Paleobiology and Extinction of North American Pleistocene Proboscideans

费舍尔9628063 10,000年前，在更新世接近尾声时，许多生活在北美的最大、最令人印象深刻的哺乳动物灭绝了。在这些受害者中，最引人注目的是乳齿象和猛犸象，这两种动物都是大象的近亲，也是哺乳动物鼻目的成员。这种“晚更新世巨型动物群灭绝”的原因一直受到古生物学家和考古学家的激烈争论，这对进化生态学、全球气候变化和生物多样性保护都有影响。对立的学派将物种灭绝归因于与上一个冰河时代结束相关的气候变化、人类的过度狩猎，或者可能是这些因素的某种组合。这里提出的项目继续了一种相对较新的方法来解决这一争议，其特点是能够将明显不同的预测与相反的灭绝机制的操作联系起来。它重点研究了乳齿象和猛犸象的象牙和磨牙的结构和组成，揭示了关于这些动物一生中的生长速度、它们的个体生殖史以及它们生活的环境的新信息。像大象一样，乳齿象和猛犸象的象牙在出生后不久就开始生长，并持续到一生，增加了一层又一层的新材料。对象牙横截面的显微分析显示出结构和组成的薄层，有点像树轮，代表着有规律的时间间隔(每天、每两周和每年)。磨牙的生长记录较短，但在其他方面具有可比性。这些层厚度的变化记录了象牙生长速度的变化，并间接反映了动物的营养和生殖状况。地层组成的变化，特别是氧同位素的变化，反映了年内和年际的气候变化，并确认了用于年龄确定的年度特征。对这些模式的分析得出了季节性死亡率、年龄、性别、生长率、性成熟年龄和产犊的数据。例如，雄性的性成熟表现为，在将成熟的雄性逐出母系家庭单位后，由于营养压力而导致的生长速度明显下降，在母系家庭中，它们是作为小牛被抚养的。雌性的产奶史表现为象牙生长速度的多年变化周期，反映了象牙生长与胎儿骨骼发育和产奶量之间的权衡。成熟期年龄和产犊间隔等生活史特征是生态压力性质和程度的敏感指标，因为它们显示出对改变的营养状态和/或捕食压力做出反应的可预测模式。有害的气候和/或植被变化往往会延缓生长和生殖时间表，而过度狩猎往往会加速生长和繁殖时间表。这项拟议的研究将使用象牙/牙齿分层和组成的模式来量化导致北美口鼻目动物灭绝时间的生活史特征的变化。在物种内部，这种趋势代表了种群自身对不断变化的条件的反应，因此构成了对当地生态压力性质的敏感记录。这项工作的目标是检验晚更新世乳齿象和猛犸象灭绝的两个相互竞争的主要因果假说：过度捕杀和气候/植被变化。除了有助于解决北美生物群历史上的一个主要问题外，这项研究的结果还将澄清人类与北美鼻目动物相互作用的性质和历史，并提供与古气候重建相关的新信息。

Fisher 9628063 Near the close of the Pleistocene epoch, 10,000 years ago, many of the largest and most impressive mammals inhabiting North America went extinct. Among the most conspicuous of these victims were mastodons and mammoths, both of which were relatives of elephants and members of the mammalian order Proboscidea. The cause of this "late Pleistocene megafaunal extinction" has been vigorously debated by paleontologist and archaeologists, with implications for evolutionary ecology, global climate change, and conservation of biodiversity. Opposing schools of thought attribute the extinction to climate change associated with the end of the last Ice Age, excessive hunting by humans , or possibly some combination of these factors. The project proposed here continues a relatively new approach to resolving this controversy, distinguished by its ability to associate clearly divergent predictions with the operation of contrasting extinction mechanisms. It focuses on the structure and composition of the tusks and molar teeth of mastodons and mammoths, revealing new information about these animal's growth rates throughout life, their individual reproductive histories, and the environments in which they lived. Mastodon and mammoth tusks, like those of elephants, began growing shortly after birth and continued throughout life, adding layer upon layer of new material. Microscopic analysis of tusks cross sections reveals structural and compositional laminae, somewhat like tree rings, representing regular intervals of time (daily, fortnightly, and annual). Records of growth in molar teeth are shorter but otherwise comparable. Variations in the thickness of these layers record changes in tusks growth rate and indirectly reflect the animal's nutritional and reproductive status. Variations in composition of layers, especially with respect to oxygen isotopes, reflect intra- and interannual climate changes and confirm annual features for use in age determination. Analyses of these patterns yield data o n seasonal mortality, age, gender, growth rates, age at sexual maturity, and calving. For example, sexual maturation in males is reflected as a pronounced drop in growth rate caused by nutritional stress following expulsion of maturing males from the matriarchal family units in which they were nurtured as calves. Calving histories of females appear as multiyear cycles of variations in tusk growth rate, reflecting a tradeoff between tusk growth on one hand and fetal bone development and milk production on the other . Life history traits such as age at maturity and calving interval are sensitive indicators of the nature and degree of ecological stress because they show predictable patterns of response to altered nutritional status and/or predation pressure. Deleterious climate and/or vegetational change tends to retard growth and reproductive schedules, while overhunting tends to accelerate them. The proposed research will use patterns of tusk/tooth lamination and composition to quantify changes in life history traits of North America proboscideans leading up to their time of extinction. Such trends, within species, represent a population's own response to changing conditions and thus comprise a sensitive record of the nature of local ecological stress. the goal of this work is to test the two major, competing causal hypotheses for the late Pleistocene extinction of mastodons and mammoths: overkill and climatic/vegetational change. In addition to contributing to the solution of a major problem in the history of the North American biota, the results of this study will clarify the nature and history of human interaction with proboscidean in North America and provide new information relevant to paleoclimate reconstruction.