Abstract
Based on the numerous new specimens of Plesiaceratherium gracile from the Early Miocene Shanwang Basin, China, we investigated the sexual dimorphism and reconstructed the body. The sexual dimorphism of P. gracile is the size of both the lower incisor i2 (length male/female ratio = 2.69, width M/F = 1.63) and the upper incisor I1 (length M/F = 1.63). Other mensural variations show their ranges greatly overlapping on both genders, with a lower M/F ratio, they hence can not be interpreted as sexual dimorphism. The large-sized body, the strong sexual difference of incisors, and the minor population gap between both gender (male/female = 29/21) suggest that P. gracile is polygynous and has a solitary life style. On the other hand, we reconstruct the body of P. gracile. The head-body length is 2796–3117 mm, the shoulder height is 1476–1627 mm, and the body weight is about 1198 kg. All skeletons have the ventral edge of the thorax flushing with the olecranon of the ulna. Meanwhile, the spinous processes of the posterior thoracic vertebrae and the lumbar vertebrae are always vertical, the metatarsal-femur ratio and the tibia-femur ratio are higher, and the two joint angles between the scapula, the humerus, and the radius are 102°–143° and 115°–160°, respectively, which indicates that P. gracile has a mediportal limb and a horse-gallop running type. Furthermore, the limb-segments comparisons of P. gracile and other rhinos demonstrate that the proximal long bones, including the scapula, the humerus and the femur, are important for the body height whatever their body is large-sized or small-sized; the distal parts of the limb, including the metacarpus and the metatarsus, are sensitive to the body mass changes.
Highlights
Intraspecific variation records nearly every aspects of individual difference within a species, the most interesting of which is sexual dimorphism. Pocock (1945) was the first to note that the secondary sexual character of the extant Asiatic rhinoceroses lies in the horns. Groves (1982) recognized more sexual differences in the body size and the skull of the Asiatic rhinoceroses, including several skull measures. Owen-Smith (1988) found that the gender of the African white rhinoceros could be judged based on the difference of the body and horn sizes. Dinerstein (1991) focused on the detailed sexual dimorphism of the greater one-horned rhinoceros, recording the difference of the mandible and the skull
Plesiaceratherium is a hornless rhinoceros, so the gender cannot be determined based on the horn difference, which is relatively evident on some horned rhinocerotids
Given that the size of incisors has long been recognized as reflecting sexual dimorphism, we first investigated this feature and found that the length of i2 is the most dimorphic character, followed by its width (Figures 1–3 and Table 1)
Summary
Intraspecific variation records nearly every aspects of individual difference within a species, the most interesting of which is sexual dimorphism. Pocock (1945) was the first to note that the secondary sexual character of the extant Asiatic rhinoceroses lies in the horns. Groves (1982) recognized more sexual differences in the body size and the skull of the Asiatic rhinoceroses, including several skull measures. Owen-Smith (1988) found that the gender of the African white rhinoceros could be judged based on the difference of the body and horn sizes. Dinerstein (1991) focused on the detailed sexual dimorphism of the greater one-horned rhinoceros, recording the difference of the mandible and the skull. Groves (1982) recognized more sexual differences in the body size and the skull of the Asiatic rhinoceroses, including several skull measures. Dinerstein (1991) focused on the detailed sexual dimorphism of the greater one-horned rhinoceros, recording the difference of the mandible and the skull. With regard to the fossil rhinocerotids, many studies have revealed that sexual dimorphism lies in the presence of the horn or the size of the lower incisors (Osborn, 1898a,b; Peterson, 1920; Cerdeño and Sánchez, 2000; Mead, 2000; Deng, 2001, 2005; Mihlbachler, 2005, 2007; Chen et al, 2010). Mihlbachler (2007) recorded the skeleton of Menoceras arikarense and suggested that the greater degree of sexual bimodality of the nasal horn may relate to the small-sized body, but found no sexual difference in body size. The gender of extinct rhinoceroses has been estimated by means of paleoproteomic analyses (Cappellini et al, 2019)
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