尽管研究人员被警告不要看某些马的口腔，但他们还是收集了有关在以往的5550万年中，北美的马的牙齿进化的大量的数据。 一项新的报告显示，牙齿磨损度（这是对磨牙的尖利度及铲磨度的描绘，人们可用其来测量一匹马口腔中其磨牙及其它牙齿因为食物而磨损的程度）的变化趋势与古时候全球气温冷却事件及植被的变化有关。 当马在跨越千年时间的演化中，它们的磨牙缓慢地由低冠磨牙变化成为高冠磨牙；研究人员认为这一事实与草原的扩大及靠近地面的对牙齿磨损度更大的食物有关联。 现在，Matthew Mihlbachler及其同事说，在演化进程中牙齿磨损度的增加确实与这种磨牙高度的增加有关联。 但是，古时候大多数的马的种群显示了其食物磨损性的高度可变性，这提示牙冠高度增加的选择压力在大多数的时间可能都是弱的，即马在其整个进化中由于获取食物方便而生活得相当轻松。 但是，研究人员确实发现，马在其整个历史中有某些时候的进食草的压力一定很高，其中包括在中新世早期（即马亚科刚出现之前），当时马亚科动物中才真正开始流行那些高冠磨牙。（生物谷Bioon.com）
生物谷推荐原文出处：
Science DOI: 10.1126/science.1196166
Dietary Change and Evolution of Horses in North America
Matthew C. Mihlbachler1,2,*, Florent Rivals3, Nikos Solounias1,2, and Gina M. Semprebon4
Abstract
The evolution of high-crowned molars among horses (Family Equidae) is thought to be an adaptation for abrasive diets associated with the spread of grasslands. The sharpness and relief of the worn cusp apices of teeth (mesowear) are a measure of dietary abrasion. We collected mesowear data for North American Equidae for the past 55.5 million years to test the association of molar height and dietary abrasion. Mesowear trends in horses are reflective of global cooling and associated vegetation changes. There is a strong correlation between mesowear and crown height in horses; however, most horse paleopopulations had highly variable amounts of dietary abrasion, suggesting that selective pressures for crown height may have been weak much of the time. However, instances of higher abrasion were observed in some paleopopulations, suggesting intervals of stronger selection for the evolution of dentitions, including the early Miocene shortly before the first appearance of Equinae, the horse subfamily in which high-crowned dentitions evolved.
