Palaeopathology mini-series: Periodontitis, hypercementosis, and enamel hypoplasia in extinct rhinoceroses

Dental paleopathology in fossil rhinoceroses: etiology and implications

Christine Böhmer and Gertrud E. Rössner

In the literature, a variety of dental anomalies have been reported in living and extinct rhinoceroses. Malpositionings, irregular eruptions, supernumerary teeth (hyperdontia), dental asymmetries and tooth rotations have been described. In comparison to anomalies found in permanent teeth, reports on anomalies in deciduous teeth are relatively rare. In combination with knowledge about the timing of dental development, dental pathologies can provide a unique opportunity to reconstruct specific phases in the life history of fossil taxa, as well as animal-environment interactions.

The dental ontogeny of the extinct rhinoceros Prosantorhinus germanicus from the Miocene (about 15 million years ago) of Germany is well-known from an exceptionally well preserved sample of juvenile dentaries (Böhmer et al. 2016, Journal of Mammalian Evolution). Two of the represented juvenile individuals each revealed a dental anomaly, which were analyzed macroscopically and via X-ray tomography and published in the Journal of Zoology.

One specimen documents decreased dental wear of deciduous cheek teeth, mandibular bone resorption as well as abnormal cemental deposition. At an early age, the extinct juvenile rhinoceros probably was affected by a gingival infection resulting from an accumulation of bacterial plaque. This led to a chronic gingivitis and eventually may have caused periodontitis. Furthermore, it suffered from inflammation-induced hypercementosis.

Figure 1
Figures from the article by Böhmer & Rössner (2017): X-rays of the jaws of (a) the fossil rhinoceros Prosantorhinus germanicus and (b) an extant rabbit, both displaying hypercementosis (indicated by arrows).

Another specimen revealed an abnormal reduction in thickness of enamel in the deciduous cheek tooth. The enamel hypoplasia indicates stress experienced during the prenatal period. During a phase of environmental stress (e.g., increased fluoride exposure) or physiological stress (e.g., malnutrition) the function of the secretory ameloblasts was disrupted in the developing embryo.

 

Figure 2.tif
Figures from the article by Böhmer & Rössner (2017): A clearly visible, deep trench in the dental enamel (hypoplasia) encompasses the deciduous tooth of the fossil rhinoceros Prosantorhinus germanicus (on the left). Schematic illustration of a longitudinal section through a tooth (on the right).

The present work did not solely diagnose the paleopathological findings, but also explores the potential processes and mechanisms behind pathologies, complemented by knowledge of the dental ontogeny in P. germanicus. Identification of the causative agent and cascade effects allow for detailed reconstruction and timing of specific harmful events in the life history of extinct mammals. Provided that samples are large enough, this may even help to reconstruct the evolutionary history of dental apparatus diseases and seasonal environmental or physiological phases of stress of a population or a community. One of the specimens revealed a periodontal disease, whereas the other specimen displays a linear enamel hypoplasia that is an indicator of a period of physiological stress during tooth development. Since enamel development (amelogenesis) of the affected deciduous tooth in P. germanicus probably took place in utero, the hypoplasia detected in the juvenile rhinoceros is an indicator of maternal stress. This shows that the analysis of dental paleopathologies is a useful means of retrospective assessment of specific phases in the life history of extinct animals and adds to the picture of the living conditions of the extinct rhinoceros P. germanicus.

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