Long bones comprise articular ends (epiphyses) joined by transitional metaphyses and a diaphysis (shaft). The structure of the latter is often viewed as regularly tubular across tetrapods (limbed vertebrates). However, assessments of the bone structure along the whole diaphysis are rare. Here I assess whole-diaphysis profiles of global compactness (bone fraction) of 164 species of extant and extinct therian mammals (marsupials + placentals) in a phylogenetically informed context. Generally terrestrial, mammals have acquired multiple times the highly specialised aerial, fully aquatic, and subterranean lifestyles, allowing to potentially associate specific traits with these lifestyles. I show that there is a consistent increase in global compactness along the diaphysis in most mammals. This pattern is modified in a limited number of specialised species: all aerial clades (gliders and bats) have rather uniform and low values, while cetaceans' humeral diaphysis is marked by a slightly more compact mid-diaphyseal region. Among subterranean clades, structure alterations are most obvious in fossorial talpids (true moles) and their highly modified humerus. These results call for the investigation of bone structure in whole skeletal elements of key fossils in order to reconstruct the patterns of evolutionary modifications associated with lifestyle transitions. This article is protected by copyright. All rights reserved.

中文翻译：

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哺乳动物的肱骨干结构

长骨包括由过渡干骺端和骨干（轴）连接的关节末端（骨骺）。后者的结构通常被视为跨越四足动物（四肢脊椎动物）的规则管状。然而，对整个骨干的骨骼结构的评估很少见。在这里，我评估了 164 种现存和灭绝的兽类哺乳动物（有袋动物 + 胎盘动物）在系统发育背景下的整体紧密度（骨分数）的整体骨干概况。通常在陆地上，哺乳动物已经多次获得高度专业化的空中、完全水生和地下生活方式，从而可能将特定特征与这些生活方式联系起来。我表明，在大多数哺乳动物中，沿骨干的整体紧密度持续增加。这种模式在数量有限的特殊物种中得到了修改：所有空中进化枝（滑翔机和蝙蝠）都具有相当均匀和较低的值，而鲸类的肱骨骨干以稍微更紧凑的骨干中部区域为标志。在地下进化枝中，结构改变在 fossorial talpids（真痣）及其高度修饰的肱骨中最为明显。这些结果要求对关键化石的整个骨骼元素中的骨骼结构进行研究，以重建与生活方式转变相关的进化改变模式。本文受版权保护。版权所有。结构改变在 fossorial talpids（真痣）及其高度修饰的肱骨中最为明显。这些结果要求对关键化石的整个骨骼元素中的骨骼结构进行研究，以重建与生活方式转变相关的进化改变模式。本文受版权保护。版权所有。结构改变在 fossorial talpids（真痣）及其高度修饰的肱骨中最为明显。这些结果要求对关键化石的整个骨骼元素中的骨骼结构进行研究，以重建与生活方式转变相关的进化改变模式。本文受版权保护。版权所有。