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.

早白垩世是鸟类早期演化历史中的关键阶段。珍稀化石表明，诸如尾综骨与龙骨突胸骨这类重要的演化新特征，早已出现在早白垩世的类群中，填补了始祖鸟与冠群鸟类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偏小，且缺乏量化分析形态演化速率的相关研究，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方法及其配套的敏感性检验，应用于大型形态学特征数据集，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支系的演化速率存在显著异质性，但难以明确识别出持续存在的显著高/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鸟胸类（Ornithuromorpha，包含所有现生鸟类及其最近共同祖先的后裔支系）与反鸟类（Enantiornithes，一支在白垩纪末灭绝的多样支系）分化的节点处，这与该关键分化事件开辟了新生态位、最终催生中生代鸟类这两大优势类群的多样性的假说相一致。