Living birds are unique among vertebrates in the formation of a female-specific bone tissue called medullary bone (MB) that is strictly associated with reproductive activity. MB is a rapidly mobilized source of calcium and phosphorus for the production of eggshell. Among living taxa, its skeletal distribution can be highly extensive such that it even exists in the ribs of some species. Due to its ephemeral nature, MB is rarely fossilized and so little is understood with regard to the origin of MB and its skeletal distribution in early taxa. Here we describe a new Early Cretaceous enantiornithine bird, Mirusavis parvus, gen. et. sp. nov., indicating that skeleton-wide distribution of MB appeared early in avian evolution. We suggest that this represents the plesiomorphic condition for the Aves and that the distribution of MB observed among extant neornithines is a product of increased pneumatization in this lineage and natural selection for more efficient distribution of MB.

中文翻译：

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髓骨的进化和分布：来自一种新的早白垩世反鸟氨酸鸟的证据

活的鸟类在脊椎动物中是独一无二的，它形成了一种称为髓骨 (MB) 的雌性特异性骨组织，该组织与生殖活动密切相关。MB 是用于生产蛋壳的钙和磷的快速动员来源。在活的分类群中，它的骨骼分布非常广泛，甚至存在于某些物种的肋骨中。由于其短暂的性质，MB 很少被化石化，因此对 MB 的起源及其在早期分类群中的骨骼分布知之甚少。在这里，我们描述了一种新的早白垩世反鸟氨酸鸟，Mirusavis parvus，将军。等。sp. 11 月，表明 MB 的骨骼范围分布出现在鸟类进化的早期。我们认为这代表了 Aves 的拟态条件，并且在现存的新鸟中观察到的 MB 分布是该谱系中增加的气动化和自然选择的产物，以便更有效地分配 MB。