Les phytolithes, marqueurs des environnements mio-pliocènes du Tchad. Reconstitution à partir du signal environnemental des phytolithes dans l'Afrique subsaharienne actuelle

Ce mémoire présente l'étude du signal phytolithique de sédiments mio-pliocènes du Tchad(Afrique Centrale) datés entre 7-2 Ma, et contemporains des Homininés anciens Sahelanthropustchadensis et Australopithecus bahrelghazali. Une calibration de la relation plantes-phytolithes-solspour l'Afrique tropicale subsaharienne actuelle a d'abord été réalisée pour apprécier la signatureenvironnementale des phytolithes dans le registre fossile. L'analyse des assemblages phytolithiques de98 espèces de graminées (Poaceae) a permis d'élaborer trois nouveaux indices phytolithiques à partirde 43 types propres aux Poaceae. Testés sur une base de 57 sols/sédiments modernes du Tchad, cesindices permettent de tracer les associations graminéennes aquatiques du Lac Tchad, les associationsmésophytiques des milieux humides soudaniens, et les associations xérophytiques des milieux secssahéliens. L'analyse des assemblages phytolithiques des sols/sédiments actuels considérés dans leurensemble a permis d'évaluer le potentiel de ce proxy à caractériser la physionomie des formationssoudano-sahéliennes modernes. La calibration a été appliquée à l'étude d'un enregistrementsédimentaire discontinu du Lac Tchad (6-2 Ma) (forage de Bol, 13N/14E) et à celle de 18 niveauxpaléontologiques du Djourab (7-3.5 Ma) (16N/17E). Les résultats indiquent la présence de savanesintermédiaires à fermées et de zones de végétation aquatique dominantes à 7 Ma dans le Djourab, puisde savanes plus ouvertes et sèches à 3.5 Ma. Une phase de bas niveau lacustre est enregistrée entre3.6-2.8 Ma à Bol, et un pic d'aridité à 3.2 Ma. Enfin, les résultats montrent l'existence de graminéesen C4 au Tchad depuis 7 Ma.This thesis dissertation is dedicated to the study of the phytolith signal of Mio-Pliocenesediments from Chad (Central Africa) dated between 7-2 Ma, and contemporary to the early Homininspecies Sahelanthropus tchadensis and Australopithecus bahrelghazali. A calibration work studyingthe relationship plants-phytoliths-soils in modern sub-Saharan tropical Africa was carried out in orderto assess the environmental significance of the phytolith signal in the fossil record. Phytolithassemblages produced by 98 sub-Saharan grass species (Poaceae) were analyzed and the results led tothe development of three new phytolith indices defined from 43 types specific to Poaceae. Tested on adatabase of 57 modern soil/sediment samples from Chad, these indices allow drawing aquatic grassassociations of Lake Chad, mesophytic grass associations of wetlands from the Sudanian domain, andxerophytic grass associations of drylands from the Sahelian domain. The analysis of modern soilphytolith assemblages (total assemblages) was used to assess the potential of this proxy to characterizethe physiognomy of the Sudano-Sahelian vegetation types. The calibration had been applied to thestudy of a discontinuous sedimentary record from Lake Chad (6-2 Ma) (Bol core, 13N/14E) and of18 paleontological levels from the Djourab (7-3.5 Ma) (16N/17E). The results indicate thedominance of intermediate to closed savannas and areas of aquatic vegetation at 7 Ma in the Djourab,and more open and dry savannas at 3.5 Ma. A limited lacustrine stage is recorded between 3.6-2.8 Maat Bol, and a shift of aridity at 3.2 Ma. Finally, the results show the existence of C4 grasses in Chadsince 7 Ma.POITIERS-SCD-Bib. électronique (861949901) / SudocSudocFranceF

New hippotragini (Bovidae, Mammalia) from the Late Miocene of Toros-Menalla (Chad)

International audienceUntil now, the pre-Pleistocene record of the bovid tribe Hippotragini was rather poor. We describe here two new taxa from the late Miocene of Toros-Menalla in northern Chad, which yielded the earliest known hominid, Sahelanthropus tchadensis. Tchadotragus sudrei n.gen. n.sp. is known by complete skulls and numerous horn-cores. It has typical hippotragine features such as long slender, curved horn-cores, weak cranial flexure, large frontal sinus, and hippotragine-like dentition, and is here taken as a basal member of the tribe, branching before the divergence between Oryx-Praedamalis and Hippotragus s.l. Saheloryx solidus n.gen. n.sp. is less well-known; it differs mainly by the lack of sinus in the frontal and horn-cores, shorter horn-cores, and rounded brain-case, but it shares with Tchadotragus a large number of features that prompt us to classify it also at the base of the hippotragine tree, perhaps as the sister-taxon of Tchadotragus. No other African taxon looks like Saheloryx, and the only one similar to Tchadotragus is from Sahabi, Libya. The abundance of hippotragines sharply distinguishes Toros-Menalla from the East African late Miocene bovid faunas

New Material of the Earliest Hominid from the Upper Miocene of Chad

Anthropolog

The Importance of Diagenetic Processes in Sandstones Facies of the Hamakoussou Sedimentary Basin in North Cameroon: Influence on Reservoir Quality.

