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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

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 Dorudon shows that foot-propelled swimming had been superseded by the tail-propelled swimming that characterizes modern whales.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.




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