A new species of Ceratogaulus from Nebraska and the evolution of nasal horns in Mylagaulidae (Mammalia, Rodentia, Aplodontioidea)

Members of the Mylagaulidae have been known for over a century to bear nasal horns; the only rodents, extinct or extant, ever to have done so. This striking feature is known from five of the over 30 species of mylagaulid rodents discovered across North America and Eurasia, all relatively large animals that were likely less fossorial than their relatives. We describe herein a sixth new species of horned mylagaulid. This new taxon from Sioux County, Nebraska, offers the opportunity to reassess the phylogenetic relationships of Mylagaulidae and test several evolutionary hypotheses. Our analyses demonstrate that horns evolved only once in Mylagaulidae, in the common ancestor of Ceratogaulus, first as short horns exapted from the thickened nasals of fossorial ancestors, and later as taller horns. The horns evolved following a positive allometric scaling with body mass that suggests a response to predation pressure in these nearly blind animals. The evolution of tall horns also corresponds to a jump in body mass. The largest mylagaulids are not horn-bearing species, however. Additional analyses of the complex pattern of body mass evolution we reveal will be necessary to explain the evolution of the largest head-lift digging rodents in Earth history. https://zoobank.org/urn:lsid:zoobank.org:pub:81FE999A-F79E-4BD4-9A81-2C7D3D5D81CD

超过一个世纪以来，学界已确认磨鼠科（Mylagaulidae）的成员均生有鼻角，该类群是迄今已知唯一兼具灭绝与现生类群且长有鼻角的啮齿动物。在北美与欧亚大陆发现的30余种磨鼠科啮齿类中，已有5个物种被记录具有这一显著特征；这些物种体型相对较大，相较于其近亲，掘土习性相对较弱。本文记述了第六个带角磨鼠科新物种，这一产自美国内布拉斯加州苏县的新分类群，为重新评估磨鼠科的系统发育关系以及检验多项演化假说提供了契机。我们的分析显示，磨鼠科的鼻角仅演化过一次，起源于角磨鼠属（Ceratogaulus）的共同祖先：最初的短角源自掘土类祖先的加厚鼻骨，属于功能预适应结构，后续进一步演化为更高大的角。鼻角的演化与体重呈现正异速生长关系，这表明该特征是对这些近乎失明的类群所面临的捕食压力的适应性演化。高大鼻角的演化同时伴随着体重的显著跃升，但体型最大的磨鼠科物种并不具有鼻角。若要解释地球历史上体型最大的抬首掘土啮齿类的演化历程，还需对我们所揭示的复杂体重演化模式开展进一步分析。https://zoobank.org/urn:lsid:zoobank.org:pub:81FE999A-F79E-4BD4-9A81-2C7D3D5D81CD