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.

诸多新发现的化石引发了学界对早期鸟类繁殖行为的研究热情，但相关直接证据仍十分匮乏。目前尚无任何一枚中生代鸟类蛋类可被明确归属于某一物种，这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。相较于现生鸟类，中生代鸟类的蛋类体型相对偏小，但中生代鸟类的蛋类是否能够承受繁殖成年个体的体重，目前尚未得到研究。我们基于耻骨联合（pubic symphysis）所界定的盆腔管宽度，估算了多个中生代鸟类类群的蛋类最大横径。结合已知的中生代鸟类蛋类伸长率，我们可以推算出蛋类的质量，进而得出其破裂前可承受的负重质量。将这些数值与基于骨骼遗骸推算的成年鸟类体质量进行对比后，基于21个化石物种的样本，我们发现：对于非鸟胸类（non-ornithothoracine）鸟类而言，其体质量可达蛋类可承受负重的130%。反鸟类（Enantiornithes）的体质量与蛋类可承受负重与现生鸟类相当，但部分早白垩世今鸟型类（ornithuromorphs）的体质量较其蛋类可承受的负重高出110%。本研究采用的间接分析方法，迄今为早期鸟类无法安全蹲坐于其蛋类之上（否则将面临蛋壳破损风险）提供了最为确凿的证据。我们据此提出，接触孵卵在鸟类演化历程中出现的时间相对较晚。