UNDERSTANDING REPTILE PALEOECOLOGY: A STABLE ISOTOPE APPROACH

今天，大多数鳄鱼目动物（鳄鱼、短吻鳄、鳄鱼和它们的近亲）生活在淡水到半咸淡水中。只有少数鳄鱼能够忍受海水，使用特殊的盐排泄腺来维持渗透平衡。然而，鳄鱼目动物的地理分布表明，一些鳄鱼类动物在其进化史上穿越了主要的海洋盆地，这使得一些工作人员得出结论，认为它们过去可能更能忍受海水。古生物学家试图确定形态或关系的趋势，以阐明鳄鱼耐盐性的起源和进化，但这些研究在很大程度上尚无定论。同位素方法为探索现代和化石动物的饮食、栖息地偏好和生理提供了有力的工具。我们将利用稳定同位素的自然变化来研究鳄鱼耐盐水性的起源和进化。我们将使用碳和锶同位素变化来监测鳄鱼进食的栖息地，并使用氧同位素变化来确定它们是饮用淡水还是海水。爬行动物还没有广泛使用同位素方法进行研究。因此，我们将仔细研究现代鳄鱼，以确保我们对古代鳄鱼的研究有一个坚实的基础。首先，我们将研究佛罗里达州的圈养鳄鱼。在这种受控环境下，我们可以测量动物摄入的所有水和食物的稳定同位素组成。这将使我们确定每种来源对鳄鱼牙釉质的贡献。然后，我们将在路易斯安那州的一个沿海野生动物保护区研究野生鳄鱼。在这里，我们将测量鳄鱼及其猎物的同位素组成，以更好地了解自然环境引入了多少“噪音”。这两个现代实验将为我们对古代鳄鱼的研究提供信息，并为其他工作人员使用同位素研究现代和化石爬行动物提供基础。最后，我们将使用这些同位素方法来评估不同化石鳄鱼的盐水耐受性和适应性。在大多数情况下，这些化石来自沿海地区，所以它们可能使用过含盐的栖息地。我们将分析在现存鳄鱼主要类群起源附近分类的化石物种。这种方法将揭示这些群体的生态，因为他们开始多样化和辐射。我们的最终目标是确定鳄鱼是一种原始的咸水动物，现在越来越局限于淡水，还是一种原始的淡水动物，有几个衍生的物种可以忍受咸水。

Understanding Reptile Paleoecology: A Stable Isotope ApproachPaul L. Koch, Univ. of California, Santa Cruz Today most crocodilians (crocodiles, alligators, gavials, and their kin) live in fresh to brackish water. Only a few crocodilians are able to tolerate marine water using special salt-excreting glands to maintain osmotic balance. Yet, the geographic distribution of crocodilians indicates that some groups crossed major ocean basins in their evolutionary history, leading some workers to conclude that they may have been more tolerant of salt water in the past. Paleontologists have tried to identify trends in morphology or relationships that would illuminate the origin and evolution of salt-water tolerance in crocodilians, but these studies have been largely inconclusive. Isotopic methods offer powerful tools for exploring the diet, habitat preferences, and physiology of modern and fossil animals. We will study the origin and evolution of salt-water tolerance in crocodilians using naturally occurring variations in stable isotopes. We will use carbon and strontium isotope variations to monitor the habitat in which crocodilians were feeding, and oxygen isotope variations to determine whether they were drinking fresh or marine water. Reptiles have not been studied extensively using isotopic methods. Therefore, we will take a close look at modern crocodilians to ensure that our studies of ancient crocodilians are on a solid foundation. First we will study captive crocodilians in Florida. In this controlled setting, we can measure the stable isotope composition of all the water and food ingested by the animal. This will let us determine the contribution of each source to the tooth enamel of a crocodilian. We will then study wild crocodilians at a coastal wildlife refuge in Louisiana. Here we will measure the isotopic composition of crocodilians and their prey to better understand how much "noise" is introduced by a natural environment. These two modern experiments will inform our study of ancient crocodilians, and serve as a basis for other workers using isotopes to study modern and fossil reptiles. Finally, we will use these isotopic methods to assess the salt-water tolerance and adaptations of different fossil crocodilians. In most cases, these fossils come from coastal settings, so they may potentially have used saline habitats. We will analyze fossil species that are classified near the origins of the major groups of living crocodilians. This approach will shed light on the ecology of these groups as they began to diversify and radiate. Our ultimate goal is to determine if the crocodilians are a primitively salt-water group that has become more restricted to freshwater, or a primitively fresh-water group with a few derived groups that tolerate salt water.