Data from: Rates of dinosaur limb evolution provide evidence for exceptional radiation in Mesozoic birds

Birds are the most diverse living tetrapod group and are a model of large-scale adaptive radiation. Neontological studies suggest a radiation within the avian crown group, long after the origin of flight. However, deep time patterns of bird evolution remain obscure because only limited fossil data have been considered. We analyse cladogenesis and limb evolution on the entire tree of Mesozoic theropods, documenting the dinosaur–bird transition and immediate origins of powered flight. Mesozoic birds inherited constraints on forelimb evolution from non-flying ancestors, and species diversification rates did not accelerate in the earliest flying taxa. However, Early Cretaceous short-tailed birds exhibit both phenotypic release of the hindlimb and increased diversification rates, unparalleled in magnitude at any other time in the first 155 Myr of theropod evolution. Thus, a Cretaceous adaptive radiation of stem-group birds was enabled by restructuring of the terrestrial locomotor module, which represents a key innovation. Our results suggest two phases of radiation in Avialae: with the Cretaceous diversification overwritten by extinctions of stem-group birds at the Cretaceous–Palaeogene boundary, and subsequent diversification of the crown group. Our findings illustrate the importance of fossil data for understanding the macroevolutionary processes generating modern biodiversity.鸟类是现存四足动物（tetrapod）中多样性最高的类群，同时也是大规模适应性辐射（adaptive radiation）的经典研究模型。现生生物学研究表明，鸟类冠群（crown group）的适应性辐射发生在飞行起源之后的漫长时段。然而，由于仅考量了有限的化石数据，鸟类演化的深时（deep time）格局始终模糊不明。本研究基于中生代兽脚类恐龙（Mesozoic theropods）的完整系统发育树，对分支演化（cladogenesis）与四肢演化展开分析，揭示了恐龙向鸟类的过渡过程以及动力飞行（powered flight）的直接起源。中生代鸟类从非飞行祖先处继承了前肢演化的约束机制，且早期飞行类群的物种分化速率并未出现加速。但早白垩世的短尾鸟类同时展现出后肢的表型解放（phenotypic release）以及升高的物种分化速率，其强度在兽脚类演化前1.55亿年（155 Myr）的所有时段中均无先例。由此可见，干群鸟类（stem-group birds）在白垩纪的适应性辐射，得益于陆地运动模块的重构，这一过程属于一项关键创新（key innovation）。本研究结果显示，鸟翼类（Avialae）的辐射演化分为两个阶段：白垩纪的物种分化被干群鸟类在白垩纪-古近纪界线（Cretaceous–Palaeogene boundary）处的灭绝事件所覆盖，后续才发生冠群鸟类的辐射分化。本研究结果阐明了化石数据对于理解塑造现代生物多样性的宏观演化（macroevolution）过程的重要意义。