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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.
全文翻譯：
罕見(jiàn)的化石記錄 胚胎與幼體被保存下來(lái)在化石記錄中是少見(jiàn)的事情。微小纖細(xì)的骨骼 通常在石化前就被食腐肉的動(dòng)物拆散了，或者被風(fēng)化作用破壞掉了。 魚(yú)龍比起陸地的動(dòng)物 有更大的幾率被保存下來(lái)，因?yàn)樗鼈冏鳛楹Ｑ髣?dòng)物常生活在腐蝕性較小的環(huán)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(dòng)物殘食，缺少混雜、沖走 小骨頭的快速水流和波浪，以及相當(dāng)快地被掩埋。 當(dāng)這些因素存在時(shí)，某些地區(qū)就會(huì)變成 一個(gè)充滿保存完好的魚(yú)龍化石的寶庫(kù)。在德國(guó)獲爾茲梅登，那兒的沉積物給人們提出了一個(gè) 有趣的分析案例。
人們?cè)诤谏�、含瀝青的海洋頁(yè)巖中發(fā)現(xiàn)了約 19，000 年前沉積下來(lái)的 魚(yú)龍化石。 幾年時(shí)間內(nèi)，在這些巖石中取得了數(shù)以千計(jì)的海洋爬行動(dòng)物、魚(yú)類以及無(wú)脊椎 動(dòng)物的標(biāo)本。 它們的保存質(zhì)量非常的好，但更令人稱奇的是保存下來(lái)的育有胚胎的魚(yú)龍化 石數(shù)目。 在獲爾茲梅登附近一個(gè)小地區(qū)的六個(gè)不同的頁(yè)巖層中分別發(fā)現(xiàn)了育有胚胎的魚(yú)龍 化石。 這表明大量的魚(yú)龍經(jīng)年累月重復(fù)使用一個(gè)特定的地點(diǎn)。 那些胚胎已經(jīng)發(fā)育得相當(dāng)完 整了。 比如，它們的蹼槳已經(jīng)完全形成了。
有一個(gè)標(biāo)本甚至被保存在產(chǎn)道中。 而且，那 塊頁(yè)巖包含著很多在 20 到 30 英寸之間的新生幼體的化石。為什么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?yàn)槠浔４尜|(zhì)量幾乎舉世無(wú)雙，采集工作的進(jìn)行一 直是一絲不茍的。大家都認(rèn)識(shí)到這些化石的價(jià)值極其珍貴，但這些因素并不能解釋這個(gè)有 趣的問(wèn)題： 為什么在一個(gè)特定的地點(diǎn)會(huì)如此集中地出現(xiàn)即將臨產(chǎn)的懷孕魚(yú)龍群呢?
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