Data from: Cope’s rule and the adaptive landscape of dinosaur body size evolution

The largest known dinosaurs weighed at least 20 million times as much as the smallest, indicating exceptional phenotypic divergence. Previous studies have focused on extreme giant sizes, tests of Cope's rule, and miniaturization on the line leading to birds. We use non-uniform macroevolutionary models based on Ornstein–Uhlenbeck and trend processes to unify these observations, asking: what patterns of evolutionary rates, directionality and constraint explain the diversification of dinosaur body mass? We find that dinosaur evolution is constrained by attraction to discrete body size optima that undergo rare, but abrupt, evolutionary shifts. This model explains both the rarity of multi-lineage directional trends, and the occurrence of abrupt directional excursions during the origins of groups such as tiny pygostylian birds and giant sauropods. Most expansion of trait space results from rare, constraint-breaking innovations in just a small number of lineages. These lineages shifted rapidly into novel regions of trait space, occasionally to small sizes, but most often to large or giant sizes. As with Cenozoic mammals, intermediate body sizes were typically attained only transiently by lineages on a trajectory from small to large size. This demonstrates that bimodality in the macroevolutionary adaptive landscape for land vertebrates has existed for more than 200 million years.

目前已知体型最大的恐龙，其体重至少为体型最小恐龙的2000万倍，这反映出极强的表型分化（phenotypic divergence）。此前的相关研究多聚焦于极端巨型体型、科普法则（Cope's rule）的验证，以及通向鸟类的支系上的体型小型化现象。本研究采用基于奥恩斯坦-乌伦贝克过程（Ornstein–Uhlenbeck）与趋势过程的非均匀宏观演化模型，对上述观测结果进行整合分析，并提出核心科学问题：究竟是何种演化速率、方向性与约束模式，能够解释恐龙体重的多样化演化历程？研究结果显示，恐龙的演化受到向离散体型最优值的吸引约束，这些最优值会发生罕见但突发的演化偏移。该模型既解释了多支系方向性演化趋势的稀有性，也阐明了尾综骨类小型鸟类与巨型蜥脚类等类群起源过程中突发方向性偏移现象的成因。绝大多数性状空间（trait space）的拓展，仅源于少数支系中罕见的突破演化约束的创新事件。这些支系会快速迁入性状空间的全新区域，偶尔会演化出小型体型，但多数情况下会朝向大型或巨型体型的方向发展。与新生代哺乳动物的演化规律一致，从中型体型向大型体型演化的支系，通常仅会短暂处于中等体型阶段。这一发现表明，陆生脊椎动物宏观演化适应性景观（adaptive landscape）的双峰分布模式，已经存在了超过2亿年。