Passage IV

NATURAL SCIENCE: This passage is adapted from the article "Tales from the Pit"by Andrew Curry(2014 by Smithsonian Institution).

The Messel Pit, located in central Germany, is known for its fossils of mammals from the Eocene epoch.

At some point around 50 million years ago, underground water came into contact with a vein of molten rock. High-pressure steam erupted, forming a crater with steep sides. As water seeped in, it created a lake shaped more like a drinking glass than a soup bowl. Any animal that fell in sank quickly to the bottom.

Still, that alone doesn't explain why so many land mammals-not to mention birds, bats and insects-perished in the lake that became the Messel Pit. One theory is that carbon dioxide periodically bubbled up from deep beneath the lake bottom, smothering animal near the shore. Another possibility is that some of the summer algae blooms were toxic, poisoning animals that had chosen the wrong time and place to slake their thirst. Or perhaps smaller animals died nearby and were washed in by small floods or rushing streams.

The lake was so deep that oxygen didn't circulate near the bottom, which meant that there were no bottom feeders around to consume the dead and dying animals. Year after year, algae scumming the lake surface bloomed and died and so layers of fine clay and dead micro-organisms drifted to the bottom. Each layer was as thick as a strand of hair. It took 250 years to build up an inch of mud. Over millions and millions of years, plants and animals were preserved like flowers pressed between the pages of a book, and the algae and other organic matter turned into oil shale.

Among the thousands of fossils that paleontolo- gists have recovered at Messel Pit are specimens repre senting nearly 45 different mammal species. Those finds are critical to understanding how warmblooded creatures evolved. Mammals and dinosaurs appeared at nearly the same time around 200 million years ago. But dinosaurs were so well suited to the environment that they crowded out any competition. Mammals lived on the margins, mostly tiny creatures eking out a living b eating insects under the cover of darkness. "They just tried to stay out of the way, " says Thomas Lehmann, a Senckenberg Research Institute paleontologist. And so it went for nearly 150 million years.

Then, in an instant, everything changed, apparently when an asteroid or comet struck Earth 66 million years ago and dramatically altered the climate, eventually wiping out the giant reptiles. The diversity of species found among the Messel Pit fossils reveals that mammals rushed to fill every empty ecological nook and cranny they could find. "They really tried every- thing flying, jumping, running, tree-dwelling, ant eating," says Lehmann. "From the point of view o evolution. Messel is a fantastic laboratory to see what life might have given us."

Might have, but in many cases didnt. Messel's most fascinating specimens may be those species that have no living relatives, though they look jarringly familiar. In the visitor center. kids crowd around to watch as a conservator armed with toothbrushes. dental picks and scalpels cleans layers of oil shale away from a fossil unearthed just a few weeks earlier. To me, the skeleton of Ailuravus macrurus looks like that of a giant squirrel. It's thi ree feet long, including its bushy tail. Near the ribs a black stain traces the creatures fossilized digestive tract Despite its tail, Ailuravus is no squirrel ancestor. It's an evolutionary dead end; Ailuravus and all of its relatives died out more than 37 million years ago. Why? Maybe they fell victim to climate changes, or a better-adapted competitor, or dis appearing food sources, or simple bad luck.

Ailuravus' resemblance to a modern squirrel is an example of evolutionary convergence. Given enough time, adaptations may lead to nearly identical solutions-bushy tails, say, or powerful, kangaroo-like hind legs-popping up in different species. "it's like using the same interlocking toy bricks to build different forms," says Lehmann.

And there are forms aplenty at the Messel Pit. The exquisitely preserved fossils have provided paleontolo- gists with unprecedented insights into the adaptive strategies-some successful, others not-adopted by mammals for feeding, movement and even reproduc tion. For instance, the contents of the tiny prehistoric horse's stomach-fossilized leaves and grape seeds- indicate that the animal was not a grazer but a browser, eating what it found on the forest floor. The paleontolo gists also found eight fossilized specimens of pregnant mares, each carrying a single foal. That discovery suggests that the early horses had already adopted herd behavior, since joint care would be the best way to guarantee the survival Il of small numbers of offspring.