标签：话题作文 话题 高中英语作文
字数：2000字
提要： 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 fos..