changing
while others stay the same," Daeschler said.
If one considers adaptation as a process of collecting tools to live in a
new environment, the new finding offers "a snapshot of the toolkit at this particular
point in this evolutionary transition," Daeschler said.
In fact, much of its value comes from this insight into the order in which
those tools appeared in fish, said Jennifer Clack of Cambridge University, an
expert unconnected with the study.
Knowing that detail about the transition from fish to land-dweller, she said, "might
help us to unravel why it happened at all. Why did creatures come out of the
water and get legs and walk away?"
It's impossible to tell if Tiktaalik was a direct ancestor of land vertebrates,
she said, but if a scientist set out to design a plausible candidate, "you'd
probably come up with something like this."
Shubin said the researchers plan to return to the small rocky outcropping
that yielded the fossils and recover more material. "We've really only begun
to sort of crack that spot," he said. The site is in Nunavut Territory, and "Tiktaalik" in
the creature's name comes from the traditional language used in the area. It
refers to a large freshwater fish seen in the shallows.
