Several scenarios have been proposed to explain rapid net size increases in some early Cenozoic mammalian lineages: sustained and gradual directional change, successive occupation of adaptive zones associated with progressively larger body sizes, and nondirectional evolution associated with branching events in combination with higher diversification potential of the larger lineages. We test these hypotheses in brontotheres, which are among the first radiations of mammals that consistently evolved multitonne sizes. Body-mass evolution in brontotheres mainly occurred during speciation and had no preferential direction. Long-term directional change stemmed from the higher survival of larger lineages in less-saturated herbivore guilds. Our study emphasizes the role of differential species proliferation in explaining the long-term phenotypic trends observed in the fossil record, which are more than an accumulation of steady microevolutionary changes.

中文翻译：

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巨型食草动物的宏观进化途径

人们提出了几种情景来解释一些早期新生代哺乳动物谱系的净尺寸快速增加：持续和逐渐的方向变化、与逐渐变大的体型相关的适应区的连续占据、与分支事件相关的非定向进化以及更高的多样化潜力。更大的血统。我们在雷兽中测试了这些假设，雷兽是最早进化出数吨大小的哺乳动物之一。雷兽的体重演化主要发生在物种形成过程中，没有优先方向。长期的方向性变化源于饱和度较低的食草动物群体中较大谱系的较高存活率。我们的研究强调了差异物种增殖在解释化石记录中观察到的长期表型趋势方面的作用，这不仅仅是稳定的微进化变化的积累。