什么是史前水栖爬行动物
1. 什么是史前水栖爬行动物
从两栖动物到爬行动物，是脊椎动物演化史上的一次革命性变革。爬行动物是自脊椎动物诞生以来，首次不再完全依赖水体的脊椎动物，它们大大拓展了自己的生存空间 。
从水生到陆生，再到空中，这是脊椎动物所经历的漫长而复杂的演化过程，但是在这个演化过程中，却有一些爬行动物选择了不同的演化路径。它们在陆上度过了无数个难忘的光阴后，终于决定重新返回水里，而不是沿着前辈的道路继续行进。
这样“反常”的演化，或许更多的是出于残酷的生存竞争压力。对于生的渴望，让它们重新回到了竞争相对较小的水里，在那里，它们会有更多的食物，更广阔的生存空间。
我们把这些动物称为水栖爬行动物。
水生爬行动物大约在二叠纪时就已出现，中龙（Mesosaurus）就是最为典型的代表，而其中很大一部分甚至生存到了现代，但是它们集中出现的时间为中生代（距今大约2.5亿年~ 6500 万年）。
本册图书所描述的水生爬行动物正是生存于中生代的水栖爬行动物， 我们称它们为史前水栖爬行动物。
它们是当时水中占据优势的物种，根据不同的身体结构和习性，生存在湖泊、河流、近海、远海、浅海、深海等不同的生存环境中， 并位于当地的食物链顶端。
虽然如此，史前水栖爬行动物依旧没能逃脱灭亡的厄运， 它们的生命在白垩纪末期都走向了终结。 其中，鱼龙目（Ichthyosauria）大约生存了 1.5 亿年，蛇颈龙目（Plesiosauria）生存了 1.85 亿年，而海中巨无霸沧龙科（Mosasauridae）仅生存了大约3000万年。
究竟是什么原因导致强大的史前水栖爬行动物走向灭绝，虽然科学家已经提出了很多假设，但到目前为止并没有定论。
2. 史前水栖爬行动物的分类
史前水栖爬行动物包罗的类别非常广，其中最为重要的有以下几类：鱼龙目（Ichthyosauria），沧龙科（Mosasauridae），海龙目（Thalattosauria），齿龙目（Placodontia），幻龙目（Nothosauroidea），蛇颈龙目（Plesiosauria），原龙目（Protorosauria），鳄形类动物（Crocodyliforms），龟鳖类（Turtles）等。
这些类别在本册图书中均有介绍。
3. 史前水栖爬行动物的特点
重新返回水中的爬行动物，首先在其身体结构上发生了非常大的改变，随之而来的还有它们的生产方式、 行为方式等。 这是一个艰难而又漫长的过程， 不过，要适应新的生存环境，它们别无选择。
史前水栖爬行动物的身体大都具有流线型，就像现在的海豚一般，脖子和身体几乎连成一线，这样有助于它们减少在水中运动的阻力。
因为可以依赖水的浮力，它们不需要再费力地支撑沉重的身体，所以它们的胸骨、肋骨、四肢等都逐渐变细。但它们的脊椎骨会变得相对粗壮，而且能够灵活地摆动，这样才能更有利于它们在水中前行。
它们的四肢在入水后会慢慢变成指（趾）间带蹼的鳍状肢，而它们的尾巴会变成宽鳍，来为它们划水提供足够的动力。
因为身体结构及生存环境的变化，它们的行为方式也会发生很大的改变，最为突出的就是它们的繁殖方式。它们中很大一部分成员不再像普通的爬行动物采用卵生的方式，而是采用全新的卵胎生的方式。不需要将自己的后代包裹在蛋壳中进行孵化，而是直接将它们的幼体产在水中。
4. 史前水栖爬行动物的研究历史
史前水栖爬行动物虽然不像恐龙那样被大众所熟知，但是它们的发现与研究历史却比恐龙要早很多。
早在 1699 年，学者们就描述了发现于英国威尔士的鱼龙化石残片。1708 年，又有两篇文章提到了鱼龙目的脊椎骨化石， 但当时并没有被科学地归为鱼龙目，人们认为它们是属于那些在圣经记载的大洪水中死去的“邪恶人类”。随后，在 1780 年，人们在荷兰发现了沧龙的头骨化石，巨大的头骨化石中清晰可见大而锋利的牙。这真是古生物发掘史上最具纪念意义的时刻之一，人们被这个巨大的头骨吓到了，一度以为它是传说中被勇士杀死的恶龙。
1811 年，玛丽安宁（Mary Anning）在英国南部的莱姆镇发现了一具完整的鱼龙化石，这终于让人们认识到在远古曾经生活着这样一种巨大而奇特的动物。
史前水栖爬行动物就这样进入人们的视线，随后人们又发现了很多鱼龙目、蛇颈龙目等的化石，而那时候人们甚至还没有听说过恐龙这个词，恐龙被理查欧文 （Richard Owen） 正式提出的时候已经是1842年了。
早期对于史前水栖爬行动物的研究大都集中在欧美，中国的研究起步相对较晚。一直到 1942 年，著名古生物学家杨钟健研究了威远中国上龙 （Sinopliosaurus weiyuanensis），中国的史前水栖爬行动物研究的序幕才真正开启。 但是近十年来， 由于贵州龙动物群的发现，而让中国成为这一研究领域的一颗闪耀的新星。这些珍贵的化石，解开了很多困扰科学家许久的谜题。
贵州龙动物群是一个主要由水栖爬行动物及鱼类组成的脊椎动物群，并共生有双壳类、菊石等多种无脊椎动物。它分布广泛、时代延续较长，富含内容丰富的三叠纪海生脊椎动物群，包括鳍龙类、鱼龙类、海龙类和原龙目等。它的发现对科学家研究史前水栖爬行动物各门类的起源、分布、系统发育以及古生物地理，都有着非常重要的意义。
