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.
英语语法大全:名词性从句知识点总结
英语语法名词性从句知识:应注意的两个问题
英语语法名词性从句知识点:关系代词怎样省略
高三英语听力 听力模拟试题 对话材料(1)
高三英语听力 听力模拟试题 段落材料(3)
英语语法名词性从句知识点:whatever引导主语从句的用法
高三英语语法和惯用法:that ,why 与 because的用法区别
英语语法名词性从句知识点:what与that引导名词性从句区别
英语语法名词性从句知识点:怎样学好名词性从句?
英语语法名词性从句知识点:宾语从句的时态呼应
高三英语听力 听力模拟试题 对话材料(3)
英语语法名词性从句知识点:名词性从句的关键点
英语语法名词性从句知识:主语从句学习指要
英语语法名词性从句知识点:形容词可用作状语吗
高三英语听力 听力模拟试题 段落材料(4)
英语语法名词性从句知识:八种宾语从句不省略that
英语语法名词性从句知识:四组名词性从句
英语语法名词性从句知识点:四类名词性从句详解
英语语法名词性从句知识:同位语边缘知识归纳
英语语法名词性从句知识点:whichever引导主语从句的用法
英语语法名词性从句知识点:whoever与no matter who的区别
英语语法名词性从句知识:概念和用法介绍
英语语法名词性从句知识点:八种宾语从句不省略that
高三英语听力 听力模拟试题 对话材料(4)
英语语法名词性从句知识:名词性从句基础
英语语法名词性从句知识点:whoever引导主语从句的用法
英语语法名词性从句知识点:that, why 与 because 引导表语从句
英语语法名词性从句知识点:whether 和if引导名词性从句
英语语法名词性从句知识:表语从句用法详解
英语语法名词性从句知识点:that与whether引导名词性从句
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