Sandrine Ladevèze

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. 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 45 Myr from the Early Miocene to the Cretaceous.

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. 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/Significance The 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.

First skull of Orthaspidotherium edwardsi (Mammalia, “Condylarthra”) from the late Paleocene of Berru (France) and phylogenetic affinities of the enigmatic European family Pleuraspidotheriidae

ABSTRACT Among the archaic ‘ungulates,’ pleuraspidotheriids are well documented by skulls and postcranial elements of Pleuraspidotherium from Berru and Cernay-lès-Reims (late Paleocene, Paris Basin, France). Nevertheless, the relationships of pleuraspidotheriids (i.e., Pleuraspidotherium, Orthaspidotherium, and Hilalia) to other ‘condylarths’ have not been conclusively settled. They have been related to the typically North American Meniscotheriidae, Phenacodontidae, and Mioclaenidae, and even to the modern ungulates Perissodactyla and Artiodactyla. We here provide additional data, especially from the first complete skull of Orthaspidotherium, and highlight a mosaic of plesiomorphic and derived characters that distinguish Pleuraspidotheriidae from all other ‘ungulates.’ Their basicranial morphology approximates the ancestral morphotype of artiodactyls, but this results from symplesiomorphies. Pleuraspidotheriids exhibit the following synapomorphies: strong processes on the petrosal and enclosure of the facial nerve by the tympanic process and tympanohyal. Although their dentition resembles that of perissodactyls, meniscotheriids, and phenacodontids, the development of a pseudohypocone from the metaconule unambiguously shows that these similarities arose by convergence. Other evolutionary tendencies are the strong reduction of the hypoconulid, molarization of last premolars, development of lingual cingulum on M2, lengthening of the snout, and presence of diastemata. The pleuraspidotheriid tarsal morphology is primitive and not uncommon among ‘condylarths’ but presents some synapomorphies such as a calcaneum with a transverse cuboid facet and an astragalus with a squatting facet, cotylar fossa, and sustentacular hinge. Our study does not indicate a close relationship of Pleuraspidotheriidae with any of the archaic or modern ‘ungulates’ mentioned above. Instead, primitive arctocyonids could represent a possible ancestral morphotype for pleuraspidotheriids.

Metatherian petrosals from the late Paleocene of Itaboraí, Brazil, and their phylogenetic implications

Abstract Four Paleocene petrosal bones from the Itaboraí fissure fillings of Brazil belong to two metatherian morphotypes. They are compared with ear regions of other fossil and recent marsupials. The results of a parsimony analysis using thirty-eight characters from the petrosal and ear region are partly congruent with recent studies and confirm that characters from the ear region are highly informative for phylogenetic reconstruction. Metatheria are diagnosed by five petrosal synapomorphies: stapedial artery absent in adults; reduced, intramural prootic canal; extrabullar internal carotid artery; inferior petrosal sinus between petrosal, basisphenoid, and basioccipital; and cava supracochleare and epiptericum completely separated. The stem group of South American and Australian metatherians is composed of the Paleocene taxa Pucadelphys, one of the oldest known metatherians from South America, and Petrosal Type II from Itaboraí. Among the South American-Australian metatherians two clades are defined here. The crown clade is composed of Australian taxa plus Dromiciops (here referred to Australidelphia sensu stricto), while they plus Caenolestes form the Australidelphia sensu lato. Petrosal Type I from Itaboraí is the most basal taxon of Australidelphia sensu lato, the latter diagnosed by three significant synapomorphies: ventral part of the mastoid rounded and bulbous owing to enlargement of the fossa subarcuata; posttemporal foramen absent; and foramen for a branch of the transverse sinus present on the posterior edge of the ventral pars canalicularis.

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. However, brain morphology of this group remains poorly documented, and major questions remain regarding the initial phase of euprimate brain evolution. Micro-CT investigation of the endocranial morphology of Plesiadapis tricuspidens from the Late Palaeocene of Europe—the most complete plesiadapoid cranium known—shows that plesiadapoids retained a very small and simple brain. Plesiadapis has midbrain exposure, and minimal encephalization and neocorticalization, making it comparable with that of stem rodents and lagomorphs. However, Plesiadapis shares a domed neocortex and downwardly shifted olfactory-bulb axis with Euprimates. If accepted phylogenetic relationships are correct, then this implies that the euprimate brain underwent drastic reorganization during the Palaeocene, and some changes in brain structure preceded brain size increase and neocortex expansion during evolution of the primate brain.

