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.
罕見的化石記錄
胚胎與幼體被保存下來在化石記錄中是少見的事情。 微小纖細的骨骼通常在石化前就被食腐肉的動物拆散了，或者被風化作用破壞掉了。 魚龍比起陸地的動物有更大的幾率被保存下來，因為它們作為海洋動物常生活在腐蝕性較小的環境中。 但是它們的石化需要一系列因素：軟組織的腐爛速度緩慢，很少被其他動物殘食，缺少混雜、沖走小骨頭的快速水流和波浪，以及相當快地被掩埋。 當這些因素存在時，某些地區就會變成一個充滿保存完好的魚龍化石的寶庫。
在德國獲爾茲梅登，那兒的沉積物給人們提出了一個有趣的分析案例。 人們在黑色的、含瀝青的海洋頁岩中發現了約19，000年前沉積下來的魚龍化石。 幾年時間內，在這些岩石中取得了數以千計的海洋爬行動物、魚類以及無脊椎動物的標本。 它們的保存質量非常的好，但更令人稱奇的是保存下來的育有胚胎的魚龍化石數目。 在獲爾茲梅登附近一個小地區的六個不同的頁岩層中分別發現了育有胚胎的魚龍化石。 這表明大量的魚龍經年累月重複使用一個特定的地點。 那些胚胎已經發育得相當完整了。 比如，它們的蹼槳已經完全形成了。 有一個標本甚至被保存在產道中。 而且，那塊頁岩包含著很多在20到30英寸之間的新生幼體的化石。
為什麼在其他地方那么稀少的懷孕雌獸和幼體在獲爾茲梅登卻那么多呢? 因為其保存質量幾乎舉世無雙，採集工作的進行一直是一絲不苟的。 大家都認識到這些化石的價值極其珍貴，但這些因素並不能解釋這個有趣的問題： 為什麼在一個特定的地點會如此集中地出現即將臨產的懷孕魚龍群呢?