Hopkins Samantha S B
Department of Integrative Biology and Museum of Paleontology, University of California, 1101 VLSB, Berkeley, CA 94720, USA.
Proc Biol Sci. 2005 Aug 22;272(1573):1705-13. doi: 10.1098/rspb.2005.3171.
Ceratogaulus, a member of the extinct fossorial rodent clade Mylagaulidae, is the only known rodent with horns and the smallest known horned mammal. The function of the large, dorsally projecting nasal horns on this burrowing animal has been the subject of wide speculation among palaeontologists; suggested uses range from sexual combat to burrowing. Mammals have evolved adaptations for digging repeatedly; horns and other cranial appendages have also evolved numerous times. These two adaptations co-occur in mammals extremely rarely: only two fossil genera (Ceratogaulus and the xenarthran Peltephilus) and no extant mammals are both horned and fossorial. Tracing the evolution of fossoriality in aplodontoid rodents (the larger clade to which Ceratogaulus belongs) reveals that Ceratogaulus descended from ancestors who dug by head-lifting. Whereas this suggests an obvious explanation for the horns of this rodent, evidence from functional morphology, anatomy, phylogeny and geologic context indicates that the horns in Ceratogaulus were used for defence, rather than digging, and evolved to offset increased predation costs associated with spending more time foraging above ground as body size increased.
角古鼠是已灭绝的穴居啮齿动物类群米拉古鼠科的一员，是已知唯一长角的啮齿动物，也是已知最小的有角哺乳动物。这种穴居动物背上突出的大鼻角的功能，一直是古生物学家广泛猜测的话题；其用途从性争斗到挖掘都有。哺乳动物已经进化出了多次挖掘的适应性特征；角和其他颅骨附属物也多次进化。这两种适应性特征在哺乳动物中极为罕见地同时出现：只有两个化石属（角古鼠和异关节类的佩尔泰菲勒斯），现存的哺乳动物中没有既是有角又是穴居的。追溯角古鼠所属的较大类群阿普洛东齿啮齿动物的穴居性进化过程，发现角古鼠是从通过抬头挖掘的祖先那里演化而来的。虽然这为这种啮齿动物的角提供了一个明显的解释，但功能形态学、解剖学、系统发育和地质背景的证据表明，角古鼠的角是用于防御而非挖掘，并且随着体型增大，为了抵消因在地面觅食时间增加而带来的更高被捕食成本而进化出来的。
