Variations in craniodental morphology have been correlated to feeding adaptations in living organisms and used as proxies for paleodiet reconstruction. Within the mammalian order Carnivora, the Miocene fossil musteloid Leptarctus has been variably interpreted as a carnivore, frugivore, herbivore, omnivore, or insectivore based on morphological comparisons with extant species. Here, we perform the first simulation of cranial biomechanics in Leptarctus primus, aiming to identify a living analogue using biomechanical capability rather than qualitative morphology. Finite element models (FEMs) of 18 extant carnivorans and two extinct outgroup taxa were used to compare known dietbiomechanics relationships with the biomechanical properties of L. primus FEMs within a phylogenetic context. Multivariate analyses of simulated bite efficiency and skull stiffness values indicate that L. primus is most similar overall to Taxidea taxus (American badger) in unilateral bite simulations. Based on biomechanical predictions, we postulate that L. primus resembled the American badger in its feeding ecology more closely than any other taxon tested and thus conclude that L. primus was dominantly a carnivore with an auxiliary feeding capability of omnivory. We also compared the L. primus FEMs with the potentially synonymous Hypsoparia bozemanensis to determine a possible range of feeding capabilities. We observed an increase in mechanical efficiency with a deepening of the zygomae of H. bozemanensis, a trait previously used to differentiate it from L. primus. Ongoing work to expand the database of cranial biomechanical simulation data across Carnivoramorpha should help to further clarify evolutionary patterns of skull biomechanical specializations in musteloids and other carnivorans.

Recent field work in the upper Permian Moradi Formation of Niger has yielded new material of the lateoccurring and largest moradisaurine captorhinid, Moradisaurus grandis. The material includes two right hemimandibles, which represent individuals that are slightly smaller than the holotype. The smaller size of these mandibles, together with evidence in the form of tooth replacement, strongly suggest that the new material is ontogenetically younger than the holotype. The available evidence implies that tooth rows were added during growth in M. grandis. The mandible of M. grandis is distinguished from other moradisaurines in the extreme lingual extension of the tooth plate, which forms a dental shelf or plateau that supports the three lingual-most rows of teeth. In addition, a mentomeckelian bone is preserved, the first to be reported in a Paleozoic reptile. In contrast to the conspicuous wear documented on the teeth of certain other moradisaurines, wear facets are not observed on the teeth of the M. grandis specimens described here. The absence of tooth wear is surprising because the articular morphology of M. grandis is virtually identical to, albeit slightly larger than, that of Labidosaurikos meachami, for which propalinal jaw movement was first inferred in a moradisaurine. A phylogenetic analysis incorporating the new anatomical information confirms the hypothesis that M. grandis forms a clade with Rothianiscus multidontus and Gansurhinus qingtoushanensis within Moradisaurinae, and positions Captorhinikos chozaensis as the basal-most member of the subfamily.

The early evolutionary history of waterfowl (Anseriformes) is poorly understood. Aside from the morphologically aberrant Presbyornithidae, the only other known early Paleogene anseriform taxon is Anatalavis oxfordi from the early Eocene of England. Here, I describe two bones from the late Paleocene of southern Mongolia (localities Naran-Bulag and Tsagaan-Khushuu), which belong to a very large swan-sized swimming waterfowl (Anseriformes) and are described as Naranbulagornis khun, gen. et sp. nov. This is the first non-presbyornithid anseriform bird found in the early Paleogene (Paleocene through early Eocene) of Asia and one of the few currently known Paleocene birds from Central Asia. It is also the largest volant bird from the Paleogene of Asia. Naranbulagornis khun documents a mosaic of autapomorphic and plesiomorphic features that are otherwise present in modern Anhimidae, Anseranatidae, and Anatidae, indicating a greater morphological diversity of early Cenozoic anseriforms than previously thought. The new taxon is morphologically closer to Anatoidea than to Presbyornithidae and represents the earliest global evidence of a spatial coexistence between presbyornithids and other basal waterfowl during the early Paleogene.

