The multicausal twilight of South American native mammalian predators (Metatheria, Sparassodonta).

Sparassodonts were the apex mammalian predators of South America throughout most of the Cenozoic, diversifying into a wide array of niches including fox-like and even saber-toothed forms. Their extinction is still controversial, with different authors suggesting competition with other predators (placental carnivorans, terror birds, and carnivorous opossums), extinction of prey, and climate change as causal explanations. Here, we analyse these hypotheses using a novel approach implicating Bayesian analyses.

Global elongation and high shape flexibility as an evolutionary hypothesis of accommodating mammalian brains into skulls.

Little is known about how the large brains of mammals are accommodated into the dazzling diversity of their skulls. It has been suggested that brain shape is influenced by relative brain size, that it evolves or develops according to extrinsic or intrinsic mechanical constraints, and that its shape can provide insights into its proportions and function. Here, we characterize the shape variation among 84 marsupial cranial endocasts of 57 species including fossils, using three-dimensional geometric morphometrics and virtual dissections.

Evolution of the hypercarnivorous dentition in mammals (Metatheria, Eutheria) and its bearing on the development of tribosphenic molars.

One major innovation of mammals is the tribosphenic molar, characterized by the evolution of a neomorphic upper cusp (=protocone) and a lower basin (=talonid) that occlude and provide shearing and crushing functions. This type of molar is an evolutionarily flexible structure that enabled mammals to achieve complex dental adaptations. Among carnivorous mammals, hypercarnivory is a common trend that evolved several times among therians (marsupials, placentals, and stem relatives).

Endocranial morphology of Palaeocene Plesiadapis tricuspidens and evolution of the early primate brain.

Expansion of the brain is a key feature of primate evolution. The fossil record, although incomplete, allows a partial reconstruction of changes in primate brain size and morphology through time. Palaeogene plesiadapoids, closest relatives of Euprimates (or crown-group primates), are crucial for understanding early evolution of the primate brain.

Earliest evidence of mammalian social behaviour in the basal Tertiary of Bolivia.

The vast majority of Mesozoic and early Cenozoic metatherian mammals (extinct relatives of modern marsupials) are known only from partial jaws or isolated teeth, which give insight into their probable diets and phylogenetic relationships but little else. The few skulls known are generally crushed, incomplete or both, and associated postcranial material is extremely rare. Here we report the discovery of an exceptionally large number of almost undistorted, nearly complete skulls and skeletons of a stem-metatherian, Pucadelphys andinus, in the early Palaeocene epoch of Tiupampa in Bolivia.

Cranial anatomy of the earliest marsupials and the origin of opossums.

Background: The early evolution of living marsupials is poorly understood in part because the early offshoots of this group are known almost exclusively from jaws and teeth. Filling this gap is essential for a better understanding of the phylogenetic relationships among living marsupials, the biogeographic pathways that led to their current distribution as well as the successive evolutionary steps that led to their current diversity, habits and various specializations that distinguish them from placental mammals.

Methodology/principal Findings: Here we report the first skull of a 55 million year old peradectid marsupial from the early Eocene of North America and exceptionally preserved skeletons of an Oligocene herpetotheriid, both representing critical groups to understand early marsupial evolution.

Petrosal anatomy in the fossil mammal Necrolestes: evidence for metatherian affinities and comparisons with the extant marsupial mole.

We present reconstructions of petrosal anatomy based on high-resolution X-ray computed tomography scans for the fossil mammal Necrolestes and for the marsupial mole Notoryctes sp. Compared with other mammals, Necrolestes exhibits a mosaic of plesiomorphic and derived characters, but most of the evidence supports its metatherian status. We revised previous descriptions and report on features of phylogenetic or functional significance.

Exceptionally preserved North American Paleogene metatherians: adaptations and discovery of a major gap in the opossum fossil record.

A major gap in our knowledge of the evolution of marsupial mammals concerns the Paleogene of the northern continents, a critical time and place to link the early history of metatherians in Asia and North America with the more recent diversification in South America and Australia. We studied new exceptionally well-preserved partial skeletons of the Early Oligocene fossil Herpetotherium from the White River Formation in Wyoming, which allowed us to test the relationships of this taxon and examine its adaptations. Herpetotheriidae, with a fossil record extending from the Cretaceous to the Miocene, has traditionally been allied with opossums (Didelphidae) based on fragmentary material, mainly dentitions.