Testing for Depéret's Rule (Body Size Increase) in Mammals using Combined Extinct and Extant Data

Whether or not evolutionary lineages in general show a tendency to increase in body size has often been discussed. This tendency has been dubbed “Copes rule” but because Cope never hypothesized it, we suggest renaming it after Depéret, who formulated it clearly in 1907. Depérets rule has traditionally been studied using fossil data, but more recently a number of studies have used present-day species. While several paleontological studies of Cenozoic placental mammals have found support for increasing body size, most studies of extant placentals have failed to detect such a trend. Here, we present a method to combine information from present-day species with fossil data in a Bayesian phylogenetic framework. We apply the method to body mass estimates of a large number of extant and extinct mammal species, and find strong support for Depérets rule. The tendency for size increase appears to be driven not by evolution toward larger size in established species, but by processes related to the emergence of new species. Our analysis shows that complementary data from extant and extinct species can greatly improve inference of macroevolutionary processes.

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. These give an unprecedented glimpse into early metatherian morphology, evolutionary relationships and, especially, ecology. The remains of 35 individuals have been collected, with 22 of these represented by nearly complete skulls and associated postcrania. These individuals were probably buried in a single catastrophic event, and so almost certainly belong to the same population. The preservation of multiple adult, sub-adult and juvenile individuals in close proximity (<1 m2) is indicative of gregarious social behaviour or at least a high degree of social tolerance and frequent interaction. Such behaviour is unknown in living didelphids, which are highly solitary and have been regarded, perhaps wrongly, as the most generalized living marsupials. The Tiupampan P. andinus population also exhibits strong sexual dimorphism, which, in combination with gregariousness, suggests strong male–male competition and polygyny. Our study shows that social interactions occurred in metatherians as early as the basal Palaeocene and that solitary behaviour may not be plesiomorphic for Metatheria as a whole.

Reassessment of the Morphology and Taxonomic Status of the Earliest Herpetotheriid Marsupials of Europe

The earliest Eocene locality of Dormaal (Belgium) has provided the oldest Cenozoic herpetotheriid marsupials of Europe. No herpetotheriid has ever been reported earlier than the Eocene in Europe, except for a questionable single upper molar from the Upper Cretaceous of the Belgian/Dutch border. The systematics of the herpetotheriids of Dormaal was formerly based on only a dozen dental specimens, which were assigned, after several revisions, to two species Peratherium constans and Amphiperatherium brabantense. Most importantly, these two species were considered at the root of most of the hepetotheriid lineages of the European Paleogene. Here we report a large sample of about 400 new dental remains that allow a better definition of both species as well as a testing of their systematic status. The evidence of significant morphological variability leads us to reconsider the diagnosis of Peratherium constans and to question the validity of Amphiperatherium brabantense. This study highlights that the primitive species Peratherium constans and Amphiperatherium brabantense are hardly distinguishable from each other, and therefore conclude that Peratherium constans was the only marsupial present at Dormaal. The important morphological variation exhibited by this herpetotheriid is similar to the variability observed in the type-species Peratherium elegans and in other fossil and extant metatherians. Consequently, our results suggest that several Amphiperatherium species from the Eocene could represent variants of the genus Peratherium. The question of the Amphiperatherium presence in Europe is therefore raised and a thorough discriminate analysis of both genera should be conducted in later works.

An exceptionally well-preserved skeleton of Palaeothentes from the Early Miocene of Patagonia, Argentina: new insights into the anatomy of extinct paucituberculatan marsupials

During the Cenozoic paucituberculatans were much more diverse taxonomically and ecomorphologically than the three extant genera of shrew-like marsupials. Among paucituberculatans, palaeothentids were abundant during the Early Miocene, although most of the fossil remains consist of isolated teeth or fragmentary jaws. We describe a new and exceptional partial skeleton of Palaeothentes lemoinei (Palaeothentidae), collected from the Santa Cruz Formation (Santacrucian age, Early Miocene) in Patagonia. Whereas the skull of P. lemoinei has more plesiomorphic traits in the face, palate, and cranial vault than that of living paucituberculatans, the dental morphology is more derived. The osseous inner ear was examined using micro-CT scanning, revealing a cochlea with 1.9 turns, the presence of a “second crus commune”, an anterior semicircular canal (SC) projecting slightly dorsally from the dorsal-most point of the posterior SC, and lateral and posterior SCs projecting laterally to the same level. On the basis of postcranial anatomy, previous studies have demonstrated that P. lemoinei was an agile cursorial form, an inference supported by study of the new postcranial elements.