Comparative Analysis of Enamel Hypoplasia in the Siwalik Artiodactyles of Pakistan

Abstract

The ecological response of the extinct artiodactyls towards vegetational and environmental changes in the early Miocene to early Pleistocene ecosystems of the Siwaliks is analyzed in seven artiodactyl families named as Giraffidae, Tragulidae, Cervidae, Hippopotamidae, Anthracotheriidae, Suidae and Bovidae by using comparative occurrence of a dental defect, enamel hypoplasia in these artiodactyls. Enamel hypoplasia is the thinning of enamel caused by depletion of ameloblasts (enamel forming cells) during tooth development. The depletion of ameloblasts is associated with the physiological or environmental stress that an animal had faced during its life history, so this dental defect can provide us a permanent and reliable record of the ecological stress faced by an animal during its tooth development. The 848 teeth of 487 extinct individuals belong to 39 artiodactyl species are used in current enamel hypoplasia analysis in order to trace out the impact of Neogene and Quaternary Stress events on these animals. These studied fossils have a chronological range of 18.3-0.6 Ma and belong to the Siwalik out croups in northern Pakistan. This is most possibly first ever order level study for comparative analysis of enamel hypoplasia in any of the Neogene deposits around the globe. The results of the present study revealed that the early Pliocene Siwalik tragulids had high occurrence of enamel hypoplasia as compared to the middle Miocene Siwalik tragulids (p<0.05) as tragulids prefer warm ecosystems. There was a gradual increase in the frequency of enamel hypoplasia for the Siwalik cervids during the Pliocene epoch (p<0.05) due to drier and cooler environmental conditions along with increased seasonality. The percentage for occurrence of enamel hypoplasia was high in the anthracotheroides throughout their existence in the Siwaliks indicating significant level of stress to anthracotheroides that ultimately lead to family level extinction of this group. Enamel hypoplasia occurrence shows increment of stress for the Siwalik hippopotamids in early Pliocene to onwards habitats. The moderate level of stress for the Siwalik giraffids, suids and bovids having no significant difference in various Siwalik intervals is inferred by enamel hypoplasia results in these artiodactyls (p>0.05). Comparatively high multiple linear enamel hypoplasia in the Pliocene Siwalik artiodactyls points out that climatic changes of the Pliocene in the Siwaliks were episodic in nature. The increased seasonality, aridity and glaciation events triggered the C3 to C4 vegetational shifts which might have posed a very high level of stress to these Siwalik artiodactyls leading to speciation and extinction of various taxa in the Siwalik region.