sample essays of IELTS

14 A Rare Fossil Record

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.