In this article, we present the proboscideans from the late Miocene (Turolian) locality Chomateri, Greece, near the classical locality Pikermi. The material consists of juvenile teeth, whose morphological features, such as the dislocation of the half-loph(id)s and the resultant alternate arrangement of the successive loph(id)s (anancoidy), permit assignment to the tetralophodont gomphothere Anancus. However, the anancoidy is rather weak and the occlusal morphology simple, both regarded as primitive features within anancines. Reexamination of the late Miocene anancines from Europe reveals that they all share primitive molar features (weak anancoidy, simple morphology, thick enamel) with the material from Hohenwarth, Austria, showing further primitive skull features, such as the longer mandibular symphysis compared with other anancines. The proper name to refer to the late Miocene anancines from Europe is Anancus lehmanni (Gaziry, 1997), with type locality Dorn-Dürkheim 1 (Turolian; Germany). The presence of Anancus in Chomateri constitutes the first late Miocene record of the genus in Greece, and the first faunal element that clearly indicates that Chomateri postdates the classical Pikermi. Finally, we discuss the biostratigraphy and the biogeography of the late Miocene anancines of the Old World. Anancus originated possibly at ∼9.0–8.5 Ma in Asia and entered Europe during the second half of the Turolian, at ∼7.2 Ma (Tortonian-Messinian boundary). The arrival of Anancus in Europe coincides with a faunal turnover in both the eastern and western sectors of the European Mediterranean region, and in the southern Balkans in particular, with the decline of the ‘Pikermian’ large-mammal fauna.

Triassic-aged fossil vertebrates have been sporadically collected from the Fremouw Formation, central Transantarctic Mountains, since their initial discovery in the late 1960s, giving paleontologists insight into high-latitude faunas in the wake of the end-Permian mass extinction event. On a recent expedition (2010–2011), a small reptile skeleton was collected from Graphite Peak, which we present here alongside novel geological data and interpretations taken on site. Antarctanax shackletoni, gen. et sp. nov., is known from a partial postcranial skeleton including cervical and dorsal vertebrae, a humerus, and both pedes. Important morphological information includes well-defined laminae and deep fossae on cervicodorsal vertebrae. The new taxon can be differentiated from previously known Fremouw Formation reptiles (e.g., Prolacerta, Procolophon), as well as those from the Karoo Basin, South Africa (e.g., Mesosuchus, Proterosuchus, Euparkeria). Our inclusion of A. shackletoni in phylogenetic analyses of early amniotes finds it as an archosauriform archosauromorph, increasing known archosauriform diversity in the Early Triassic. The fauna of the lower Fremouw Formation traditionally has been considered to represent a subset of the Lystrosaurus Assemblage Zone of the Karoo Basin, with differences largely a result of pronounced differences in sampling intensity. However, a review of recent changes to the fauna, as well as a reassessment of occurrences based on older literature, indicates that significant discrepancies, including the co-occurrences of taxa known from both earlier and later in time and the presence of endemic forms in Antarctica, exist between the faunas of the Lystrosaurus Assemblage Zone and lower Fremouw Formation.

Isolated cheek teeth of Sciuridae (Mammalia, Rodentia) from 15 early and middle Miocene localities in Anatolia (Turkey) are described. The localities range in age from local zone B to local zone H, where zones are correlated to European MN zones 1 to 7 + 8. The material represents the two subfamilies: Sciurinae (ground and tree squirrels) and Pteromyinae (flying squirrels). The number of different species found at a single locality ranges from one to four. The Sciurinae were found to belong to the genera Palaeosciurus, Dehmisciurus (formerly called ?Ratufa), Spermophilinus, Tamias, and Atlantoxerus. Members of the Pteromyinae are Hylopetes, Miopetaurista, Aliveria, Albanensia, and Blackia. The MN 2 locality Harami 1 has yielded the oldest Spermophilinus and Miopetaurista known so far. Atlantoxerus adroveri from Bağiçi and Yenieskihisar (MN 7 + 8) is the first member of the Xerini tribe in Anatolia. The squirrel from Keseköy (MN 3), described as Palaeosciurus aff. feignouxi, shows close morphological resemblance to early Miocene Protospermophilus kelloggi from North America. This may suggest that European Palaeosciurus and American Protospermophilus are closely related genera. Three MN 7 + 8 localities (Sarıçay, Bağiçi, and Yenieskihisar) yielded two Spermophilinus species, which shows that Spermophilinus developed, at least locally, along two parallel lines. Similarity between Anatolian and European squirrel assemblages confirms that Anatolia was connected to the European part of the Eurasian continent in early and middle Miocene times. The lacustrine character of the sediments and the presence of both ground and/or tree and flying squirrels indicate that most of the localities represent a wet, but forested environment.

