Hippopotamoids are herbivorous mammals that originated in the late middle Eocene. This taxon includes animals with a great variety of sizes and body proportions, from small and gracile forms with slender limbs to heavy massive ones. Many hippopotamoids have previously been considered semi-aquatic but recent studies have highlighted a diversity of ecologies. This study focuses on bone microanatomy, one of the various proxies that enable inferring the ecology of extinct taxa. The comparative analysis of the inner structure of the stylopod bones in various hippopotamoids, based on both transverse and longitudinal virtual sections, highlights a diversity of patterns and clarifies previously proposed hypotheses about the ecology of the sampled hippopotamoids. The filling of the medullary area by spongious deposits in the pygmy hippopotamus, Choeropsis liberiensis, appears associated with frequent incursions into the water by an animal that essentially forages in forests. The common hippopotamus, Hippopotamus amphibius, which spends most of the day submerged in water, shows a greater filling of the medullary area by spongious bone and a thicker cortex. These observations coupled with comparisons with diverse terrestrial and semi-aquatic mammals of various sizes confirm that semi-aquatic lifestyle and heavy weight-bearing are associated with similar microanatomical specializations causing an increase in bone mass. However, for a given mass, comparisons enable determining if an additional increase in bone compactness occurs, as in Hippopotamus amphibius, in which case a semi-aquatic lifestyle could be inferred. Accordingly, this study suggests an essentially terrestrial lifestyle for Microbunodon minimum, Bothriodon velaunus, Elomeryx borbonicus, Merycopotamus medioximus, Paenanthracotherium bergeri, and probably also Saotherium cf. S. mingoz, a slight degree of water dependence in Brachyodus onoideus, and a stronger one in Libycosaurus bahri and Hexaprotodon garyam, though less intense than in Hippopotamus amphibius. Comparisons with other large terrestrial and semi-aquatic taxa, and based on a large part of the diaphysis, are required to better decipher the microanatomical changes associated with a semi-aquatic lifestyle from those linked to loading in heavy quadrupedal mammals.

河马类是起源于始新世中期的草食性哺乳动物。这个分类单元包括大小和身体比例各异的动物，从四肢细长的小而柔软的形状到沉重的笨重的动物。许多河床类动物以前被认为是半水生的，但最近的研究强调了生态的多样性。这项研究的重点是骨骼微解剖学，这是能够推断出已灭绝的类群生态的各种代理之一。基于横断面和纵断面的虚拟剖面，对各种河南拟南芥中节肢动物骨骼的内部结构进行了比较分析，突出了多种模式，并阐明了先前提出的关于采样的河南北半部生态学的假设。侏儒河马中的海绵状沉积物填充了髓质区域，拟南芥（Lichoeross liberiensis）似乎与动物在森林中觅食的频繁侵入水中有关。常见的河马，河马两栖动物，大部分时间都沉浸在水中，海绵状骨头和较厚的皮质显示出髓质区域更大的充盈。这些观察结果以及与各种大小的各种陆生和半水生哺乳动物的比较证实了半水生生活方式和沉重的体重与引起骨骼质量增加的相似的微解剖学专长有关。但是，对于给定的质量，通过比较可以确定是否发生骨致密性的进一步增加，如两栖河马，在这种情况下，可以推断出半水生生活方式。因此，这项研究表明了一个基本的地面生活方式Microbunodon最小，沟齿兽velaunus，Elomeryx borbonicus，Merycopotamus medioximus，Paenanthracotherium bergeri，大概也Saotherium比照 S. mingoz，轻微程度的在水中的依赖Brachyodus onoideus，以及更强的一个在Libycosaurus巴赫里和Hexaprotodon garyam，虽然比在不太激烈河马。需要与其他大型陆生和半水生类群进行比较，并以大部分骨干生物为基础，以便更好地从与四足动物重负荷有关的方面解读与半水生生活方式相关的微观解剖学变化。