Paleontologists have discovered the fossil remains of a
375-million-year-old fish that fills a crucial gap in the evolutionary
history of Earth’s four-limbed creatures.
Among the fish’s most distinguishing features: the skeletal
beginnings of shoulders, wrists, and legs that signal a future move to
land.
Gently scratched from soil layers on desolate Ellesmere Island in
Canada’s Nunavut Territory, the new fossils could join the ancient bird
archaeopteryx as an icon for evolution in action, some researchers say.
“It’s really an amazing find,” says Hans-Dieter Sues, associate Scientists have long surmised that four-limbed land animals evolved
director of science at the Smithsonian Institution’s National Museum of
Natural History in Washington.
from early fish. In recent years, they have been slowly closing the
fossil gap between these fish and the first land vertebrates. But
existing fossils have failed to fill an important, 10-million-year hole
in the record, when the fish were undergoing significant physical
changes.
Thus, the appearance of the new species is likely to become “a
textbook example of the transition” from fish to four-limbers, known as
tetrapods, says Edward Daeschler, a paleontologist with the Academy of
Natural Sciences in Philadelphia. He and Neil Shubin, of the University
of Chicago, led the team that made the discovery, which will publish
its findings Thursday in the journal Nature.
Scientists can look at fossils on either side of the gap, then sit
back “and philosophize on what intermediate forms should look like,”
Dr. Sues says. This single find, he continues, clears much of the fog
from that view.
The team named the species Tiktaalik roseae, using an Inuit
word for large, shallow-water fish. During its heyday, it lived in
freshwater streams and ponds under conditions far warmer and more lush
than today – at the time, what is now Ellesmere Island was part of the
continent Laurentia, which straddled the equator.
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The sharp-toothed meat-eater’s head gives it a crocodile-like
profile – suggesting a fish even Julia Child would have found tough to
turn into chowder. But because of the excellent state of preservation,
the researchers say they have been able to identify several key
features that serve as significant way marks along the path between
fish and tetrapod.
Like its immediate ancestors, Tiktaalik is scaly and displays gill
and fin features that indicate it lived mostly in the water. But it
lost its gill cover and its snout grew, suggesting that changes were
under way in the creature’s breathing mechanism and in its food
sources. It boasts a heftier rib cage than its ancestors, presumably to
support it when it leaves the water. It’s skull has lost related bones
associated with fish but missing in tetrapods. Another gill feature, a
tiny slit that became part of the ear in tetrapods, has grown wider.
And the bones in the fins along its sides point to proto-limbs with
enough strength and flexibility at the shoulder, elbow, and wrist to
allow Tiktaalik to lift itself off the bottom and perhaps temporarily
move about on land.
The blending of fish and tetrapod features prompted the team to tag
the species with the street name “fishapod,” according to Dr. Shubin.
The discovery caps a search that spans five field trips over six
years aimed at filling this gap in the fossil records. The team knew
the kind of rock formations they needed to search, pored over detailed
geological maps, then settled on a set of sites to search. Twin Otter
aircraft carried the team and its gear from Resolute Bay to the island,
then a helicopter ferried them the rest of the way.
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