Cavenderichthys talbragarensis (Woodward, 1895) is a common signature freshwater stem teleost in the Upper Jurassic of Australia. Our discovery of numerous otoliths in the Talbragar Fish Bed establishes Cavenderichthys as only the second known Jurassic teleost and the third known stem teleost with otoliths in situ and provides an important calibration point for the interpretation of isolated Mesozoic otoliths. It corroborates the low degree of morphological diversification of stem teleost otoliths. The abundance of otoliths (isolated, in coprolites and in situ) allows the mapping of ontogenetic effects and of intraspecific variability for the first time in attributable stem teleost otoliths. Here, we describe 284 otoliths, mostly from Cavenderichthys. Otoliths in situ in Cavenderichthys document the presence of sagitta, lapillus, and asteriscus. Three other, much rarer otolith types were also found that have a teleost otolith pattern more primitive than that of Leptolepidiformes; the origin of these is unknown, but it is likely that they stem from pholidophoriform fishes of the family Archaeomenidae. If confirmed, this otolith pattern would provide a further highly diagnostic and synapomorphic character to define teleosts at the level of the Leptolepidiformes and above. Different mineral replacement processes in fossils from the Talbragar Fish Bed aided us in finding many otoliths, particularly otoliths in situ. Given their abundance and distinct preservation, it is surprising that otoliths in the Talbragar Fish Bed have not been recognized before.

The East Asian records of Amphicyon are rare, fragmented, and unclear. Heretofore, no material could be definitely assigned to this genus. Here, we report a maxillary fragment with P4–M2 and the M3 alveolus from the early middle Miocene Dingjia'ergou fauna, Tongxin, northern China. The enlarged and anteroposteriorly elongated M2 clearly allows us to assign the specimen to Amphicyon as Amphicyon zhanxiangi, sp. nov. The new species is closest to European A. giganteus (e.g., presence of the parastyle and a posteriorly located protocone of P4) and represents an immigrant from Europe during the late early or the earliest middle Miocene. The morphological differences between Amphicyon zhanxiangi, Amphicyon ulungurensis, and North American Amphicyon species suggest at least three dispersal events of the genus from Europe to East Asia during the early and middle Miocene. Amphicyon sp. from the late Miocene site in the Lower Irrawaddy near Magway, Myanmar, shares the enlarged and anteroposteriorly elongated M2 with Amphicyon zhanxiangi and could be a descendant of the latter species.

The Mylodontinae from the late Pleistocene (Lujanian Stage/Age) of Argentina, represented by Lestodon armatus, Mylodon darwinii, and Glossotherium robustum, are known from numerous remains, whereas early and middle Pleistocene Mylodontinae are less well characterized owing to insufficient material. In this work, we describe a new genus and species of Mylodontinae from the Ensenadan Stage/Age based on a cranium and comparisons with South and North American Mylodontinae. The new genus is similar to the taxon Mylodon darwinii, but the presence of plesiomorphic characteristics support a more primitive status for this novel sloth. The addition of this new taxon, Archaeomylodon sampedrinensis, gen. et sp. nov., increases the diversity of Mylodontinae during the middle Pleistocene. This new mylodontine in the middle Pleistocene is an important reference point for interpretations of increasing body size in South American Mylodontinae during the Pliocene–Pleistocene. These changes in body size may have been influenced by the arrival of Holarctic immigrants to South America.

The teeth of two megatooth macro-predatory shark species (Carcharocles chubutensis and Carcharocles megalodon; Otodontidae, Chondrichthyes) occur within the Miocene Chesapeake Group of Maryland, U.S.A. Definitive separation between all the teeth of Carcharocles chubutensis and Carcharocles megalodon is impossible because a complex mosaic evolutionary continuum characterizes this transformation, particularly in the loss of lateral cusplets. The cuspleted and uncuspleted teeth of Carcharocles spp. are designated as chronomorphs because there is wide overlap between them both morphologically and chronologically. In the lower Miocene Beds (Shattuck Zones) 2–9 of the Calvert Formation (representing approximately 3.2 million years, 20.2–17 Ma, Burdigalian) both cuspleted and uncuspleted teeth are present, but cuspleted teeth predominate, constituting approximately 87% of the Carcharocles spp. teeth represented in our sample. However, in the middle Miocene Beds 10–16A of the Calvert Formation (representing approximately 2.4 million years, 16.4–14 Ma, Langhian), there is a steady increase in the proportion of uncuspleted Carcharocles teeth. In the upper Miocene Beds 21–24 of the St. Marys Formation (representing approximately 2.8 million years, 10.4–7.6 Ma, Tortonian), lateral cusplets are nearly absent in Carcharocles teeth from our study area, with only a single specimen bearing lateral cusplets. The dental transition between Carcharocles chubutensis and Carcharocles megalodon occurs within the Miocene Chesapeake Group. Although this study helps to elucidate the timing of lateral cusplet loss in Carcharocles locally, the rationale for this prolonged evolutionary transition remains unclear.

