Data from: Pelvis morphology suggests that early Mesozoic birds were too heavy to contact incubate their eggs

Numerous new fossils have driven an interest in reproduction of early birds but direct evidence remains elusive. No Mesozoic avian eggs can be unambiguously assigned to a species, which hampers our understanding of the evolution of contact incubation, which is a defining feature of extant birds. Compared to living species eggs of Mesozoic birds are relatively small, but whether the eggs of Mesozoic birds could actually have borne the weight of a breeding adult has not yet been investigated. We estimated maximal egg breadth for a range of Mesozoic avian taxa from the width of the pelvic canal defined by the pubic symphysis. Known elongation ratios of Mesozoic bird eggs allowed us to predict egg mass and hence the load mass an egg could endure before cracking. These values were compared to the predicted body masses of the adult birds based on skeletal remains. Based on 21 fossil species, we show that for non-ornithothoracine birds body mass was 130% of the load mass of the eggs. For Enantiornithes body mass and egg load mass were comparable to extant birds, but some early Cretaceous ornithuromorphs were 110% heavier than their eggs could support. Our indirect approach provides the best evidence yet that early birds could not have sat on their eggs without running the risk of causing damage. We suggest that contact incubation evolved comparatively late in birds.

诸多新化石引发了学界对早期鸟类繁殖行为研究的兴趣，但相关直接证据仍较为匮乏。目前尚无任何一枚中生代鸟类卵能够被明确归属于某一具体物种，这一现状阻碍了我们对接触孵卵（contact incubation）演化历程的认知——而接触孵卵是现生鸟类的标志性特征之一。与现生鸟类的卵相比，中生代鸟类的卵相对偏小，但中生代鸟类的卵是否能够承受成年个体的体重，目前尚未有相关研究探讨。我们基于耻骨联合（pubic symphysis）所界定的骨盆通道宽度，对一系列中生代鸟类类群的最大卵宽进行了估算。结合已知的中生代鸟类卵的长宽比（elongation ratios），我们得以预测卵的质量，进而推算出卵在破裂前可承受的负载质量。随后将这些数值与基于骨骼遗骸估算的成年鸟类体质量进行对比。基于21个化石物种的分析结果显示：非鸟胸类（ornithothoracine）鸟类的体质量可达其卵负载质量的130%；反鸟类（Enantiornithes）的体质量与卵负载质量与现生鸟类相当，但部分早白垩世今鸟型类（ornithuromorphs）的体质量较其卵可承受的负载高出110%。我们的间接研究方法提供了迄今为止最为有力的证据，表明早期鸟类若直接伏在卵上孵卵，将面临导致卵破裂的风险。据此我们提出，接触孵卵在鸟类演化历程中出现的时间相对较晚。