托福写作高分范文：Fossil Record

英语自学网

　　The preservation of embryos and juveniles is a rate occurrence in the
fossil record. The tiny, delicate skeletons are usually scattered by scavengers
or destroyed by weathering before they can be fossilized. Ichthyosaurs had a
higher chance of being preserved than did terrestrial creatures because, as
marine animals, they tended to live in environments less subject to erosion.
Still, their fossilization required a suite of factors: a slow rate of decay of
soft tissues, little scavenging by other animals, a lack of swift currents and
waves to jumble and carry away small bones, and fairly rapid burial. Given these
factors, some areas have become a treasury of well-preserved ichthyosaur
fossils.
        　　The deposits at Holzmaden, Germany, present an interesting case for
analysis. The ichthyosaur remains are found in black, bituminous marine shales
deposited about 190 million years ago. Over the years, thousands of specimens of
marine reptiles, fish and invertebrates have been recovered from these rocks.
The quality of preservation is outstanding, but what is even more impressive is
the number of ichthyosaur fossils containing preserved embryos. Ichthyosaurs
with embryos have been reported from 6 different levels of the shale in a small
area around Holzmaden, suggesting that a specific site was used by large numbers
of ichthyosaurs repeatedly over time. The embryos are quite advanced in their
physical development; their paddles, for example, are already well formed. One
specimen is even preserved in the birth canal. In addition, the shale contains
the remains of many newborns that are between 20 and 30 inches long.
        　　Why are there so many pregnant females and young at Holzmaden when they are
so rare elsewhere? The quality of preservation is almost unmatched and quarry
operations have been carried out carefully with an awareness of the value of the
fossils. But these factors do not account for the interesting question of how
there came to be such a concentration of pregnant ichthyosaurs in a particular
place very close to their time of giving birth.