Postcanine enamel microstructure of Eocene Sparnotheriodontidae (Mammalia: Litopterna) from Antarctica and southern South America compared to that of other South American Native Ungulates

Abstract

This study examines the postcanine enamel microstructures in three Eocene Sparnotheriodontidae taxa (Notiolofos arquinotiensis, Phoradiadius divortiensis, Sparnotheriodontidae indet.), comparing their schmelzmuster with other early-diverging South American Native Ungulates (SANUs) exhibiting vertical Hunter-Schreger bands (HSB). All three sparnotheriodontid taxa exhibit a highly derived schmelzmuster with three zones, inner and outer radial enamel and thick vertical HSB with broad transition zones. This latter trait differs from the narrow transition zones in most comparative taxa,except the lower molar of Astrapotherium. The Andean Phoradiadius divortiensis shares enamel features with the Antarctic Notiolofos arquinotiensis (acute-angled IPM in inner radial enamel and in HSB) and Sparnotheriodontidae indet. (enamel thickness, prism-parallel IPM in outer radial enamel). However, P. divortiensis uniquely shows low and steep prism inclination in the outer enamel. This acute-angled IPM, absent in comparative SANU taxa, further suggests Sparnotheriodontidae as anapomorphic branch within Litopterna, akin to specialised families in Perissodactyla. Comparisons with Astrapotheria, Carodnia vieirai (Xenungulata), Pyrotherium romeroi (Pyrotheria), and Didolodontidae (possible Litopterna) highlight the unique features of Sparnotheriodontidae, including exclusively vertical HSB combined with acute-angled IPM. Didolodontidae show plesiomorphic features such as no HSB or transverse HSB, and therefore occupy a basal evolutionary position. Pyrotherium and Carodnia exhibit divergent schmelzmuster traits, reinforcing the distinctiveness of Sparnotheriodontidae. Our findings suggest Sparnotheriodontidae represents one of the stratigraphically oldest SANU clades to develop schmelzmuster with exclusively vertical HSB, underscoring their unique evolutionary trajectory within the broader context of Litopterna.