MARGINALIA
Missing Links and Found Links
In and out of the water, transitional forms from the fossil record illuminate the nuts and bolts of evolution
Pat Shipman
Enter the Fishapod
I am equally enamored of another found link, the fossil skeleton of
Tiktaalik roseae, described on April 6, 2006, in the
journal Nature. Tiktaalikis a name suggested by
the elders of the Nunavut people, who live where the fossils were
found on Ellesmere Island in the Canadian Arctic; it means
"large, shallow-water fish." This 375-million-year-old
fish shows a delicious combination of unexpected features, some
inherited from its fishy ancestors and some typical of later
land-dwelling tetrapods (four-footed animals). Neil Shubin of the
University of Chicago, co-leader of the discovery team, jokingly
calls the newly discovered species a "fishapod."
Tiktaalik's fins, gills, scales and primitive jaw show it
was a fish. Unlike fish and like tetrapods, it had a distinct neck,
so its head moved independently of its body. Its flattened head and
broad body make Tiktaalik look somewhat like a weird, scaly
crocodile, an impression enhanced by its four-to-nine-foot length.
Its skeleton differs markedly from those of crocodiles or
alligators, though, despite the overall resemblance in body shape.
Tiktaalik's front fins hold the biggest surprise. Each was
a sort of half-fin, half-leg containing the bony elements found in a
limb—with a functional wrist, elbow and shoulder—and yet
retaining the bony "rays" of a fish fin. According to team
member Farish Jenkins, Jr., of Harvard University, the front fins
were sturdy enough to support the creature in very shallow water or
on land for brief trips.
Its broad and robust ribs were imbricated, like tiles on a roof.
They helped to support the body on land and probably housed lungs to
supplement the gills. The presence of lungs is expected because many
of the primitive fish in Tiktaalik's ancestry had lungs for
gulping air at the water's surface as well as gills. Soft tissues
are rarely preserved in fossils, so the lack of fossilized lungs is
unremarkable. With or without lungs, Tiktaalik was uniquely
adapted to moving between land and water.
"We were absolutely surprised at the features of the
specimens," Ted Daeschler of the Academy of Natural Sciences,
co-leader of the team, told me. "That is one of the beauties of
this material. We knew the end points—fish at the beginning
and tetrapods at the end—but we could not have predicted the
sequence in which those anatomical changes occurred."
Discovering the unexpected is one of the joys of paleontology.
A dramatic change in habitat—becoming a land animal when your
ancestors lived in water—required many anatomical changes.
Sturdy limbs replaced flexible fins. New foods had to be found, and
new means of getting them had to be developed. In this case, when
Tiktaalik crawled up on land it probably preyed upon
insects. Predatory fish in the past and present often suck aquatic
food into their mouths using the same mechanism that passes water
across the gills. But Tiktaalik does not have a bony gill
cover, which means there was less water flow over the gills and a
less effective sucking mechanism. Too, its snout is longer than in
its predatory ancestors. Both of these changes suggest that
Tiktaalik was snapping up prey, perhaps from the air,
rather than gulping down prey along with water.
Eventually tetrapods left the water and relied solely on lungs for
respiration, abandoning their gills. By simply being,
Tiktaalik not only proves that such major adaptive changes
occurred but also reveals how this specific transition from water to
land occurred.
This remarkable fossil shows us something else: that the transition
was not an all-or-nothing affair. "Land" or
"water" is too simple a dichotomy for the realities of
ecosystems. There are many habitats—swamps, or shallow,
plant-choked streams, or ponds that shrink seasonally and
occasionally dry up—that require a range of adaptations to
both land and water. Tiktaalik may have been at home in
such places.
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