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A Rare Fossil Record
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.
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 19，000 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?