The prehistoric aquatic reptiles
1. What are the prehistoric aquatic reptiles?
The transition from the amphibians to reptiles is a revolutionary change in the evolutionary history of the vertebrates. Ever since the beginning of vertebrates, the reptiles are the first group of vertebrates that have truly freed themselves from an aquatic existence and hence tremendously expanded their living spaces.
The ver tebrates have undergone a long and complicated course of evolution, from the water to the land and then to the air. However, in this evolutionary process, some of the reptiles took a different trajectory. Having spent a long time on the land, they re-entered the water, instead of following their predecessors’ steps to tread the land.
This “reversed” course of evolution might be due more to the brutal “struggle for existence” on the land. Perhaps out of the fond “memories” of a less competitive aquatic environment, they returned to the water where food was more plentiful and living places more accommodating. We called them aquatic reptiles.
The aquatic reptiles appeared around the Permian, and Mesosaurus is a typical representative. A considerable number of them have even survived to this day. However, the majority of them lived during the Mesozoic Era, about 250 to 65 million years ago.
The aquatic reptiles described in this book are the ones that lived in the Mesozoic, and we call them the prehistoric aquatic reptiles.
They were the dominant species in the water at that time. Dependent on their different structures and habits, they lived in different water bodies such as lakes, rivers, coastal waters, open seas, shallow seas, and deep oceans, and were located at the top of the local food chain.
Even so, the prehistoric aquatic reptiles could not escape the fate of demise and eventually became extinct at the end of Cretaceous. Among them, the Ichthyosauria lived for about 150 million years, the Plesiosauria 185 million years, and the ocean giant Mosasauridae only about 30 million years.
What had led the powerful prehistoric aquatic reptiles to extinction? Although speculations abound, scientists have not yet reached consensus.
2. Classification of the prehistoric aquatic reptiles
The prehistoric aquatic reptiles contain diverse groups. The most important ones are as follows: the Ichthyosauria, the Mosasauridae, the Thalattosauria, the Placodontia, the Nothosauroidea, the Plesiosauria, the Protorosauria, the Crocodyliforms, and the Turtles etc.
This book deals with all of these groups.
3. Characteristics of the prehistoric aquatic reptiles
The reptiles that returned to the water must have first changed their anatomical structures along with the changes in the modes of reproduction and behavior. This was a difficult and long process, and they had no choice but strived to adapt to new environment.
The prehistoric aquatic reptiles all had a streamlined body, just like that of the dolphin, with their neck and body joined smoothly into a streamline, which helped reduce the resistance of the water while swimming.
As the buoyancy of the water eased their support of a heavy body, their sternum, ribs, and four limbs gradually became thinner. However, their spinal column became more robust, stronger, and more flexible, which helped them move more easily in the water.
After they returned to the water, their four limbs gradually became webbed ippers and their tail changed into a wide “fin”, which provides sufficient power for pushing themselves through the water.
With changes in the anatomical structures and the living environment came the great changes in their behavior, the most striking being the change in the mode of their reproduction. Many of them no longer laid eggs like other reptiles. Instead they developed ovoviviparity. As a result, they did not have to incubate their babies in the eggs but delivered their babies directly in the water.
4. History of the prehistoric aquatic reptile studies
Although the prehistoric aquatic reptiles are not as well-known as the dinosaurs, their discoveries and studies got started much earlier than those of the dinosaurs.
As early as in 1699, scholars reported the finding of some fragmentary ichthyosaur fossils in Wales, the Great Britain. In 1708, the fossil vertebrae of ichthyosaur were mentioned in two more papers, but these fossils were not yet scientifically recognized as ichthyosaurs. Instead, they were considered the “evil people” who died in the flood as recorded in the Bible. Then in 1780, a large mosasaur skull was discovered in Netherlands, in which large and sharp teeth were clearly observed. It was a rather historic moment in the history of paleontological discoveries, because people were frightened and thought the large skull belonged to the evil dragon killed by a warrior as is told in a legend.
In 1811, Mary Anning found a complete ichthyosaur fossil near her hometown of Lyme Regis in southern England, which brought to light this huge and bizarre animal that had lived in the prehistoric time.
This was how the prehistoric aquatic reptiles were brought to people’s attention. Soon afterwards, many more fossils of the Ichthyosauria and Plesiosauria were discovered, when the word “dinosaur” had not even been coined yet. The word “dinosaur” was proposed by Sir Richard Owen in 1842.
Early studies of the prehistoric aquatic reptiles were done mainly in Europe and the United States, and they did not get started in China until 1942, when renowned Chinese pale
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