Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK.
Proc Biol Sci. 2013 Aug 14;280(1768):20131780. doi: 10.1098/rspb.2013.1780. Print 2013 Oct 7.
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中最多样化的群体，是大规模适应辐射的典范。新的研究表明，在飞行起源很久之后，鸟类冠群内部就发生了辐射。然而，由于只考虑了有限的化石数据，鸟类进化的深层时间模式仍然不清楚。我们分析了中生代兽脚亚目恐龙的整个系统发育树中的分支发生和肢体进化，记录了恐龙到鸟类的过渡以及有动力飞行的直接起源。中生代鸟类从不会飞行的祖先那里继承了对前肢进化的限制，而最早会飞行的类群的物种多样化率并没有加速。然而，早白垩世的短尾鸟类表现出后肢形态的显著释放和多样化率的增加，在兽脚亚目恐龙演化的前 1.55 亿年中的任何其他时期都无与伦比。因此，在陆栖运动模块的结构重组的基础上，实现了基干鸟类的白垩纪辐射适应，这代表了一个关键的创新。我们的研究结果表明，在平胸类中存在两个辐射阶段：白垩纪的多样化被白垩纪-古近纪之交的基干鸟类灭绝所覆盖，随后是冠群的多样化。我们的研究结果说明了化石数据对于理解产生现代生物多样性的宏观进化过程的重要性。
