Dinosaurology, pt 2

The world has existed for at least 4,600 million years, and fossils have been found dating back 3,800 million years.

Evidence for complex life forms does not appear until about 600 million years ago.

Dinosaue fossils have, to date, only been found in rocks of the Mesozoic era, which extends from 245 to 65 million years ago.

Unfortunately, dinosaur fossils are rather rare. Shelly fossils , such as those of clams and snails, are more abunfant and wide spread.

Anything that was once alive can leave fossil traces, but some organisms are better candidates for fossilization than others. Usually, only the hard parts - such as bones - end up as fossils. Muscles, skin and internal organs are rarely preserved.

The shells of animals such as clams and snails an dthe bones of vertebrates are much more likely to be prserved than are the bodies of soft animals such as worms and jellyfish.

There are a number of ways fossils can form.

1. Most fossils involve watery environments and result from the burial of an organism's remains in the sediments of a river, lake or sea. Once the soft tissues have rotted away, the bones or shell become encased in the surrounding muds and silts. As time passes, these sediments harden into rocks, and the bones or shells that are trapped within create an impression of their once-living form.Sometimes the actual remains are totally replaced, cell by cell, with minerals that wash through the enveloping rock. This is called PETRIFICATION.

In other cases, the whole bone is dissolved, leaving behind a hole-a natural mold in the rock that can later fill up with minerals.

2. Other fossils are created in less usual ways. Insects and small animals can become trapped in tree sap that eventually hardens into the semiprecious stone amber and seals in a perfect copy of the entombed animal.

Sometimes, the mineral silica can fill the impressions in the rock left behind by an animal, resulting in a fossil shell or skeleton that glitters with the fire of precious opal.

On rare occasions, the scalding ashes from a volcano can encase a creature. The resulting fossil is a cavity in the shape of the creature.

By far the greatest number of fossils are the remains of shelled creatures that lived in shallow seas. Corals, clams, snails and a host of other invertebrate (lacking backbone) animals make up the bulk of the world's fossil collection.