●首页 加入收藏 网站地图 热点专题 网站搜索 [RSS订阅] [WAP访问]  
语言选择:
英语联盟 | www.enun.cn
英语学习 | 英语阅读 | 英语写作 | 英语听力 | 英语语法 | 综合口语 | 考试大全 | 英语四六 | 英语课堂 | 广播英语 | 行业英语 | 出国留学
品牌英语 | 实用英语 | 英文歌曲 | 影视英语 | 幽默笑话 | 英语游戏 | 儿童英语 | 英语翻译 | 英语讲演 | 求职简历 | 奥运英语 | 英文祝福
背景:#EDF0F5 #FAFBE6 #FFF2E2 #FDE6E0 #F3FFE1 #DAFAF3 #EAEAEF 默认  
阅读内容

A Rare Fossil Record

[日期:2008-02-05]   [字体: ]

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.

   免责声明:本站信息仅供参考,版权和著作权归原作者所有! 如果您(作者)发现侵犯您的权益,请与我们联系:QQ-50662607,本站将立即删除!
 
阅读:

推荐 】 【 打印
相关新闻      
本文评论       全部评论
发表评论

点评: 字数
姓名:
内容查询

热门专题
 图片新闻