Data from: Millions of years behind: slow adaptation of ruminants to grasslands

The Late-Cretaceous appearance of grasses, followed by the Cenozoic advancement of grasslands as dominant biomes, has contributed to the evolution of a range of specialized herbivores adapted to new diets, as well as to increasingly open and arid habitats. Many mammals including ruminants, the most diversified ungulate suborder, evolved high–crowned (hypsodont) teeth as an adaptation to tooth–wearing diets and habitats. The impact of different causes of tooth wear is still a matter of debate, and the temporal pattern of hypsodonty evolution in relation to the evolution of grasslands remains unclear. We present an improved time–calibrated molecular phylogeny of Cetartiodactyla, with phylogenetic reconstruction of ancestral ruminant diets and habitats, based on characteristics of extant taxa. Using this timeline, as well as the fossil record of grasslands, we conduct phylogenetic comparative analyses showing that hypsodonty in ruminants evolved as an adaptation to both diet and habitat. Our results demonstrate a slow, perhaps constrained, evolution of hypsodonty towards estimated optimal states, excluding the possibility of immediate adaptation. This augments recent findings that slow adaptation is not uncommon on million–year time scales.白垩纪晚期草本植物的出现，以及随后新生代时期草原跃升为优势生物群系，推动了一系列适应全新食性与愈发开阔干旱生境的特化植食动物的演化。包括物种多样性最高的有蹄类亚目——反刍动物在内的诸多哺乳类，演化出高齿冠（hypsodont）牙齿，以适应易磨损的食性与生境。目前，关于牙齿磨损不同诱因的影响仍存在争议，而高齿冠性状演化与草原扩张之间的时间模式仍不明确。本研究基于现存类群的特征，构建了优化后的鲸偶蹄目（Cetartiodactyla）时间校准分子系统发育树，并对反刍动物祖先的食性与生境进行了系统发育重建。结合该时间线与草原化石记录，我们开展了系统发育比较分析，结果表明反刍动物的高齿冠性状演化是对食性与生境的共同适应。研究结果显示，高齿冠性状朝着预估最优状态的演化过程缓慢且可能受到约束，排除了快速适应性演化的可能性。这一发现进一步支持了近期研究结论——在百万年的时间尺度上，缓慢适应性演化并非罕见现象。