The family Isotemnidae, now considered paraphyletic by some, is composed of the most basal members of the suborder Toxodontia. They were moderate to large, plantigrade or semidigitigrade mammals, with features related to weight bearing. Several isolated postcranial remains from the Sapoan Fauna (early Eocene) have been referred to one of the earliest isotemnid genera, Isotemnus. This is the largest notoungulate genus found in the Sapoan Fauna, and isotemnid remains can be distinguished from those of similarly-sized notostylopids. Here, the anatomy of these remains is compared with those of later isotemnids, Thomashuxleya and Periphragnis. Isotemnus was one of the smallest isotemnids (around 50 kg, inferred from the astragalus) while Thomashuxleya and Periphragnis were four times larger (∼200 kg). Thomashuxleya and Periphragnis share features not present in other Eocene notoungulates such as Notohippidae or Notostylopidae, including: a distal humerus with a sharp, salient medial trochlear crest; radial bicipital tuberosity not present or barely insinuated; astragalus with wide and low trochlea, short neck, and variable nuchal crest; and a calcaneus with a rectangular fibular facet, half the length of the ectal facet, and an unusually thick sustentaculum. Isotemnus shares some of these features, but in other aspects, it is more similar to smaller typotherian taxa; the astragalar neck is better developed, and a crest is present on the calcaneal body that is absent in Periphragnis. These differences could be due to the smaller size of Isotemnus and its earlier divergence from other toxodontians and could help elucidate the most basal morphology of Notoungulata.

Henricosborniids are a group of South American native ungulates mainly found in Argentina (several Patagonian localities, Jujuy, and Mendoza), Brazil (São José de Itaboraí), and Bolivia (Tiupampa) that have a biochron ranging from the Danian (Tiupampan SALMA) to the late Bartonian (Barrancan subage of the Casamayoran SALMA). The record of henricosborniids in Mustersan levels must be confirmed. The family includes the genera Henricosbornia, Peripantostylops, Othnielmarshia, Orome, and Simpsonotus. Their fragmentary fossil record, mainly limited to isolated teeth, has constrained their utility for obtaining phylogenetically useful characters; for that reason, only few taxa (i.e., Henricosbornia, Simpsonotus) are usually included in this type of analysis. Even though there is consensus about the basal position of the group within the order, the family is recovered as polyphyletic or, more commonly, paraphyletic. The analysis of specimens from the early Eocene of Las Violetas and Las Flores (Itaboraian SALMA), the oldest Patagonian localities yielding notoungulate remains, and Paso del Sapo, along with the implementation of both implied weighting and landmark information, allow us to discuss several features for use in henricosborniid phylogenetic analysis and identification: 1) ectoloph of the molars with well-marked paracone and metacone folds; 2) molars with crochet present, oblique to the ectoloph, with variable development; 3) triangular third upper molar, with poorly developed hypocone; 4) reduced trigonid with short paralophid; 5) entoconid more like an isolated cusp and not completely crested. Although useful, these characters need to be evaluated in a more exhaustive phylogenetic context.

Archaeopithecidae encompasses small Eocene notoungulates defined by Ameghino and characterized by an incipient trend towards hypsodonty. Although their remains are very abundant in early Eocene outcrops from Patagonia, their taxonomic diversity is poor, and very few species had been recognized. Teratopithecus elpidophoros gen. et sp. nov. from the early Eocene levels of Paso del Sapo, Las Violetas, and Cañadón Vaca is here described. It has a stylar cusp labial to the ectoloph, which is a novel feature not previously identified in notoungulates. We also describe other archaeopithecid remains from the Paso del Sapo area that are referred to Archaeopithecus cf. A. rogeri and discuss the biochron of the family. The study of previous collections made by Feruglio, Brandmayr, Simpson, and Pisano during the first half of the XX century allows confirmation of the presence of Archaeopithecidae in the Itaboraian, Riochican, and Mustersan Mammal Ages as well as in the Sapoan fauna.

Until the discovery of the genus Punahyrax (Geste Formation) in different localities of Catamarca and Salta provinces, “archaeohyracids” were unknown in northwestern Argentina. Here we analyze the record of these notoungulates in the region through specimens known for the Geste Formation and new remains recovered from the lower levels of the Quebrada de Los Colorados Formation, assessing their diversity and significance in the context of the pre-Deseadan radiation of Typotheria. We identify three “archaeohyracid” morphotypes for Barrancan levels and four for Mustersan levels in northwestern Argentina, implying the possible presence of as much as seven “archaeohyracid” taxa in the region for the last third of the Eocene, and illustrating a striking diversity, especially considering the scarce pre-Tinguirirican record known until now. These new records underline the faunal differences of the Geste and Quebrada de los Colorados formations regarding the highly fossiliferous Lower and Upper Lumbrera formations. In this sense, although the existence of a sample bias should not be completely dismissed, the possibility of important biogeographic barriers acting in the area by the late middle Eocene needs to be analyzed with independent evidence. The record of Pseudhyrax eutrachytheroides in northwestern Argentina represents the first notoungulate taxon common between this region and typical Patagonian localities and represents a useful and presumably reliable new basis for the assignment of a Mustersan age for the middle section of the Geste Formation.

