Data from: Rates of morphological evolution are heterogeneous in Early Cretaceous birds

The Early Cretaceous is a critical interval in the early history of birds. Exceptional fossils indicate that important evolutionary novelties such as a pygostyle and a keeled sternum had already arisen in Early Cretaceous taxa, bridging much of the morphological gap between Archaeopteryx and crown birds. However, detailed features of basal bird evolution remain obscure because of both the small sample of fossil taxa previously considered and a lack of quantitative studies assessing rates of morphological evolution. Here we apply a recently available phylogenetic method and associated sensitivity tests to a large data matrix of morphological characters to quantify rates of morphological evolution in Early Cretaceous birds. Our results reveal that although rates were highly heterogeneous between different Early Cretaceous avian lineages, consistent patterns of significantly high or low rates were harder to pinpoint. Nevertheless, evidence for accelerated evolutionary rates is strongest at the point when Ornithuromorpha (the clade comprises all extant birds and descendants from their most recent common ancestors) split from Enantiornithes (a diverse clade that went extinct at the end-Cretaceous), consistent with the hypothesis that this key split opened up new niches and ultimately led to greater diversity for these two dominant clades of Mesozoic birds.

早白垩世（Early Cretaceous）是鸟类早期演化史中的关键阶段。珍稀化石表明，尾综骨（pygostyle）与具龙骨突的胸骨（keeled sternum）等重要演化新特征已于早白垩世的类群中出现，填补了始祖鸟（Archaeopteryx）与冠群鸟类（crown birds）之间大量的形态学鸿沟。然而，由于此前研究的化石类群样本量有限，且缺乏针对形态演化速率的定量分析，基干鸟类演化的详细特征仍不甚明晰。本研究将新近开发的系统发育分析方法及其配套的敏感性检验，应用于大规模形态特征数据矩阵，以量化早白垩世鸟类的形态演化速率。研究结果显示，尽管不同早白垩世鸟类演化支系间的演化速率存在显著异质性，但难以精准识别出持续存在的显著高或低演化速率模式。尽管如此，演化速率加速的最强信号出现在今鸟型类（Ornithuromorpha）与反鸟类（Enantiornithes）分化的节点处：今鸟型类包含所有现生鸟类及其最近共同祖先的后裔，反鸟类则是一个在白垩纪末灭绝的多样演化支。这一结果与“该关键分化事件开辟了新的生态位，最终推动这两大中生代鸟类优势类群实现更大多样性”的假说相符。