Macroevolution and geochemistry of Cenozoic giant sharks

摘要鲨科鲨鱼化石具有丰富的新生代全球历史，以大白鲨（Carcharodon carcharias）等物种为代表。在中新世，已经灭绝的巨齿鲨达到了真正巨大的体型。据估计，这种已经灭绝的物种的平均体型约为40吨，与一些恐龙和鲸鱼相媲美，是有史以来最大的动物之一。我们想要回答的基本问题是。像巨齿鲨这样的鳍状鲨是如何进化出巨大体型的？相对于体型较小的祖先，巨齿鲨的个体可以通过生长得更快（增加了速度）、更长的时间（增加了寿命），或者这两者的结合来获得非常大的体型。这些生长模式将通过七个已灭绝和现代的Carcharodon物种及其姊妹分类群Isurus与已灭绝的外群Otodus obliquus进行比较来确定。宏观进化是研究在密切相关的物种群体（枝）中观察到的进化模式和过程。为了理解lamids体型的宏观进化，需要确定任何给定个体的两个关键特征：(1)它死的时候有多少岁？这将通过研究保存在椎体中心的化学特征来完成；它有多大？这将由相关的牙齿来确定，它们的尺寸与体型高度相关。鲨鱼的骨骼主要由软骨组成，不容易变成化石。然而，本研究所需的两个元素是这一规则的例外：(1)椎体中心，在生命中用骨代替软骨；(2)坚固的牙齿。虽然保存在中央的增量生长环经常是环状的，但有时也不是。因此，对年轮的简单计数可能会导致错误的个体年龄测定。为了避免这个问题，将使用保存在centra的氧同位素特征所代表的代用季节和年周期来校准每个标本的个体年龄。这将在对中心进行地球化学分析后进行，以确定相对现代的层状岩，成岩作用的程度（石化过程中的蚀变）以及这一过程如何影响化石中心中存档的氧同位素特征。这些分析将包括物理和化学特性的测定，如结晶度、碳酸盐含量、微量元素、微量元素和稀土元素浓度以及氧同位素。我们的初步研究表明：(1)氧同位素特征即使在高度改变的情况下也能保持在中部；(2)已知存在于未改变骨中的某些元素和化合物可以用作示踪剂来模拟化石骨的成岩作用。知识价值。该项目将整合古生物学和地球化学的概念、方法和数据。体型宏观进化是科学家们最感兴趣的，例如，它如何与柯普“定律”（进化分支中体型增加的进化）的概念联系起来。这项研究将提供一个案例研究，使用一个进化出可能是有史以来最大体型的进化枝。本项目的相关地球化学部分将量化骨骼中的成岩作用，并校准化石鲨鱼的增量生长。这些结果将有助于对用于其他已灭绝脊椎动物的古生物学解释的同位素特征进行更明智的解释。更广泛的影响。一名研究生将使用这个项目的一部分作为她的博士研究。将招聘一名本科生研究助理。这项研究将在研究生研讨会上向佛罗里达大学的学生展示，并在全国会议和受邀研讨会上向其他同事/学生展示。K-12教师将参与暑期研究体验。鲨鱼牙齿受到化石收藏家的高度重视，巨齿鲨引起了公众的极大兴趣。这项研究将通过一般讲座、化石节、实地考察、通讯和佛罗里达自然历史博物馆的展览向公众传播。我们将在每年有720万次网络访问（6000万次点击）的网站（www.flmnh.ufl.edu）上进行宣传。

ABSTRACTFossil sharks of the Family Lamnidae have a rich Cenozoic global history and are represented today by species such as the great white (Carcharodon carcharias). During the Miocene, the extinct lamnid shark C. megalodon attained truly enormous proportions. With a mean estimated body size of ~40 tons, this extinct species rivals some dinosaurs and whales as one of the largest animals that ever lived. The fundamental question that we seek to answer is .how did large body size evolve within lamnid sharks like C. megalodon?. Relative to its smaller ancestor, individuals of C. megalodon could have attained very large body size by growing faster (increased rate), for a longer time (increased longevity), or a combination of these two. These growth modes will be determined within the lamnid clade using seven extinct and modern species of Carcharodon and its sister-taxon Isurus compared to the extinct outgroup Otodus obliquus. Macroevolution is the study of evolutionary patterns and processes observed within closely related groups of species (clades). In order to understand lamnid body size macroevolution, two key features need to be determined for any given individual: (1) How many years old was it when it died? This will be done by studying chemical signatures preserved in vertebral centra; (2) How large was it? This will be determined from associated teeth, whose dimensions are highly correlated to body size. Shark skeletons are composed mostly of cartilage, which is not prone to fossilization. Nevertheless, the two elements needed for this study are exceptions to this rule: (1) vertebral centra, which during life replace cartilage with bone; and (2) durable teeth. Although incremental growth rings preserved in centra are oftentimes annular, they sometimes are not. A simple counting of the rings, therefore, could potentially lead to erroneous individual age determinations. To circumvent this problem, proxy seasonal and annual cycles represented by the oxygen isotopic signatures preserved in centra will be used to calibrate the individual age of each specimen. This will be done after centra are analyzed geochemically to determine, relative modern lamnids, the extent of diagenesis (alteration during fossilization) and how this process affected oxygen isotopic signatures archived in fossil centra. These analyses will include = determination of physical and chemical characteristics such as crystallinity, carbonate content, trace, minor, and rare earth elemental concentrations, and oxygen isotopes. Our pilot studies indicate that: (1) oxygen isotopic signatures can remain in centra even when highly altered; and (2) certain elements and compounds known to occur in unaltered bone can be used as tracers to model diagenesis in fossil bone. Intellectual Merit.--This project will integrate concepts, methods, and data from paleobiology and geochemistry. Body size macroevolution is of fundamental interest to scientists, e.g., how it might relate to the concept of Cope's "Law" (evolution of increased body size within clades). This study will provide a case study using a clade that evolved what may be the largest body size ever. The related geochemical part of this project will quantify diagenesis in bone and calibrate incremental growth in fossil sharks. These results will facilitate more informed interpretations of the isotopic signatures used forpaleobiological interpretations in other extinct vertebrates.Broader Impacts. One graduate student will use part of this project for her Ph.D. research. An undergraduate student research assistant will be recruited. This research will be presented to UF students in graduate seminars and to other colleagues/students at national meetings and invited seminars. K-12 teachers will be involved in summer research experiences. Shark teeth are highly prized by fossil collectors and inmegalodonlt evokes much public interest. This research will be disseminated to the public through general lectures, fossil festivals, field trips, newsletters, and exhibits at the Florida Museum of Natural History. The public outreach will be promoted on our website (www.flmnh.ufl.edu), which receives 7.2 million cybervisits (60 million iihitsls) per year.