The extinct South American native ungulates are widely known as a particular evolutionary experiment of mammalian herbivory. Among their representatives, forms with bunodont dentition have received little attention, being usually regarded as primitive and placed in the paraphyletic order “Condylarthra”. Here we describe a new Didolodontidae Saltaodus sirolli gen. et sp. nov. from the Eocene of Lumbrera Formation (Santa Bárbara Subgroup) near San Antonio de los Cobres, Salta Province, Argentina. Saltaodus is represented by a small left dentary with a canine alveolus and a brachydont dentition, with p1-2, the roots of p3 and p4-m2. The jaw is broken mesial to the canine alveolus, which is circular and mesially projected. There is only one root for p1 and p2. The specimen has some similarity with North American archaic ungulates but more resembles South American bunodont ungulates. In contrast to Kollpaniinae, Saltaodus presents more separate, slender and higher cusps with well-developed cristids; the talonid basin is wide and not filled by the base of the hypoconid or the distal wall of the metaconid. Several derived character states as compared to Kollpaniinae, such as the more molarized p4, with a clear difference between trigonid and talonid, molars with a talonid equal to or wider than the trigonid, and well separated hypoconid, hypoconulid and entoconid, allow the placement of Saltaodus among Didolodontidae. Saltaodus represents one of the few known small didolodontids (< 5 kg) from the lower latitudes of South America.

Astrapotheres were a clade of unusual early to middle Cenozoic herbivorous mammals endemic to South America. Neogene astrapotheres were large, tusked mammals that probably had a short proboscis and may have preferred mesic lowland habitats; they were widespread during the early Miocene, became restricted to the tropics during the middle Miocene, and apparently did not persist into the late Miocene. The geologically youngest astrapotheres pertain to the subfamily Uruguaytheriinae, and in this report, we describe a partial uruguaytheriine astrapothere cranium with well-preserved postcanine dentition that we identify as Granastrapotherium cf. snorki. This specimen was collected from fluviolacustrine strata in the Tumbes Region of extreme northwestern Peru that likely pertain to the Zorritos Formation. At present, the temporal range of Granastrapotherium snorki is restricted to the late middle Miocene (Serravallian Age; ca. 13.6–12.8 Ma), which suggests a similar age for the fossil-bearing sediments of the upper Zorritos Formation. The fossil locality, which is ca. 25 km southwest of the city of Tumbes, is ca. 1,000 km distant from other sites in Peru and Colombia where G. snorki has been recorded and extends the geographic range of the species westward more than 550 km. We estimate the body mass of G. snorki at 1,800–2,500 kg based on head-body length of 3.75 m; this is lower than dentition-based body mass estimates but still suggests G. snorki was the largest terrestrial mammal in South America at the time.

In this work, we describe two new species of proterotheriid litopterns, Olisanophus riorosarioensis gen. et sp. nov. and Olisanophus akilachuta sp. nov. from the middle Miocene (Laventan SALMA) of Quebrada Honda, Bolivia. When incorporated into a recently published phylogenetic analysis (40 taxa; 92 characters), they plot as sister taxa, partially supported by their connected metaconule and protocone on M3. Additionally, we revise the taxonomy of two contemporaneous proterotheriids from La Venta, Colombia. ‘Prolicaphrium' sanalfonensis is reassigned to Mesolicaphrium gen. nov., with a prominent protocone on M3 as an autapomorphy of the genus. We revalidate the genus Neodolodus for Neodolodus colombianus, a species referred to ‘Prothoatherium' or Lambdaconus by previous authors. We used the paleotree R package to examine evolutionary trends in diversity and body size (using m1 length as a proxy) in proterotheriid and macraucheniid litopterns in a phylogenetic context. Proterotheriids were more diverse in the Paleogene than their fossil record indicates; their diversity peaked in the early Miocene and gradually declined until the Pleistocene. Macraucheniids experienced two peaks in diversity, in the early and late Miocene, but were still fairly diverse in the Pleistocene, unlike proterotheriids. Multiple proterotheriid lineages became larger during the Paleogene, but body size was roughly static during the Neogene, with no obvious link between phylogeny and size. Macraucheniids can be grouped into three size classes that are phylogenetically conserved and roughly correspond temporally to Eocene (small Polymorphis spp.), Miocene–Pliocene (medium-sized ‘cramaucheniines’ and early macraucheniines, e.g., Theosodon, Promacrauchenia), and Pleistocene (large macraucheniines, e.g., Macrauchenia) species.

We describe a new interatheriid notoungulate, Juchuysillu arenalesensis gen. et sp. nov., based on six partial upper and lower dentitions from the early to middle Miocene Nazareno Formation of southern Bolivia. A specimen is also referred to J. arenalesensis from the early middle Miocene (Langhian) locality of Cerdas, Bolivia (ca. 100 km to the northwest). The new species is distinguished by its very small size (ca. 15% smaller than Protypotherium minutum) and the unique combination of shallow ectoloph sulci on P3-4, length of M1 > M2 > M3, trapezoidal upper molars, and absence of a buccal talonid sulcus on m3. A second, larger interatheriid species is present at Nazareno but is not represented by remains sufficiently complete for a more precise identification. A phylogenetic analysis indicates that J. arenalesensis represents a unique lineage within Interatheriinae that diverged after Protypotherium sinclairi but before Miocochilius anomopodus, Caenophilus tripartitus, Miocochilius federicoi, and several species of Protypotherium, including Protypotherium australe. Juchuysillu arenalesensis is one of two small (< ca. 3 kg) typotheres at Nazareno; its body mass is estimated at 1.1 kg. An updated faunal list for the Nazareno Formation includes 19 species of mammals pertaining to seven orders and 15 families; fragmentary turtle (Testudinidae?) and bird (Phorusrhacidae) remains are also present. The precise age of Nazareno Formation vertebrates is unknown, but the presence of at least three notoungulate species shared with Cerdas suggests a similar (∼16–15 Ma) age for the fossil-bearing levels.