Three juvenile specimens of Prosaurolophus maximus, represented by articulated to disarticulated skeletons, are the smallest known individuals for the taxon. Cranial anatomy of the juvenile specimens indicates that diagnostic characters of P. maximus are ontogenetically variable. In the smallest individual, the crest and deeply excavated fossa at the caudal margin of the circumnarial depression are poorly developed or absent. The crest approaches adult-like morphology in large juveniles, whereas crest robusticity and the deep excavation of the circumnarial depression occur only in subadult and adult individuals. The shape of the caudal margin of the circumnarial depression is consistent between juvenile and adult individuals, potentially making this feature a reliable character for taxonomic identification at younger ontogenetic stages. The crest of P. maximus grows isometrically during ontogeny, unlike the positive allometric growth of lambeosaurine hadrosaur crests, suggesting that this taxon may have had soft tissue structures associated with the narial-crest region, rather than the bony crest itself, selected for sexual display. Recovered from sediments of the Bearpaw Formation deposited during the Baculites compressus ammonite zone and magnetochrons 33n.3n to 33n.2n, the juvenile specimens are stratigraphically younger than P. maximus specimens from the Dinosaur Park Formation (Alberta) and contemporaneous with most specimens from the Two Medicine Formation (Montana), extending the temporal range of the taxon to 75.7–74 Ma. The occurrence of P. maximus in well-drained terrestrial deposits of the Dinosaur Park and Two Medicine formations and marine sediments of the Bearpaw Formation indicates that this taxon inhabited various paleoenvironments in western North America.

The core of the fossil record of Teleosauridae, a family of thalattosuchian crocodylomorphs, is well known from western Tethyan marine deposits of the Jurassic. Outside this province, their fossil record is patchy and in need of revision, with specimens from Russia, Madagascar, and Asia. Peipehsuchus teleorhinus is known from the Early or Middle Jurassic of China and teleosaurid specimens have been mentioned or preliminarily described from two Jurassic localities in Thailand, yet they were not assigned to a given taxon. Thanks to recent field work, at least 10 individuals represented by cranial material were excavated and prepared from a single Jurassic locality known as Phu Noi in the lower Phu Kradung Formation of northeastern Thailand. Here, we describe these specimens, together with disarticulated postcranial elements, and erect a new taxon, Indosinosuchus potamosiamensis, gen. et sp. nov. Phylogenetic analyses confirm the teleosaurid affinities of the new species, which does not form an exclusive clade with the Chinese teleosaurid Peipehsuchus teleorhinus. The presence of teleosaurids at Phu Noi and a preliminary account of its faunal content favor a Middle to Late Jurassic age for the fossilbearing horizon. In contrast, Cretaceous deposits in Thailand are characterized by goniopholidids and pholidosaurids, indicating a faunal turnover in Southeast Asia across the Jurassic–Cretaceous. As previously shown by isotope data, the new teleosaurid species was a resident of the freshwater environment and co-occurs with remains of exclusively terrestrial taxa such as sauropod, ornithopod, and theropod dinosaurs and freshwater tetrapods such as turtles and temnospondyls.

Neoepiblemidae (Caviomorpha) includes South American hystricognath rodents that together with Chinchillidae and Dinomyidae compose the clade Chinchilloidea. Despite the considerable advance in knowledge of the past decades, these extinct rodents are still poorly studied. To contribute to the taxonomy, systematics, and ontogeny of this group, in this paper we study the cheek tooth morphology of the genus NeoepiblemaAmeghino, 1889, from upper Miocene deposits through qualitative and quantitative analyses. For this purpose, we describe the anatomical variation, perform a quantitative analysis using linear measurements, and provide comments on the dental replacement. Based on the cheek tooth morphology, our interpretations indicate that there are two species of Neoepiblema that can be differentiated from each other. Neoepiblema ambrosettianus is not a valid name, and N. horridula, the first described Neoepiblema species, is the senior synonym. Hence, N. acreensis is a valid name for the second species of the genus. In neoepiblemids, the premolar is replaced during the early postnatal period, similar to euhypsodont dinomyids and in contrast to some other euhypsodont caviomorphs (e.g., cavioids), which replace the premolar during the intrauterine stage. These data on dental replacement in neoepiblemids contribute to knowledge about the ontogeny of this extinct rodent group.

