Extinctions

Recent news that scientists had recovered actual marrow from a dinosaur fossil aroused much amazement. But the achievement is not a surprise. In fact, it doesn't even set a new record for the oldest pieces of life. Most of us think of fossils as dead things that have been petrified, turned to stone. But that doesn't have to be. A fossil is defined only as any evidence of life from the prehistoric or geologic past that is preserved in the Earth's crust. A prejudice against preservation may have kept scientists from looking for meat in the ancient bones, but now we know better, and a race is on to find ever-older tissues.

Ötzi, the 5,000-year-old man found in an Alpine glacier in 1991, is the best-known example of a frozen fossil. Mammoths and other extinct polar animals are also known from permafrost. These fossils are not as pretty as the food in your freezer, as they undergo a kind of slow mummification in the frozen condition. It's a geologic version of freezer burn in which ice migrates out of the tissues into the surroundings.

The desert preserves dead matter by desiccation. Ancient humans have been naturally mummified this way, such as the 9,000-year-old Nevadan known as Spirit Cave Man. Older material is preserved by various desert packrats, which have the habit of making piles of plant matter cemented into rock-hard bricks by their viscous urine. When preserved in dry caves, these packrat middens can last tens of thousands of years. The beauty of packrat middens is that they can yield deep environmental data about the American West during the late Pleistocene: vegetation, climate, even the cosmic radiation of the times. Similar middens are being studied in other part of the world.

In a few places plant matter has been preserved in sediment for many millions of years. The Clarkia beds of northern Idaho are between 15 and 20 million years old, putting their origin in the Miocene Epoch. Tree leaves can be split from these rocks still displaying their seasonal colors, green or red. Biochemicals including lignins, flavonoids and aliphatic polymers can be extracted from these fossils, and DNA fragments are known from fossil liquidambar, magnolias and tulip trees (Liriodendron).

Mary Schweitzer, the North Carolina State University professor who documented soft tissues in Tyrannosaurus rex leg bones, has been exploring biomolecules in ancient fossils for several years. The presence of those in the 68-million-year-old bones was not the oldest of her finds, but actual tissues of this age are unprecedented. The discovery challenges our notions of how fossils form. Surely more examples will be found, perhaps in existing museum specimens.