DISSERTATION RESEARCH: Macroevolutionary drivers of digit reduction in fossil horses

这项研究利用丰富的马家族化石记录，结合马进化过程中栖息地变化和解剖学的信息，来测试足趾减少的原因和后果。家养的马和它们的亲戚（斑马和驴）是大型的食草动物，四肢细长，末端只有一个手指（蹄）。今天的演变？马的进化传统上被解释为对草原的简单持续适应：随着草原逐渐取代森林，早期狗大小的三趾马进化成大型单趾食草动物。他们的牙齿也变得更长，以处理粗糙的草，而他们的手指减少，以便他们可以更快或更有效地跑过开阔的空间。然而，这种教科书式的解释过于简单化，而且在很大程度上未经检验。这项研究将揭示一个重大的进化转变，最终在人类之一？最具文化和经济价值的物种。研究结果将在哈佛自然历史博物馆（HMNH）展出，该博物馆的马化石展览目前展示了过于简化的进化模式。他们还将通过旨在与公众参与的讲座分享，以及将该项目带入课堂的HMNH教师教育活动。各种各样的假设已被提出的选择性压力驱动数字减少在马血统：速度捕食者逃逸，经济的运动在开放的草原资源稀缺，补偿身体的大小增加。然而，没有一个是用现代方法明确测试过的。通过整合化石出现，性状变化，系统发育关系，生态和环境的信息，该项目旨在直接测试马趾减少的潜在进化原因。具体而言，本研究将使用生态学和系统发育比较方法来评估以下问题：1）三趾马和单趾马在空间和时间上是否重叠，或者它们是否占据单独的生态位？2)马的趾形态、体重、牙齿高度或栖息地的变化是否与马的发育相关？3)足趾形态或环境的变化会影响化石记录中的物种形成和灭绝率吗？该项目的结果将为化石马的足趾减少的原因提供定量支持（或缺乏定量支持），填补了我们理解这一重大进化转变发生原因的关键空白。

This study uses the rich fossil record of the horse family, combined with information about changing habitats and anatomy in horse evolution, to test the causes and consequences of digit reduction. Domestic horses and their relatives (zebras and donkeys) are large, grazing animals with slender limbs that end in a single digit (the hoof). The evolution of today?s horses is traditionally explained as simple ongoing adaptation to grasslands: as grasslands gradually took over from forests, the early dog-sized, three-toed horses evolved into large, single-toed grazers. Their teeth also became longer to deal with abrasive grass, while their digits reduced so they could run faster or more efficiently across open spaces. However, this textbook explanation is oversimplified and remains largely untested. This study will shed light on a significant evolutionary transition that culminated in one of humans? most culturally and economically important species. Results will be featured in the Harvard Museum of Natural History (HMNH), whose fossil horse exhibit currently shows the oversimplified pattern of evolution. They will also be shared via lectures designed to engage with members of the public, as well as teacher education events at the HMNH that will bring this project into the classroom. A variety of hypotheses have been proposed for the selective pressures driving digit reduction in the horse lineage: speed for predator escape, economy of movement over open grasslands with scarce resources, and compensation for increasing body size. However, none have been explicitly tested with modern methods. By integrating information on fossil occurrences, trait change, phylogenetic relationships, ecology, and environment, this project aims to directly test potential evolutionary causes of digit reduction in horses. Specifically, this research will use ecological and phylogenetic comparative methods to assess the following questions: 1) Did tridactyl and monodactyl horses overlap in space and time or did they occupy separate niches? 2) Are changes in digit morphology, body mass, tooth height, or habitat correlated across the horse phylogeny? 3) Do changes in digit morphology or environment influence speciation and extinction rates in the fossil record? The results of this project will provide quantitative support (or lack thereof) for the cause of digit reduction in fossil horses, filling in a key gap in our understanding of why this major evolutionary transition happened.