Missing Links and Found Links
In and out of the water, transitional forms from the fossil record illuminate the nuts and bolts of evolution
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
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shows that foot-propelled swimming had been superseded by the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.