Comparative Studies on Enamel Hypoplasia in the Siwalik Rhinocerotidae (Mammalia)

Enamel Hypoplasia (failure for the enamel to form properly), a tooth defect during development, provides a permanent record of systematic stress during early life. Research on enamel defects can provide an insight into environmental conditions present during the growing years of an extinct animal’s life. Anthropologists and paleontologists have carried out studies on incidence and distribution of Linear Enamel Hypoplasia to assess the health status of past populations. The present study on Enamel Hypoplasia in Siwalik Rhinoceroses is being conducted for the first time on Siwalik mammals. Dental defects are known in many mammalian taxa but their potential use in paleontological interpretations has not previously been explored in Siwalik mammals. This study is based on examination of a total of 1754 Rhinocerotid teeth housed in major museums and institutes of Pakistan, France, UK and the USA. The Neogene Rhino collections collected from the Potwar Plateau, Sulaiman Range, Bugti Hills, Kirthar Range and the Siwalik Hills housed at the GSP, PMNH, PUPC, MNHN, MHNT, AMNH, PMHU, YPNHM, and the NHM, London, were investigated. Recent Rhino teeth have also been examined at MNHN, Paris and the Harvard Museum of Comparative Zoology (MCZ).Each and every tooth in the collections was examined for the presence or absence of Enamel Hypoplasia (EH) and description of each defect, its position on the tooth crown, the number of occurrences on the tooth, and the position of the defected tooth in each jaw were recorded. A total 1754 Rhinocerotid teeth comprising 846 fossils Rhino teeth and 908 recent Rhino teeth were examined. The 846 fossil Rhinocerotid teeth included 21 incisors, 2 canines, 43 deciduous premolars, 283 premolars, and 497 molars, whereas the recent Rhino teeth included 15 incisors, 32 canines, 486 premolars, and 375 molars. The 846 fossil teeth calculated for MNI (minimum number of individuals) indicated 337 animals whereas the recent Rhinos teeth are from 45 animals. In fossil Rhinos, 34 teeth had hypoplasia and in recent only 6 teeth are found with EH. EH are recorded almost in equal numbers on the buccal as well as on the lingual side in the fossil or recent specimens studied. Most of the EH are of Linear type which are more prominent and common. 5 cases of semicircular EH have also been noted which, except one, are on the lingual side. The teeth having hypoplasia in this study show that 87% of EH occurs on permanent teeth, whereas 13% are in deciduous teeth. Among the deciduous teeth, 60% occurrences are on the dP4, which is the last one to erupt among the deciduous teeth of rhinoceroses. EH position on the crown from the cementoenamel junction (i.e. neck), indicate EH in most of the teeth occurred at a late developmental stage. One possible inference, based on the location of EH on the tooth and the position of the tooth in the jaw, is that Enamel Hypoplasia occurred when the animal was not dependent upon mother’s nutrition. Therefore, the animal was under some sort of physiological stresses perhaps triggered by external factors.The ~25 Myr to about 2 Myr fauna of Rhinocerotids dental material examined and analyzed in this study, covers a wider geographical region from the Bugti Hills in central Pakistan to the Pabbi Hills in north-eastern Pakistan, and all the way to the Siwalik Hills in India. This study includes 14 Rhino species from the earliest radiation in the late Oligocene in the Bugti Hills to the still living Rhinoceros sondaicus in the Upper Pliocene rocks of the Pabbi Hills and the Siwalik Hills. The 34 species showing hypoplasia occur almost at all the intervals of the Neogene. It is difficult to directly correlate the hypoplasia occurrences with global or regional climate changes but there exists some relationship, which is discussed here. The Rhino species with EH are apparently more prevalent at four time periods; around 22-20 Myr, ~16 Myr, 12-8 Myr and ~2 Myr in the Pliocene. It has been argued that climate, especially seasonality with prolonged draught periods, might have been the cause of stress for these animals having hypoplasia. It would, however, bring credence to the hypothesis proposed here that climate change has caused the EH in Rhinos if other mammalian taxa are also examined for the same time span