Taking the Plunge
In 1995 Christian de Muizon of the National Museum of Natural History in Paris and his colleagues announced the discovery of sloth fossils in Peru dating back somewhere between three and seven million years. The rocks in which they found the bones had formed in the sea; the same rocks have yielded other ocean-going creatures including fish, sea lions, and weird dolphins with walrus-like tusks. The sloths, de Muizon concluded, were aquatic as well. Terrestrial sloths have much longer lower leg bones than upper ones, but the Peruvian sloths had reversed proportions. Manatees and otters also have reversed legs, which suggests that the sloths' limbs were adapted for powerful swimming strokes. The front of their skull was manatee-like as well: its jaws extended out well beyond its front teeth, with a rich supply of blood vessels. Like manatees, de Muizon argued, the sloths had powerful muscular snouts they used to root out sea grass.
In their initial report, the paleontologists dubbed the fossils Thalassocnus natans. But it was already clear that they might have more than one species on their hands. In the years since, they've dug into the Peruvian rocks and found hundreds of sloth fossils, which they have been carefully studying and comparing. The new papers are not the last word on Thalassocnus, but the sloths are already shaping up as a great illustration of a transition to the water.
Carl Zimmer/The Loom
Jun.21.04
Jun.21.04
Brilliant popularization of exciting front-line science. What this is for me is like sight, like gaining depth perception. Millions of years deep.
