Department of Geological Sciences, University of Texas at Austin, Austin, TX, USA.
Evolution. 2021 Jun;75(6):1415-1430. doi: 10.1111/evo.14245. Epub 2021 May 12.
Birds share an array of unique characteristics among extant land vertebrates. Among these, external and microstructural characteristics of extant bird eggs have been linked to changes in reproductive strategy that arose among non-avian theropod dinosaurs. More recently, differences in egg proportions recovered in crown birds relative to other dinosaurs were suggested as possibly linked to avian flight, but dense sampling close to its proposed origin was lacking. Here we assess the evolution of eggshell thickness in a targeted sample of 114 dinosaurs including birds, and test the relationship of eggshell thickness with potential life history correlates and locomotor mode using phylogenetic comparative methods. Only egg mass and flight are identified as significant predictors of eggshell thickness. While a high correlation between egg mass and eggshell thickness is expected, that relationship is much stronger in flying taxa, which show a significantly higher slope and lower residual variance than flightless species. This suggests stabilizing selection of eggshell thickness among theropods, as recovered for other traits in extant birds (e.g. genome size, metabolic rate). Within living birds, Eufalconimorphae present an apomorphic increase in relative eggshell thickness which remains unexplained, as few morphological synapomorphies of this clade have been identified.
鸟类在现存的陆地脊椎动物中拥有一系列独特的特征。其中，现生鸟类卵的外部和微观结构特征与非鸟兽脚亚目恐龙中出现的生殖策略变化有关。最近，有人提出，相对于其他恐龙，冠群鸟类的卵比例差异可能与鸟类的飞行有关，但在其提出的起源附近缺乏密集的采样。在这里，我们评估了 114 种恐龙（包括鸟类）的蛋壳厚度的进化，并使用系统发育比较方法测试了蛋壳厚度与潜在的生活史相关因素和运动模式的关系。只有卵质量和飞行被确定为蛋壳厚度的重要预测因子。虽然卵质量和蛋壳厚度之间存在高度相关性是可以预期的，但在飞行类群中，这种关系要强得多，它们的斜率明显更高，残差方差更低，这表明在兽脚亚目中存在蛋壳厚度的稳定选择，这在现生鸟类的其他特征中也有发现（例如基因组大小、代谢率）。在现存鸟类中，鹰形目鸟类表现出相对蛋壳厚度的特化增加，但这种增加仍然无法解释，因为这个分支的很少形态学共衍征被识别出来。
