A Rare Fossil Record
[size=3][color=#29384e][font=Times New Roman]The preservation of embryos and juveniles is a rate occurrence in the fossil record. The tiny[/font][font=宋体]，[/font][font=Times New Roman] 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[/font][font=宋体]，[/font][font=Times New Roman] as marine animals[/font][font=宋体]，[/font][font=Times New Roman] they tended to live in environments less subject to erosion. Still[/font][font=宋体]，[/font][font=Times New Roman] their fossilization required a suite of factors[/font][font=宋体]：[/font][font=Times New Roman] a slow rate of decay of soft tissues[/font][font=宋体]，[/font][font=Times New Roman] little scavenging by other animals[/font][font=宋体]，[/font][font=Times New Roman] a lack of swift currents and waves to jumble and carry away small bones[/font][font=宋体]，[/font][font=Times New Roman] and fairly rapid burial. Given these factors[/font][font=宋体]，[/font][font=Times New Roman] some areas have become a treasury of well-preserved ichthyosaur fossils.[/font][/color][/size][size=3][color=#29384e][font=Times New Roman] [/font][/color][/size]
[size=3][color=#29384e][font=宋体] [/font][font=Times New Roman]The deposits at Holzmaden[/font][font=宋体]，[/font][font=Times New Roman] Germany[/font][font=宋体]，[/font][font=Times New Roman] present an interesting case for analysis. The ichthyosaur remains are found in black[/font][font=宋体]，[/font][font=Times New Roman] bituminous marine shales deposited about 190 million years ago. Over the years[/font][font=宋体]，[/font][font=Times New Roman] thousands of specimens of marine reptiles[/font][font=宋体]，[/font][font=Times New Roman] fish and invertebrates have been recovered from these rocks. The quality of preservation is outstanding[/font][font=宋体]，[/font][font=Times New Roman] 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[/font][font=宋体]，[/font][font=Times New Roman] suggesting that a specific site was used by large numbers of ichthyosaurs repeatedly over time. The embryos are quite advanced in their physical development[/font][font=宋体]；[/font][font=Times New Roman] their paddles[/font][font=宋体]，[/font][font=Times New Roman] for example[/font][font=宋体]，[/font][font=Times New Roman] are already well formed. One specimen is even preserved in the birth canal. In addition[/font][font=宋体]，[/font][font=Times New Roman] the shale contains the remains of many newborns that are between 20 and 30 inches long.[/font][/color][/size]
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[color=#29384e][font=宋体][size=10.5pt] [/size][/font][size=10.5pt]Why are there so many pregnant females and young at Holzmaden when they are so rare elsewhere[/size][font=宋体][size=10.5pt]？[/size][/font][size=10.5pt] 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[/size][/color]
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