Anatomy, Sexual Dimorphism, and Ontogeny of the Tragulid Skull

Abstract

Fossil tragulids are small ruminant artiodactyls characterized by large saber-like upper canines and a stout body. Although they have existed since the late Eocene and display considerable morphological diversity, their evolutionary history and biology remain poorly understood. The genus Dorcatherium is particularly affected by taxonomic uncertainty, with many species historically placed in a wastebasket taxonomy. This thesis aims to clarify the taxonomy and paleobiology of the type species of Dorcatherium, D. naui. The study is based on complete and partial skulls from the middle to late Miocene locality of Eppelsheim and the earliest late Miocene locality of Hammerschmiede (Germany), complemented by comparative material of the closely related extant tragulid Hyemoschus aquaticus. For the first time, the cranial morphology of D. naui is described in detail, allowing the identification of diagnostic autapomorphies independent of dental characters. Comparative analysis reveals a previously unknown sexual dimorphism in cranial ornamentation, with more strongly developed cranial crests in males. Moreover, using 13 skulls from Hammerschmiede, an ontogenetic series based on dental wear stages was established using modern H. aquaticus as an analogue, enabling comparisons at similar developmental stages supported further by µCT data. The results indicate that cranial ornamentation in fossil tragulids was markedly more pronounced and sexually dimorphic than in extant forms, with males of D. naui exhibiting strongly developed cranial crests. In contrast, males and females of the extant H. aquaticus display largely similar cranial morphology. Taken together, this pattern suggests a paedomorphic reduction of cranial ornamentation in living tragulids and implies differences in male combat behavior. Comparative ontogenetic analyses further show that the development of cranial ornamentation coincides with the onset of sexual maturity in extant tragulids. By analogy, a similar onset of sexual maturity is inferred for Dorcatherium naui, based on the comparable development of cranial crests observed in similar ontogenetic stages. Beyond cranial morphology, a large comparative dataset of isolated teeth and mandibles from Hammerschmiede was used to investigate body size differences, mortality patterns, and survivorship at the population level. These analyses indicate a survivorship pattern consistent with a litter size of one offspring, comparable to that of the extant H. aquaticus. A difference in body size between the fossil and extant tragulid was noticed leading to the conclusion that D. naui might have lived in more open riparian woodlands compared to H. aquaticus, who lives in dense undergrowth of the African rainforests. Overall, this thesis provides the first integrative taxonomic and paleobiological framework for Dorcatherium naui and establishes a robust comparative basis for future studies on fossil tragulids, their life-history evolution, and phylogenetic relationships.