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
Though missing links are often talked about, it's the found ones that
hold a special place in my heart. Found links are fossils that
illustrate major transitions during evolutionary history. More than
that, such creatures offer unexpected glimpses of the
never-predictable twists and turns taken by evolution. Their
discovery and surprise bring sheer fun to paleontology and biology.
I have always loved the iconic Archaeopteryx, a beautiful
fossil recognized in 1860 that unmistakably combines features of two
major groups of animals: birds and reptiles. The exquisite feathered
wings of Archaeopteryx bear most unbirdlike claws; its
birdlike skull contains an avian brain but carries sharp reptilian
teeth, not a beak; and its feathered tail is underlain by a long
bony tail typical of a small dinosaur, not a bird. Still, the
feathers and wings on these 150-million-year-old fossils qualify
Archaeopteryx for the title of First Bird.
Archaeopteryx is a found link in another sense, because the
anatomy of this extraordinary species reveals how creatures evolved
from propelling themselves along solid substrates, such as the
ground or tree limbs, to moving through the air. It was a difficult
transition. Archaeopteryx fascinates me in part because its
anatomy is not that of a skillful, modern bird, yet it competed with
contemporary pterodactyls, which flew using different anatomical
structures. I often wonder why birds survived and those wonderful
pterodactyls went extinct.
At the time of its discovery, Archaeopteryx was hailed by
the anatomist Thomas Henry Huxley as stony proof of evolutionary
theory. Decades later, Archaeopteryx was trumped by an
extraordinary plethora of feathered dinosaurs—some
nonflying—that tell different stories about the evolution of
avian features.
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.
Longer and Longer Swims
Tiktaalik's discovery made me reconsider the much later and
opposite transition—from land back to the sea—which is
documented in the excellent record of fossil whales. For example,
Pakicetus is a 50-million-year-old species with whalelike
teeth and a whalelike skull. Its skull possesses neither the
anatomical adaptations for deep diving nor those for hearing
underwater as well as modern whales do, suggesting
Pakicetus used both land and shallow water environments.
Most of its skeleton is still unknown, but the part of its pelvis
that is known shows aquatic adaptations. Whether or not its limbs
and feet were adapted for land or sea won't be known until a more
complete specimen is found. Phil Gingerich of the Museum of
Paleontology at the University of Michigan has found remains of
Pakicetus and many other spectacular whale fossils.
"We are looking for a skeleton of
Pakicetus," he says with a grin, emphasizing the need
for an intact specimen, "knowing that what we find might turn
out to be quite different from what we expect."
Slightly younger Rodhocetus was better adapted to the
water, with ankles like land mammals' that were connected to
enlarged hind feet specialized for swimming. Its front feet retained
land-adapted hooves.
From 45 million years ago, the fossil whale Dorudon had a
less mobile (more fishlike) neck, front legs modified into flippers,
vestigial hind legs and a powerful whale tail. Dorudon
shows that foot-propelled swimming had been superseded by the
tail-propelled swimming that characterizes modern whales. The
features of these three species—plus those of a dozen other
related species—can be used to sketch out the way in which
land species returned to the sea and reevolved their aquatic
adaptations, eventually evolving into whales.
Both transitions, water-to-land and land-back-to-water, occurred in
a mosaic fashion and quite possibly used those intermediate habitats
to which Tiktaalik is adapted. In fish and in whales, first
the shape and design of the head evolved, then the forelimbs and
finally the hindlimbs and tail. Are these parallels meaningful or
simply coincidental? Did other lineages preserved in the far north
evolve from a fishy life to a tetrapodal one in another way? Only
more analysis and more fossils will tell.
Grasping the Brass Link
Finding a missing link is a life's ambition for many
paleontologists, often taking years of hard work, travel to remote
regions and more than a generous dollop of luck. The aftermath of
finding a missing link is more subtle than might be supposed. The
paleontological record will always be sparse compared to the total
number of creatures that ever lived, because fossilization is a very
rare event. Millions of animals are born and die every day, but only
a few of their bodies find habitats suitable for fossilization and
preservation. Of those few creatures that die in the right place at
the right time, many are preserved in places so thinly inhabited by
human beings that no one who could recognize the fossil for what it
is will ever see it. Not only does an organism have to be
fossilized, but it must be found and recognized by a trained eye to
add to the sum of scientific knowledge.
Although the discovery of a missing link is cause for celebration,
it is also cause for more and deeper studies. Ironically, even as
one link is found, two new missing links are
"created"—one the immediate ancestor and one the
immediate descendent of the newly found creature. But slowly, as
discoveries proceed, paleontologists are able to compile an
ever-clearer record of the evolution of life on Earth.
The discovery of Tiktaalik will encourage paleontologists
to continue their intrepid searches in far-flung areas. With luck,
its existence may spark others to rethink their position, too.
Intelligent design advocates and creationists claim that too many
links are missing for evolution to be credible; they see only the
abrupt appearance of new forms created by an Intelligent Designer.
Stephen Meyer, director of the Center for Science and Culture of the
Discovery Institute, asserts that "the transitional life forms
that ostensibly occupy the nodes of Darwin's branching tree of life
are unobservable...."
The highly observable Tiktaalik is exactly the sort of
transitional form that Meyer maintains does not exist. The fossils
are real and solid evidence that you can hold in your hand. If you
are willing to take the time to study their anatomy, you can see for
yourself the evolutionary adaptations that were made over time. This
ancient fish mutely tells a story of mosaic changes, of piecemeal
adaptation to a new ecological niche. It joins myriad other
"found links" that document transitions from one type of
creature to another or from one habitat to another. Together these
found links form a stony edifice in support of evolutionary theory.