Awell-preserved astragalus of the anomaluroid rodent Pondaungimys anomaluropsis is described from the late middle Eocene Pondaung Formation of central Myanmar. This specimen is the first postcranial element of a rodent from the Pondaung Formation and the oldest postcranial fossil currently known for Anomaluroidea. It illuminates plesiomorphous postcranial conditions in early anomaluroids and provides a basis for reconstructing the evolution of arboreal locomotion in this group of rodents. In contrast to those of living anomaluroids, the astragalus of Pondaungimys bears features indicating a less mobile ankle, specifically including a reduced range of plantarflexion at the upper ankle joint and a diminished capacity for inversion at the lower ankle joint. Its anatomy suggests that early anomaluroids were generalized quadrupeds, with intermediate arboreal adaptations between those of paramyids and modern anomaluroids. A cladistic analysis based on astragalar characters corroborates a basal position for Pondaungimys among anomaluroids. The phylogenetic signal derived from astragalar morphology is consistent with recent assessments of relationships among living anomaluroids based on craniodental characters and molecular data. Stem anomaluroids such as Pondaungimys apparently lacked the ability to glide, a locomotor pattern that is retained in extant Zenkerella.

The first intact North American Macroelongatoolithus specimen was excavated from sediments in the Wayan Formation of southeastern Idaho and initially reported by Krumenacker et al. (2017, Historical Biology 29:170–186). Here, we present a description of the specimen, including a zonal analysis of microstructural variation and comparison with an egg pair from the Liangtoutang Formation of Zhejiang, China. We assign egg pair IMNH 2428\49608 from the Albian–Cenomanian Wayan Formation of Idaho and egg pair ZMNH M8705 from the Liangtoutang Formation of China to the oospecies Macroelongatoolithus carlylei. The eggs are 39.8–41.7 cm long and 10.8–14.3 cm wide, and the eggshell consists of two structural layers of calcite demarcated by a distinct, undulating boundary. Eggshell thickness is 1.01–2.17 mm, and the continuous layer to mammillary layer ratio (CL:ML) ranges from 2.05:1 to 7.68:1. Identification of intact Macroelongatoolithus specimens from North America supports synonymy of M. xixiaensis with M. carlylei, as proposed by Zelenitsky et al. (2002) on the basis of eggshell fragments. Due to the substantial microstructural variation of the specimens reported here, we also regard M. goseongensis and M. qiongensis as junior synonyms of M. carlylei. The degree of microstructural variation within the new specimens expands the expected range of variation typically used in diagnosing oospecies and has implications for the naming of new oospecies on the basis of fragments or egg portions alone. The discovery of Macroelongatoolithus eggs from the latest Early Cretaceous of Idaho confirms the presence of this extraordinary oogenus outside of Asia and provides nonskeletal evidence of a Gigantoraptor-sized oviraptorosaur in western North America.

In rodents, and other vertebrates in general, the morphology of tarsal bones, especially the astragalus and calcaneus, has been shown to be tightly linked to locomotor movements. As a result, it has been used to infer locomotor behaviors in extinct species. Recent expeditions in Peruvian Amazonia have led to the discovery of the oldest caviomorph rodent fossils in South America, including two calcanei and one astragalus. The morphologies of these three tarsal bones are described in detail and compared with other extant and extinct caviomorphs. In order to assess and infer the locomotor behaviors of these rodents, linear measurements were taken on these tarsal bones and analyzed via multivariate analyses based on a previously assembled large data set. Both qualitative and quantitative analyses consistently suggest that the osteological adaptations of the astragalus enhance movements for climbing, those of one calcaneus rather enhance movements indicating terrestrial and partly fossorial lifestyle, whereas the other calcaneus may have belonged to a generalist form with a tendency toward a semiaquatic lifestyle. These results fit well with the associated paleoenvironments and hint at ecological diversity early in caviomorph history.

