Rare FossilRecord The preservation of embryos and juvenilesis a rare occurrence in the fossil record.The tiny,delicate skeletons areusually scattered by scavengers(食腐动物) weathering beforethey can be fossilized.Ichthyosaurs(鱼龙) had a higher chance of being preserved.thandid terrestrial creatures because,as marine animals,they tended to live inenvironments less subject to erosion.Still,their fossilization required a suiteof factors: a slow rate of decay of soft tissues,little scavenging by otheranimals,a lack of swift currents and waves to jumble and carry away smallbones,and fail rapid burial.Given these factors,some areas have become atreasury of well-preserved ichthyosaur fossils. The deposits at Holzmaden,Germany,presentan interesting case for ysis.The ichthyosaur remains are found inblack,bituminous marine shales(页岩)deposited about 190million years age.Over the years,thousands of' specimens of marinereptiles,fish and invertebrates have been recovered from these rocks.Thequality of preservation is oiitstanding,but what is even more impressive is the number ofichthyosaur fossils containing preserved emloryos. Ichthyosaurs with embryos have beenreported from 6 different levels of the shale in a small area aroundHolzmaden,suggesting that a specific site was used by large numbers ofichthyosaurs repeatedlyover time.Theembryos are quite advanced in their physical development; their paddles,forexample,are already well formed.One specimen is even preserved in the birthc.In addition, the shale contains theremains of many newborns that are between 20 and 30 inches long. Why arc there so many pregnant females andyoung at Holzmaden when they are so rare elsewhere? The quality of preservationis almost unmatched and quarry operations have been canrried out carefully withan awareness of the value of the fossils.But these factors do not account forthe interesting question of how there came to be such a concentration ofpregnant ichthyosaurs in a particular place very closes to their time of givingbirth.12. Which of the following is true of the fossil deposits discussed in the passage?