Raw isotope values from fossil herbivore enamel from sites located below 37° latitude within the contiguous United States

Paleoecological interpretations are based on our understanding of dietary and habitat preferences of fossil taxa. While morphology provides approximations of diets, stable isotope proxies provide insights into the realized diets of animals. We present a synthesis of the isotopic ecologies (δ13C from tooth enamel) of North American mammalian herbivores since ~7 Ma. We ask: 1) Do morphological interpretations of dietary behavior agree with stable isotope proxy data?; 2) Are grazing taxa specialists, or is grazing a means to broaden the dietary niche?; and, 3) How is dietary niche breadth attained in taxa at the local level? We demonstrate that while brachydont taxa are specialized as browsers, hypsodont taxa often have broader diets that included more browse consumption than previously anticipated. It has long been accepted that morphology imposes limits on diet; this synthesis supports prior work that herbivores with “grazing” adaptions, such as hypsodont teeth, have the ability to consume grass but are also able to eat other foods. Notably, localized dietary breadth of even generalist taxa can be narrow (~30 to 60% of a taxon’s overall breadth). This synthesis demonstrates that “grazing-adapted” taxa are varied in their diets across space and time, and this flexibility may reduce competition among ancient herbivores.

Methods
Isotopic data include all published stable isotope analyses of carbon from the carbonate portion of tooth enamel hydroxylapatite (δ13C) from herbivorous mammals (i.e., Perissodactyla, Artiodactyla, and Proboscidea) since the late Miocene (~7 million years ago) that occur in the contiguous United States below 37 °  latitude. Bulk data (1 sample taken parallel to a tooth’s growth axis, per individual, typically < 1 cm in length) and average values from serially sampled teeth (i.e., a series of samples taken perpendicular to a tooth's growth axis) were gathered via a Web of Science search using key words that included isotope, fossil, teeth, and other iterations of these words.

Newly acquired samples for this study (indicated in Dataset S1) were collected from the tooth enamel of late-erupting teeth using a low speed drill and carbide dental burrs. The powdered enamel was pre-treated with 30% hydrogen peroxide for at least 24 hours and 0.1N acetic acid for 18 hours, to remove organics and secondary carbonates, respectively. Samples (~1mg) were analyzed in the Department of Geosciences at the University of Florida. Analytical precision is ±0.1‰ based on replicate analyses of samples and NBS-19 standards. Stable isotope data were normalized to NBS-19 and are reported in conventional delta (δ) notation for carbon obtained from enamel (δ13C), where δ13C (parts per mil, ‰) = ((Rsample/Rstandard)-1)*1000, and R= 13C/12C. The standard is VPDB (Pee Dee Belemnite, Vienna Convention).

古生态学解释（Paleoecological interpretations）基于我们对化石类群（fossil taxa）食性与栖息地偏好的认知。形态学手段虽可对食性作出近似推断，但稳定同位素代用指标（stable isotope proxies）能够揭示动物的实际食性（realized diets）。本研究整合了约7 Ma以来北美植食性哺乳动物牙釉质（tooth enamel）的碳同位素（δ¹³C）生态数据。我们提出三个研究问题：其一，食性行为的形态学解释是否与稳定同位素代用指标数据一致？其二，放牧类群为特化类群，还是"grazing"行为可拓宽其食性生态位？其三，局部尺度下类群的食性生态位宽度是如何形成的？

研究证实，低冠齿（brachydont）类群为特化的食叶动物，而高冠齿（hypsodont）类群的食性往往更宽泛，其取食的叶片类食物占比远超此前预期。长期以来学界公认形态学对食性存在限制，本整合研究验证了此前的结论——带有"grazing"适应性特征（如高冠齿）的植食动物不仅能够取食草本植物，同时也可摄食其他食物。值得注意的是，即便广食性类群，其局部尺度的食性宽度也可能较为狭窄（仅为该类群总生态位宽度的30%至60%）。本整合研究表明，"grazing-adapted"类群在不同时空尺度下的食性存在差异，这种食性灵活性或许可降低古代植食动物间的种间竞争。

## 研究方法
本研究的同位素数据涵盖晚中新世（late Miocene，约700万年前）以来产于美国本土37°纬度以南地区的植食性哺乳动物，即奇蹄目（Perissodactyla）、偶蹄目（Artiodactyla）及长鼻目（Proboscidea）牙釉质羟磷灰石碳酸盐部分的已发表碳稳定同位素分析结果。通过Web of Science数据库，以“同位素”“化石”“牙齿”及相关关键词进行检索，收集了单样本（每个个体取1个平行于牙齿生长轴的样本，长度通常小于1cm）的批量数据，以及序列采样牙齿（即垂直于牙齿生长轴采集的一系列样本）的平均值。

本研究新增的样本（详见数据集S1（Dataset S1））采用低速钻头与碳化钨牙科钻针，从晚萌出牙齿的牙釉质中采集。粉末状牙釉质先后经30%过氧化氢浸泡至少24小时、0.1N乙酸浸泡18小时，分别去除有机质与次生碳酸盐。约1mg的样品在佛罗里达大学地球科学系进行分析。基于样品与NBS-19标准物质的重复分析，本研究的分析精度为±0.1‰。稳定同位素数据以NBS-19为基准进行归一化，采用常规δ标记法报告牙釉质碳同位素（δ¹³C），其中δ¹³C（单位为千分点，‰）= ((R样本/R标准)-1)×1000，R为¹³C/¹²C比值。本次研究的标准物质为VPDB（维也纳佩德贝莱姆石标准，Vienna Pee Dee Belemnite）。