Taxonomic Identification of Equid Fossils from the Khorat Sand pits

Abstract

In July, 2017, fossils from Phimai and Chalermprekiet sand pits (Nakhon Ratchasima province) were collected by paleontologists from Chulalongkorn University cooperated with the Department of Mineral Resources (DMR). The paleontological excavation has yielded 16 specimens of equid fossils consisting of 6 isolated teeth (4 upper and 2 lower teeth, 1 maxilla, and 9 mandibles. We identify here these equid fossils at the genus level. In order to understand the morphological changes through wear, we use a Computerized Tomography Scan (CT-scan) method to observe occlusal sections at the different high levels of tooth crowns because the cusps of teeth have morphologically changed in relation to different wear stages through the crown height of fossil equids. As a result of the study, we identify equid specimens as belonging to Hipparion cf. chiai and Hipparion sp. The former species is characterized by elongated and sub angular protocones, sub-angular shapes of hypocones, and short enamel plications of pre- and postfossettes. In addition, the morphological patterns of occlusal surface changes in each stages of wear in lower cheek teeth are similar to those of Hipparion cf. chiai (Li et al., 2017). Hipparion cf. chiai is widely distributed in the middle region of China and smaller-sized Hipparion sp. that has a rounded protocone and incomplete plication in pre- and posfossettes. Double knot in lower teeth of Hipparion sp. differs from Hipparion in Pakistan and India, but mostly similar to Hipparion in China. According to the presence of these equids, the paleoenvironments of Khorat sand pits corresponded to a forest and the age of this genus is thus considered to be the Late Miocene.