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.