Unexpected cranial sexual dimorphism in the tragulid Dorcatherium naui based on material from the middle to late Miocene localities of Eppelsheim and Hammerschmiede (Germany)

Tragulids, chevrotains or mouse deer, were common faunal elements during the Miocene. During that time, Dorcatherium was the most abundant genus, with D. naui being the first described species. Besides their abundance, until recently only very limited cranial material was available for investigation. Here we present a redescription of the first complete skull of D. naui from the middle to late Miocene locality of Eppelsheim, Germany, based on micro-computed tomography. Furthermore, we present a description and comparison of two additional, new skulls of D. naui from the late Miocene hominid locality Hammerschmiede, Germany. Within Dorcatherium, so far, only three other complete skulls are known, all belonging to D. crassum. A comparison between the three skulls of D. naui and the already known skulls of D. crassum shows that these two species differ in morphological features of the skull, such as laterally facing orbitae, separation of supraorbital foramen from supraorbital groove by a bony bridge, well-developed parietal plateau, prominent nuchal tubercle, less-developed nuchal crests, and the presence of an occipital crest. Moreover, two different osteological morphotypes are present in the skulls of D. naui that can be interpreted as a previously unknown sexual dimorphism. Very similar features are observed in D. crassum, which can be likewise related to the same dimorphism. However, males of D. naui differ from males of D. crassum by the presence of frontoparietal bulges, which were probably used for sexual display and during male-male combats in males of D. naui. For the first time, sexual dimorphism in Dorcatherium is described based on skull characteristics, which are, so far, unknown from any other fossil or extant tragulid.