The present paper describes a disarticulated skull of Allqokirus australis Marshall & Muizon, 1988, a basal sparassodont (Metatheria, Mammalia) from the early Palaeocene (c. 65 Ma.) of Tiupampa (Bolivia). The specimen includes the rostrum and palate with right premaxilla, both maxillae, left lacrimal, palatines and most upper teeth. The second largest element includes the frontals, the left squamosal, the parietals, the supraoccipital, the basisphenoid, the presphenoid, the alisphenoid, and part of the pterygoids. The nasals, basioccipital and exoccipitals are missing. Other elements are the left petrosal, the right jugal and squamosal, and both dentaries. The elements of the specimen allow for a good reconstruction of the skull, which is thoroughly described and compared to that of other sparassodonts and to the Tiupampa pucadelphyids, Pucadelphys and Andinodelphys. The dental morphology of Allqokirus australis is extremely similar to that of Patene simpsoni from the early Eocene of Itaboraí (Brazil) and presents distinct (although incipient) carnivorous adaptations. Furthermore, some characters of the ear region (e.g. medial process of the squamosal, deep groove for the internal carotid artery at the ventral apex of the petrosal) are also present in most other sparassodonts and in the pucadelphyids from the same locality. A parsimony analysis performed on the basis of a data matrix of 364 characters and 38 taxa placed Allqokirus in a sparassodont clade (the Mayulestidae) that also included Mayulestes and Patene. This family constitutes the sister group of all other sparassodonts. Our analysis also retrieved a large clade composed of the sparassodonts and the pucadelphyids, formally named Pucadelphyda n. superord. This superorder represents the large metatherian carnivorous radiation of the Tertiary of South America, which is first known at Tiupampa, and which started to diversify probably slightly earlier, during the late Cretacous in South America. So far, no representative of Pucadelphyda has been discovered in North America. At Tiupampa, Allqokirus and Mayulestes are the largest metatherians of the fauna and they fill the predaceous mammalian ecological niche. They are the earliest representatives of Sparassodonta, a successful metatherian carnivorous radiation which persisted in South America until the late Pliocene, i.e., during more than 63 Ma.

Alcidedorbignya inopinata Muizon & Marshall, 1987 is a basal pantodont (Placentalia, Mammalia) of small body size, from the early Palaeocene of the Santa Lucia Formation at Tiupampa, Bolivia. Tiupampa is the type locality for the Tiupampan, a South American Land Mammal age (SALMA), which is assigned an age equivalent to the basal Torrejonian 1 of North America (c. 65 Ma). Alcidedorbignya is known by exceptionally preserved specimens, which are described here. The two major specimens are an almost complete skeleton (MHNC 8372) and a partial skull (MHNC 8399), the former representing one of the best-preserved fossil placentals from the early Palaeocene and probably the oldest placental skeleton that is so completely known. These specimens are also the first eutherian skulls and skeleton ever discovered at Tiupampa, a locality which has yielded numerous metatherian skulls and partial skeletons. The remarkable preservation of the two skulls allows a detailed description of the cranial anatomy with well-identified sutures and foramina, including those of the auditory region. Through CT scanning of the skulls, the arterial and venous circulation pattern in the basicranium as well as the bony labyrinth of the inner ear were tentatively reconstructed. A thorough description of the postcranial skeleton of MHNC 8372 is also provided. Among pantodonts, Alcidedorbignya presents the closest morphological similarities with Pantolambda, the oldest and earliest diverging North American pantodont (known by skulls and skeletons), from the late early Palaeocene (Torrejonian 2 and 3) of New Mexico. Alcidedorbignya is one-third the size, much more gracile, and clearly exhibits more plesiomorphic features than Pantolambda. It is also at least 3 Ma older. Interesting similarities are also observed between the skull of Alcidedorbignya and several "condylarths", such as Maiorana, Baioconodon, Arctocyon, and Arctocyonides. The basicranium of Alcidedorbignya is also similar to that of some extant afrotheres (e.g., Tenrec) or Lipotyphla (e.g., 398GEODIVERSITAS • 2015 • 37 (4) Muizon C. de et al. Solenodon), but most of these similarities may represent placental symplesiomorphies. In fact, the cranial anatomy of Alcidedorbignya, beyond the simple thorough description of a basal pantodont, sheds light on the cranial anatomy of placentals from the earliest Paleocene, previously unknown in this detail. The postcranial skeleton of MHNC 8372 together with some isolated specimens referred to A. inopinata, is compared to adequate morphofunctional models (e.g., Solenodon, Dendrohyrax, Sciurus), which indicates that it was a moderately agile, plantigrade, generalized terrestrial mammal with good climbing ability (scansorial) and occasionally capable of standing in a bipedal position. The scutiform ungual phalanges were probably bearing nail-like hooves (or primate-like nails) and because of the absence of claws, fossorial habits are unlikely.A parsimony analysis of a data matrix including 72 taxa and 426 characters (cranial and postcranial...

