The preservation of embryos and juveniles is
rare 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 if 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 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.
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