Daniel Alfredo García López

South America underwent a span of isolation in the Cenozoic, during which an endemic fauna developed, including a diverse group of native ungulates. Among them, Proterotheriidae (Litopterna, Pan-Perissodactyla) were small to medium-sized hoofed herbivores mainly documented from the Oligocene to the Pleistocene. The study of proterotheriid mammals, specifically their cranial morphology, has been challenging due to the limited availability of materials, leading to a heavy reliance on dental evidence in their descriptions. This issue is particularly evident for late Neogene records, where comprehensive cranial studies have been lacking. Herein, we study a juvenile proterotheriid individual referred to Neobrachytherium intermedium recovered from Early Pliocene outcrops of northwestern Argentina. Our research focuses on cranial morphology, particularly on the dorsal and orbitotemporal regions, mesocranium, and basicranial structures. By employing noninvasive techniques, we describe the petrosal anatomy and compare it in a broader phylogenetic context. We also provide insights into ontogenetic stages within the genus. We find singular arrangements and structures for the genus (e.g., tuberosities in the postglenoid process) as well as traits shared with other mammals that are distinct from those found in other SANUs and pan-perissodactyls, such as Notoungulata (e.g., ringlike ectotympanic, gracile tympanohyal, poorly developed epitympanic wing and medial flange, large hiatus Fallopii). Traits such as the position and shape of the m. tensor tympani fossa and the development of the jugular notch may be synapomorphies of Litopterna, but this cannot be determined without resolving the phylogenetic position of Indalecia grandensis, which has been suggested to be a basal Litopterna. Considering ontogenetic observations, features like the frontal sulcus, the supraorbital foramen, and the palate configuration exhibit age-related changes. The occlusal patterns of teeth undergo substantial changes with wear, affecting loph morphology. Variations in deciduous premolars are observed within the same individual. These findings emphasize the importance of considering ontogenetic and individual variations when analyzing cranial and dental remains for taxonomic and phylogenetic purposes. The study provides valuable information for understanding late Neogene proterotheriids and the broader context of cranial evolution within Litopterna, highlighting the necessity for more comprehensive research.

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We present a detailed description of the anatomy of the dentary and lower teeth of a new specimen ofCallistoe vincei, a large carnivorous metatherian from the Eocene (?Ypresian) of northwestern Argentina. The recently collected specimen is a young adult represented by a partial right dentary with the canine, p1, roots of p3, and very well-preserved m1 to m4. The description includes a comparison with the holotype specimen, a much older individual, and other closely related large sparassodonts (e.g.,Arminiheringia). The analysis of this new material allowed identifying plesiomorphic molar features inCallistoe, such as the presence of a reduced metaconid on the m3 and a tricuspated, basined talonid on m1–m3. We also described the mesowear facets in the lower dentition, showing that the self-sharpening facet typically present in extinct and extant placental and some marsupial carnivorous forms, was absent inCallistoe. The presence of a short-term cutting edge in the trigonid related to the thinness of the enamel layer, and the associated tooth wear susceptibility, were likely compensated by a dental mechanism (overeruption) to maintain occlusal contact among antagonist teeth. This process could explain the marked extrusion of the tooth roots observed inCallistoeas well as in other large closely related members of the group.