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.
英语六级晨读美文100篇:What Every Writer Wants(91)
英语六级晨读美文100篇:Rain of Seattle II(39)
英语六级晨读美文100篇:Stress and Relaxation(55)
英语六级晨读美文100篇:The Fascinating Moonrise(77)
英语六级晨读美文100篇:Kindness of Strangers(66)
英语六级晨读美文100篇:Choosing an Occupation(53)
英语六级晨读美文100篇:Rain of Seattle II(40)
英语六级晨读美文100篇:Of Media(24)
英语六级晨读美文100篇:Napoleon to Josephine(45)
英语六级晨读美文100篇:Olympic Games(36)
名师传授四六级听力绝招,听力材料“浑身是宝”
英语六级晨读美文100篇:The Definition of a Gentleman (63)
英语六级晨读美文100篇:Benjamin Franklin(60)
英语六级晨读美文100篇:Inaugural Speech(70)
英语六级晨读美文100篇:The Price of Perfection(62)
英语六级晨读美文100篇:One Way to Become an Artist(48)
英语六级晨读美文100篇:Sorrow of the Millionaire(51)
英语六级晨读美文100篇:Failure Is a Good Thing(69)
英语六级晨读美文100篇:When Heaven & Earth Kiss(46)
英语六级晨读美文100篇:Address at Gettysburg(52)
英语六级晨读美文100篇:Two Views of the River(43)
英语六级晨读美文100篇:Dining Etiquette When Dating(54)
英语六级晨读美文100篇:The Pain of Youth(Ⅱ)(68)
英语六级晨读美文100篇:Learn to Live in the Present Moment(79)
英语六级晨读美文100篇:Three Days to See(86)
英语六级晨读美文100篇:Washington’s Address(57)
英语六级晨读美文100篇:The Reasons We Fight over Finance(56)
英语六级晨读美文100篇:Beauty Is Meaningless(71)
英语六级晨读美文100篇:Life Lessons(37)
英语六级晨读美文100篇:A Woman’s Tears(33)
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |