Dental histology of late Miocene hipparionins compared with extant Equus, and its implications for Equidae life history

Abstract

Hipparionins were a dominant element of the late Miocene faunas of Europe; however, their biology and ecology remain incompletely understood. In this paper, we explore the pace of life history of different-sized hipparionin horses, using dental histology, and compare it with extant equids. In doing so we consider (i) the size diversity of hipparionins, (ii) their generally smaller size compared to extant equids, and (iii) the allometric coupling between size and life history. In particular, we reconstruct the dental growth in lower first/second molars and in later-formed lower third molars for three hipparionin taxa: two dwarf species (Hipparion periafricanum and H. gromovae), and a larger species (H. concudense). We also analyze dental growth in an extant zebra (Equus quagga) for comparative purposes. Our results reveal that, within each species, there are differences in enamel growth parameters between the first/second molars and third molars. These results illustrate the differences in the developmental timing of these teeth and the existence of a relationship between dental growth parameters with somatic growth. We also find that hipparionin teeth grow at slower rates and tend to erupt later in time than in extant Equus. Dwarf hipparionins, moreover, exhibit lower enamel extension rates than the larger species, but similar formation and eruption times. Considering the link between dental development and life history, these results suggest a slower pace of growth of selected hipparionins compared to extant equids, and a further slower life history than expected for their size in the two dwarf forms.