Paleobiology and Extinction of North American Pleistocene Proboscideans

1万年前，在更新世末期，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，它们都是大象的亲戚，都是哺乳动物目长鼻目的成员。古生物学家和考古学家对这种“晚更新世巨型动物灭绝”的原因进行了激烈的争论，并对进化生态学、全球气候变化和生物多样性保护产生了影响。相反的思想流派将灭绝归因于与最后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采用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截然不同的灭绝机制的运作联系起来。研究重点是乳齿象和猛犸象的长牙和臼齿的结构和组成，揭示了这些动物一生中生长速度、个体繁殖史和生活环境的新信息。乳齿象和猛犸象的象牙和大象的象牙一样，在出生后不久就开始生长，并持续一生，不断增加新物质。象牙横截面的显微分析揭示了结构和成分层，有点像树木的年轮，代表了规律的时间间隔（每天，每两周和每年）。臼齿的生长记录较短，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，反映了年际和年内的气候变化，并确认了用于年龄测定的年际特征。对这些模式的分析得出季节性死亡率、年龄、性别、生长率、性成熟年龄和产犊的数据。例如，雄性的性成熟反映为在母系氏族单位将成熟的雄性驱逐后，营养压力导致的生长速度明显下降。在母系氏族单位中，雄性被养育为小牛。母象的产犊史表现为长牙生长速率变化的多年周期，反映了长牙生长与胎儿骨骼发育和产奶量之间的权衡。生活史特征，如成熟年龄和产犊间隔，是生态应激性质和程度的敏感指标，因为它们显示出对改变的营养状况和/或捕食压力的可预测的反应模式。有害的气候和/或植被变化往往会延缓生长和繁殖时间表，而过度狩猎往往会加速它们的生长和繁殖时间表。拟议的研究将使用象牙/牙齿层压和组成的模式来量化北美长鼻目动物在其灭绝时间之前的生活史特征变化。物种内部的这种趋势代表了一个种群自身对变化条件的反应，因此构成了当地生态压力性质的敏感记录。这项工作的目的是测试两个主要的、相互竞争的关于乳齿象和猛犸象晚更新世灭绝的因果假设：过度杀戮和气候/植被变化。本研究结果不仅有助于解决北美生物群历史上的一个重大问题，还将阐明北美人类与长鼻虫相互作用的性质和历史，并为古气候重建提供新的信息。

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.