罕见的化石记录

1、罕见的化石记录(Rare fossil record)罕见的化石记录the preservation of embryos and juveniles is a rate occurrenee in the fossil record. the tiny, delicate skelet ons are usually scattered by scave ngers or destroyed by weatheri ng before they can be fossilized. ichthyosaurs had a higher chanee of being preserved tha

2、n did terrestrial creatures because, as marine animals, they tended to live in environments less subject to erosi on. still, their fossilizatio n required a suite of factors:a slow rate of decay of soft tissues, little scavenging by otheranimals, a lack of swift currents and waves to jumble and carr

3、y away small bones, and fairly rapid burial. given these factors, some areas have bee a treasury of well-preserved ichthyosaur fossils.the deposits at holzmade n, germa ny, prese nt an in teresti ng case for an alysis. the ichthyosaur rema ins are found in black, bitu minous marine shales deposited

4、about 190 milli on years ago. over the years, thousa nds of specimens of marine reptiles, fish and invertebrateshave beenrecovered from these rocks. the quality of preservatio n is outsta nding, but what is eve n more impressive is the nu mber of ichthyosaur fossils containing preserved embryos. ich

5、thyosaurs with embryos have been reported from 6 differe nt levels of the shale in a small area around holzmade n, suggesti ng that a specific site was used by large nu mbers of ichthyosaurs repeatedly over time. the embryos are quite advaneed in their physical development; their paddles, for exampl

6、e, are already well formed. one specime n is eve n preserved in the birth can al. in additi on, the shale contains the rema ins of many n ewbor ns that are betwee n 20 and 30 in ches long.why are there so many preg nant females and young at holzmade n whe n they are so rare elsewhere? the quality of

7、 preservationis almostunm atched and quarry operati ons have bee n carried out carefully with an aware ness of the value of the fossils. but these factors do not acco unt for the interestingquestion of how there cameto be such a concentrationof preg nant ichthyosaurs in a particular place very close

8、 to their time of giving birth.罕见的化石记录胚胎与幼体被保存下来在化石记录中是少见的事情。微小纤细的骨骼通常在石化前就被食腐肉的动物拆散了，或者被风化作用破坏掉了。鱼龙比起陆地的动物有更大的几率被保存下来，因为它们作为海洋动物常生活在腐蚀性较小的 环境中。但是它们的石化需要一系列因素：软组织的腐烂速度缓慢，很少被其 他动物残食，缺少混杂、冲走小骨头的快速水流和波浪，以及相当快地被掩埋。 当这些因素存在时，某些地区就会变成一个充满保存完好的鱼龙化石的宝库。在德国获尔兹梅登，那儿的沉积物给人们提出了一个有趣的分析案例。人们在黑色的、含沥青的海洋页岩中发现了约 19,

9、 000年前沉积下来的鱼龙化石。 几年时间内，在这些岩石中取得了数以千计的海洋爬行动物、鱼类以及无脊椎动物的标本。 它们的保存质量非常的好，但更令人称奇的是保存下来的育有胚胎 的鱼龙化石数目。在获尔兹梅登附近一个小地区的六个不同的页岩层中分别发 现了育有胚胎的鱼龙化石。这表明大量的鱼龙经年累月重复使用一个特定的地 点。那些胚胎已经发育得相当完整了。比如，它们的蹼桨已经完全形成了。有一个标本甚至被保存在产道中。而且，那块页岩包含着很多在 20到30英寸之间的新生幼体的化石。为什么在其他地方那么稀少的怀孕雌兽和幼体在获尔兹梅登却那么多呢?因为其保存质量几乎举世无双，采集工作的进行一直是一丝不苟的。大家都认识到这些化石的价值极其珍贵，但这些因素并不能解释这个有趣的问题：为什么在一个特定的地点会如此集中地出现即将临产的怀孕鱼龙群呢 ？-三挥关虫龄三生的三三壬合起案咗以理的教三研:改：:宰关:器：?个作公正、公本着评价减境学生负担 f -改上狠*。校赛紀组加虽步三:Tif,本升芸兰亍弐.3写.“文并一学模：芯主；提急活动按二历HvS-s，认真完三丑三综情的去方三专识要到耳：为顾壬三文