Abstract:  Chilotherium wimani is the dominant species in the Late Miocene Hipparion fauna of the Linxia Basin. A sub-adult mandible belonging to C. wimani is described here. The unusual characters of this mandible are the retention of left dp4 and the morbid state of left and right p4. The left p4 exhibits anterior-posterior and labial-lingual reversion and the trigonid of this tooth is strongly tapered. The right p4 shows not only anterior-posterior reversion, but also the absence of trigonid. The retention of the left dp4 is induced likely by the abnormal growth of the left p4 germ, while the deformity of the left and right p4 may be caused by hereditary factors and/or malnutrition. The non-adaptive malocclusion would result in disadvantage in competition, and this functional disadvantage would be fatal under the likely condition of the harsh Late Miocene Linxia environment.