The mammalian sternum plays key roles in structural support, locomotion and ventilation, but its evolutionary history has rarely been addressed. Unlike most other synapsids, the therian presternum lacks a discreet interclavicle, and the mesosternum is composed of a series of sternebrae. Here, CT scans of fossil and living therians are used to confirm that the therian presternum incorporates multiple elements, including the interclavicle and the anteriormost sternal bands of ancestral synapsids. A previously unrecognized transitional element that articulates with rib 1 is variably present and is hypothesized to permit the direct integration of the first rib and the sternum, despite their origins in different developmental domains. Sternebrae are found at sites where primaxial ribs interact directly with the abaxial sternum and locally inhibit skeletal maturation. Both presternal fusion and mesosternal subdivision are the products of small developmental changes that allowed the adaptation of ancestral structures to the lifestyles of therians.

中文翻译：

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侧体节前沿的therian胸骨：复合结构的演变

哺乳动物胸骨在结构支撑、运动和通风方面起着关键作用，但其进化历史很少被提及。与大多数其他联节纲动物不同，therian presternum 缺乏谨慎的锁骨间，并且mesosternum 由一系列胸骨组成。在这里，化石和活的 therian 的 CT 扫描被用来确认 therian presternum 包含多种元素，包括锁骨间和祖先突触的最前胸骨带。一个以前未被识别的与第 1 肋骨连接的过渡元素是可变的，并且被假设允许第一肋骨和胸骨的直接整合，尽管它们起源于不同的发育领域。胸骨位于原轴肋直接与背轴胸骨相互作用并局部抑制骨骼成熟的部位。胸骨前融合和胸骨间质细分都是微小发育变化的产物，这些变化允许祖先结构适应 therians 的生活方式。