Darin Andrew Croft

The early Oligocene Tinguiririca Fauna from the Abanico Formation, central Chile, is the earliest mammal fauna dominated by high-crowned (hypsodont) herbivores known globally. The drivers of early hypsodonty in South America are thus of considerable interest. We reconstruct the palaeoenvironment for Tinguiririca based on: (1) generalised linear latent variable models (GLLVMs), which compare body mass distributions of Tinguirirican and modern South American herbivores; and (2) reconstructed diets of Tinguiririca fauna notoungulates inferred from dental mesowear. Based on GLLVMs, the Tinguiririca Fauna’s palaeoenvironment is interpreted as relatively warm (22˚C mean annual temperature), dry (1200 mm yr ^-1 mean annual precipitation) and open (23% tree cover). The Brazilian Caatinga is the closest modern South American analogue to the dry, low-vegetation-density palaeoenvironment inferred for Tinguiririca. We interpret the fauna’s notoungulates as browsers, based on their sharp ectoloph wear facets. To test this interpretation, we also analysed the mesowear angles and relative crown heights of a broad array of leontiniid and homalodotheriid notoungulates, two clades specialised for high browsing that never attained the levels of hypsodonty seen in other notoungulates. Both have sharp mesowear and greater relative crown heights than modern high browsers. Our data suggest that neither gradual habitat change nor increased levels of volcaniclastic sedimentation alone caused the early attainment of hypsodonty in notoungulates. Alternative explanations, such as one or more rapid pulses of environmental change driving the increase in hypsodonty or the unique taxonomic composition of South American faunas, must be invoked to explain the marked increase in crown height from preceding time periods into the Tinguirirican South American Land Mammal Age.

Astrapotheres (Astrapotheria) are an order of South American native ungulates (SANUs), and the geologically youngest astrapotheres belong to the subfamily Uruguaytheriinae (Astrapotheriidae). In this study, we: (i) analyze uruguaytheriine remains from the late Middle Miocene Quebrada Honda Basin (QHB) of southern Bolivia; and (ii) discuss paleoecology of Bolivian astrapotheres based on new dental mesowear angle data and enamel stable carbon isotope (δ13C) data from these and other specimens. New material consists of a partial left maxilla preserving DP2–3 and an associated deciduous lower incisor. Two newly described specimens include a mostly complete m3 and a partial palate preserving left and right DP2–4. The QHB deciduous premolars are the first described for a uruguaytheriine and among the few described for astrapotheres. We conclude that the QHB specimens represent a new but unnamed species that likely does not pertain to any presently recognized genus. It differs from other uruguaytheriines in its intermediate size, relatively high-crowned teeth, presence of a lingually open M3 central valley, and absence of m3 hypoflexid, among other features. Astrapothere mesowear angle data from the QHB and slightly older Bolivian sites (Cerdas and Nazareno) suggest that Middle Miocene astrapotheres were browsers, perhaps resembling the extant black rhino (Diceros bicornis). New and updated enamel stable carbon isotope data suggest that QHB astrapotheres and toxodontid notoungulates fed on isotopically similar vegetation slightly more enriched (~1‰) than vegetation consumed by proterotheriid litopterns (Olisanophus spp.) and the notoungulate Hemihegetotherium trilobus. These data support paleopedology- and paleoichnology-based habitat reconstructions for the QHB that suggest it was more densely vegetated than Cerdas. Relatively enriched δ13C samples (> -7.0‰) from Cerdas and Quehua (Late Miocene) suggest that some Bolivian notoungulates were grazing on C4 vegetation, which casts doubt on the proposal that the southern Central Andean Plateau experienced significant uplift prior to ~9 Ma.

The Middle Miocene fauna of La Venta, Colombia, offers a grand opportunity to understand low-latitude South American ecosystems prior to the late Cenozoic Great American Biotic Interchange (GABI). We present new material of two proterotheriid litopterns, Villarroelia totoyoi and Mesolicaphrium sanalfonense, and a macraucheniid litoptern, Theosodon, from La Venta. During the GABI, North and South American faunas intermixed, after which some lineages proliferated and others died out. We conducted an ecomorphological analysis of 11 North and South American faunas pre-dating and post-dating the GABI by scoring all mid- to large-sized mammalian herbivores on their body size, tooth morphology, and feeding height. The fossils studied here offer new information on the deciduous dentition of M. sanalfonense and mandible of the La Venta Theosodon. Pre-GABI South American faunas were more ecomorphologically diverse than their North American counterparts. The post-GABI Pleistocene fauna exhibited similar ecomorphological diversity to pre-GABI South American faunas, but modern faunas show this diversity was mostly lost in the end-Pleistocene extinctions. The new litoptern fossils provide previously unknown information on these species’ morphology, but they do not resolve outstanding systematic and phylogenetic questions. Discrepancies in ecomorphological diversity between pre-GABI North and South American faunas is attributable to the presence of small, high-feeding (arboreal) taxa and hypselodont (ever-growing cheek teeth) taxa in South America, which North America lacked. Arboreal herbivores (porcupines and primates) experienced some success in North America after dispersing during the GABI. Although hypselodont xenarthrans were successful in post-GABI North America, the fact that most other hypselodont lineages went extinct during and after the GABI suggests that dental morphology may not fully capture the ecomorphological diversity in diet of North American herbivores. Future studies could examine faunas immediately before and after the GABI to uncover the precise dynamics of the interchange and why certain lineages succeeded while others failed.

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.

El sitio de Pampa Castillo en el sur de Chile ha producido una amplia variedad de restos de mamíferos del Mioceno Temprano (Burdigaliano; Edad-mamífero Santacrucense; ∼18 millones de años), incluyendo metaterios, roedores, xenartros y ungulados nativos sudamericanos. Aquí, documentamos los restos de los Folivora (perezosos) de la fauna de Pampa Castillo: un molde endocraneano natural que carece de los bulbos olfatorios y está ligeramente distorsionado y otros cuatro especímenes que en conjunto incluyen un diente molariforme parcial, un astrágalo casi completo, y varios huesos postcraneales parciales. Todos los especímenes de perezosos de Pampa Castillo son similares en tamaño y morfología, y los referimos a una sola especie del género megaterioideo Hapalops, basado principalmente en el astrágalo y el molde endocraneano. La especie de Hapalops de Pampa Castillo era relativamente pequeña, comparable a especies de la Formación Santa Cruz como H. angustipalatus, H. elongatus y H. ruetimeyeri. Estimamos su cociente de encefalización en 0,40, cercano a los valores de H. indifferens y Eucholoeops ingens (0,48–0,58), menor que los valores de los perezosos actuales (0,68–0,90) y mayor que los valores de perezosos pleistocénicos (0,16–0,31). La escasez de especímenes de perezosos en Pampa Castillo (<1% de los especímenes identificados) caracteriza a varios otros sitios contemporáneos del sur de Sudamérica (por ejemplo, la Formación Pinturas y la Upper Fossil Zone de Gran Barranca en Argentina y Sierra Baguales en Chile) y contrasta marcadamente con su abundancia en la Formación Santa Cruz, probablemente reflejando alguna combinación de diferencias climáticas y/o florísticas, tal vez relacionadas con variaciones temporales y/o biogeográficas. Hasta donde sabemos, el ejemplar de Pampa Castillo es el primer molde endocraneano natural descripto de un mamífero extinto de Chile.

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