BackgroundThe 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 FindingsHere 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. A comprehensive phylogenetic cladistic analysis of Marsupialia including the new findings and close relatives of marsupials show that peradectids are the sister group of living opossums and herpetotheriids are the sister group of all living marsupials.Conclusions/SignificanceThe results imply that North America played an important role in early Cenozoic marsupial evolutionary history and may have even been the center of origin of living marsupials and opossums. New data from the herpetotheriid postcranium support the view that the ancestral morphotype of Marsupialia was more terrestrial than opossums are. The resolution of the phylogenetic position of peradectids reveals an older calibration point for molecular estimates of divergence times among living marsupials than those currently used.

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. Analysis of the new material reveals that several aspects of the cranial and postcranial anatomy, some of which suggests a terrestrial lifestyle, distinguish Herpetotherium from opossums. We found that Herpetotherium is the sister group to the crown group Marsupialia and is not a stem didelphid. Combination of the new palaeontological data with molecular divergence estimates, suggests the presence of a long undocumented gap in the fossil record of opossums extending some 45Myr from the Early Miocene to the Cretaceous.

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The description of new specimens of kollpaniines "condylarths" from Tiupampa (early Palaeocene of Bolivia) represents a significant addition to the knowledge of the earliest fauna of South American ungulates. Several partial mandibles and maxillae of Molinodus suarezi and Simoclaenus sylvaticus are described. The morphology of the lower premolars of Molinodus, being associated to lower molars, is established and a previous referral of an isolated p4 is rejected. A maxilla of Simoclaenus reveals the morphology of the so far unknown P1-4 of this taxon and allows a discussion on the development of the protocone in Palaeocene "condylarths". The subvertical maxilla-premaxilla suture and the vertical implantation of the P1/p1 confirm the shortness of the snout of Simoclaenus, whereas the procumbency of the p1 of Molinodus indicates a longer rostrum. The upper molars of Molinodus confirm the presence of a tendency to duplication of the protocone, which is regarded as the incipient development of a pseudohypocone. The various patterns of formation of a hypocone (or pseudohypocone) are considered and, among other South American Native Ungulates, a protocone-derived pseudohypocone (i.e. Molinodus-like) is hypothesized in Lamegoia, Raulvaccia, and notoungulates, whereas a postcingulum-derived, hypocone is present in didolodontids and litopterns. The new specimens confirm the conspicuous small size of the M1/m1 of Molinodus and Simoclaenus as compared to the M2/m2. Consequently, we examined the relative proportions of molars in these taxa as compared to a variety of extant and extinct euungulates. Their proportions were plotted into the 'developmental' morphospace based on the predictive mathematical model of Kavanagh et al. (2007) (Inhibitory Cascade Model, or IC model), which might explain a large part of the mammalian diversity in molar proportions. Based on the upper molars, the Tiupampa kollpaniines were retrieved in a separate area of the predicted morphospace with other North American "condylarths" with large M2; this departure is also consistent with previous results concerning the lower molars (large m2). These peculiar molar proportions were found distinct from many other mammals, and might represent clade-specific differences: the large size of both the upper and lower second molars relative to other molars thus possibly representing a derived character state shared by some "condylarths" and kollpaniines.

New isolated petrosals from the Itaboraí beds of Brazil (late Palaeocene or early Eocene) are here described and referred to the early diverging litoptern Miguelsoria parayirunhor, based on phylogenetic, size, and abundance arguments. Both the external and internal anatomy of these specimens were investigated, which for the first time document many details of the auditory region of a Palaeogene litoptern. Our cladistic analysis, which included our new observations, failed to recover a monophyletic Litopterna but did not exclude it. A constrained analysis for the monophyly of this order showed that several features such as a (sub)quadrangular and anteroposteriorly elongated tensor tympani fossa and a large notch in the vicinity of the external opening of the cochlear canaliculus may constitute synapomorphies for Litopterna. The evolution of several other auditory characters amongst Litopterna is discussed and the relative dimensions of the inner ear and surrounding petrosal in the group were also investigated. This allowed detection of negative allometry of the bony labyrinth within the petrosal, which was confirmed by measurements and regression analysis across a larger sample of placental mammals. This scaling effect probably has an important influence on several characters of the bony labyrinth and petrosal, amongst which are the length of the vestibular aqueduct and cochlear canaliculus. It demonstrates that many aspects of the morphological variation of the bony labyrinth need to be thoroughly investigated before being incorporated into phylogenetic analyses.  © 2015 The Linnean Society of London