We describe Tupelocetus palmeri, a new genus and species of archaeocete whale, based on a single specimen from the Cross Member of the middle Eocene Tupelo Bay Formation (Bartonian). The holotype consists of a partial cranium with complete petrosals, posterior processes of tympanics, the posterior ends of the nasals, a part of the right orbit, and what are interpreted as the right and left P2s. The incomplete skull was collected from the Martin Marietta Cross Quarry (Berkeley County, South Carolina), which has also furnished the holotype of Carolinacetus gingerichi. The new species differs from all other protocetids in having a deep cavity on the occiput, as well as having the following combination of features: large nasal processes of frontals, single-cusped P2, and premaxilla terminates at level of P2 or P3. A phylogenetic analysis places T. palmeri in a large polytomy along the cetacean stem, more closely related to crown Cetacea than Maiacetus, Artiocetus, and Rodhocetus, but more basal than Georgiacetus, Babiacetus, and Eocetus. Another member of this polytomy is Natchitochia jonesi, and although specimens of Natchitochia and T. palmeri do not share any elements, what is preserved suggests that they are similar in size and among the largest of all protocetids. Although we are unable to differentiate these two species, we outline future data that could resolve this question.

Adaptations for feeding underwater were crucial to the success of pinnipeds (seals, sea lions, and walruses) in their transition from terrestrial to aquatic habitats. Extant phocids (true seals) use multiple feeding strategies—biting, filter, and suction feeding—to capture and consume prey, and each strategy is associated with cranial, mandibular, and dental adaptations. However, little is known about feeding strategies in stem pinnipeds. The objectives of this study were to investigate feeding strategies used by some extinct pinnipeds based on cranial and mandibular morphologies and use this framework to examine the evolution of phocid feeding strategies. Three-dimensional cranial and mandibular landmark data were collected from 249 extant and fossil pinnipeds. Principal component analysis and canonical variate analysis were performed to describe the major axes of variation and compare overlap of fossil and extant taxa in morphospace. Stem pinnipeds had morphologies associated with biting and filter feeding. Several fossil taxa were most similar to extant biters, suggesting that biting was a common and important feeding strategy for early phocids. One fossil taxon, Homiphoca capensis, was potentially a filter feeder, because it consistently overlapped with extant filter feeders in cranial morphospace. No fossil taxa had morphological adaptations for suction feeding, indicating that suction feeding is a more derived strategy in phocids. Extant phocids and their ancestors have cranial and mandibular adaptations for multiple feeding strategies, which allowed these animals to move into diverse aquatic niches and likely contributed to their successful transition from terrestrial to aquatic ecosystems.

Represented by a nearly complete cranium with associated mandible, teeth, and vertebrae, Yaquinacetus meadi is a new genus and species of archaic homodont odontocete from the latest Oligocene–early Miocene (24–19.2 Ma) of Oregon, U.S.A. The new species is characterized by a moderately elongated rostrum bearing approximately 51 alveoli per tooth row and a knob-like, rectangular vertex. Together with Squaloziphius emlongi from the early Miocene of Washington State, Y. meadi constitutes a new odontocete family, Squaloziphiidae, fam. nov., diagnosed by a unique combination of characters, including transversely wide dorsal opening of the mesorostral groove at base of rostrum, followed posteriorly by an abrupt narrowing; thickened lateral margin of the maxilla in the antorbital region making a long and laterally concave crest; and massive, anteroposteriorly and ventrally long postglenoid process of the squamosal. Although sharing with Ziphiidae the presence of transverse premaxillary crests on the vertex, Squaloziphiidae differs in the pterygoid sinus fossa being shorter anteriorly and ventrally; the tubercule of the malleus being less reduced; and lacking a pair of enlarged alveoli for mandibular tusks. Our phylogenetic analysis confirms the sister-group relationship between S. emlongi and Y. meadi, either as late diverging stem odontocetes or as early crown odontocetes, but distant from Ziphiidae. These results confirm the northeastern Pacific as a center of diversification for several groups of archaic homodont odontocetes during the late Oligocene–early Miocene.

The Mylodontinae from the late Pleistocene (Lujanian Stage/Age) of Argentina, represented by Lestodon armatus, Mylodon darwinii, and Glossotherium robustum, are known from numerous remains, whereas early and middle Pleistocene Mylodontinae are less well characterized owing to insufficient material. In this work, we describe a new genus and species of Mylodontinae from the Ensenadan Stage/Age based on a cranium and comparisons with South and North American Mylodontinae. The new genus is similar to the taxon Mylodon darwinii, but the presence of plesiomorphic characteristics support a more primitive status for this novel sloth. The addition of this new taxon, Archaeomylodon sampedrinensis, gen. et sp. nov., increases the diversity of Mylodontinae during the middle Pleistocene. This new mylodontine in the middle Pleistocene is an important reference point for interpretations of increasing body size in South American Mylodontinae during the Pliocene–Pleistocene. These changes in body size may have been influenced by the arrival of Holarctic immigrants to South America.