Published studies in the Hamakoussou reservoir sandstones are very few and the characterization of the reservoir quality including diagenesis is unknown. In this paper, after lithological reports, classical petrographic techniques have been used to study the diagenesis and reservoir quality of the Hamakoussou sandstones: Diagenetic processes within and around detrital grains show that early cementation by calcite come from volcanic veins and late cementation originating from silicification. Diagenetic phenomena (early cementation, compaction, fracturation and late cementation) show that these sandstones have a low porosity due to the blockage of intergranular pore spaces by cement. Intense volcanic activity associated with the circulation of fluids (silica and calcite) as well as the dissolution along the contacts of quartz grains are the principal sources of early and late cements which are responsible for the decrease in porosity observed in these sandstones. The immediate consequence is the sudden drying up of boreholes drilled for water supply

Inner ear sensory system changes as extinct crocodylomorphs transitioned from land to water

© 2020 National Academy of Sciences. All rights reserved. Major evolutionary transitions, in which animals develop new body plans and adapt to dramatically new habitats and lifestyles, have punctuated the history of life. The origin of cetaceans from land-living mammals is among the most famous of these events. Much earlier, during the Mesozoic Era, many reptile groups also moved from land to water, but these transitions are more poorly understood. We use computed tomography to study changes in the inner ear vestibular system, involved in sensing balance and equilibrium, as one of these groups, extinct crocodile relatives called thalattosuchians, transitioned from terrestrial ancestors into pelagic (open ocean) swimmers. We find that the morphology of the vestibular system corresponds to habitat, with pelagic thalattosuchians exhibiting a more compact labyrinth with wider semicircular canal diameters and an enlarged vestibule, reminiscent of modified and miniaturized labyrinths of other marine reptiles and cetaceans. Pelagic thalattosuchians with modified inner ears were the culmination of an evolutionary trend with a long semiaquatic phase, and their pelagic vestibular systems appeared after the first changes to the postcranial skeleton that enhanced their ability to swim. This is strikingly different from cetaceans, which miniaturized their labyrinths soon after entering the water, without a prolonged semiaquatic stage. Thus, thalattosuchians and cetaceans became secondarily aquatic in different ways and at different paces, showing that there are different routes for the same type of transition

Inner ear sensory system changes as extinct crocodylomorphs transitioned from land to water

Major evolutionary transitions, in which animals develop new body plans and adapt to dramatically new habitats and lifestyles, have punctuated the history of life. The origin of cetaceans from land-living mammals is among the most famous of these events. Much earlier, during the Mesozoic Era, many reptile groups also moved from land to water, but these transitions are more poorly understood. We use computed tomography to study changes in the inner ear vestibular system, involved in sensing balance and equilibrium, as one of these groups, extinct crocodile relatives called thalattosuchians, transitioned from terrestrial ancestors into pelagic (open ocean) swimmers. We find that the morphology of the vestibular system corresponds to habitat, with pelagic thalattosuchians exhibiting a more compact labyrinth with wider semicircular canal diameters and an enlarged vestibule, reminiscent of modified and miniaturized labyrinths of other marine reptiles and cetaceans. Pelagic thalattosuchians with modified inner ears were the culmination of an evolutionary trend with a long semiaquatic phase, and their pelagic vestibular systems appeared after the first changes to the postcranial skeleton that enhanced their ability to swim. This is strikingly different from cetaceans, which miniaturized their labyrinths soon after entering the water, without a prolonged semiaquatic stage. Thus, thalattosuchians and cetaceans became secondarily aquatic in different ways and at different paces, showing that there are different routes for the same type of transition.Facultad de Ciencias Naturales y Muse

Les Orycteropodidae (Mammalia ; Tubulidentata) du Mio-Plio-Quaternaire d'Afrique

La famille des Orycteropodidae a longtemps été l'objet de discussions sur son statut, son origine et sa position au sein du clade des Mammifères. Ces études sont souvent handicapées par la connaissance très partielle de l'histoire évolutive des Tubulidentata et de la seule espèce vivante de l'ordre : Orycteropus afer. Cette étude est la première révision systématique et phylogénétique de l'ordre des Tubulidentata à l'échelle du continent africain. L'étude de nombreux spécimens d'O. afer permet de remettre en question la validité des 18 sous-espèces et de proposer trois écotypes. L'estimation de la variabilité intra-spécifique montre l'absence de dimorphisme sexuel et permet de définir les stades ontogéniques. La révision systématique des Orycteropodidae du Mio-Plio-Pléistocène d'Afrique bénéficie de la découverte des premiers spécimens d'Afrique Centrale (Tchad). Ces squelettes sub-complets apportent des informations majeures pour l'étude des formes fossiles. Trois nouvelles espèces sont mises en évidences à la suite de la description des spécimens inédits du Tchad et du Kenya. Les affinités de ces formes sont étudiées à l'aide de la première analyse cladistique réalisée pour l'ordre. Cette étude conduit, d'une part, à réorganiser les espèces de Tubulidentata en créant un nouveau genre et, d'autre part, permet d'exclure le genre malgache Plesiorycteropus de l'ordre des Tubulidentata. Dans le cadre des recherches menées sur l'environnement des hominidés anciens, et à partir des résultats de l'analyse phylogénétique, l'intérêt biochronologique et paléobiogéographique des Tubulidentata est réévalué. Des phénomènes de dispersions intercontinentales sont mis en évidences au sein des Tubulidentata. Les adaptations au fouissage et au régime myrmécophage sont analysées d'un point de vue morphologique.The status, the origin and the phylogenetic position of the Orycteropodidae among Mammals have been debated for a long time. Studies have been often hindered by a lack of knowledge on the Tubulidentata evolutionary history and on the extant form: Orycteropus afer. This work is the first systematic and phylogenetic revision ever made on the African Tubulidentata. The validity of the 18 extant sub-species is invalided and three ecotypes are suggested for O. afer. No sexual dimorphism has been observed by the study of the intra-specific variability, and ontogenetic stages are determined. The discovery of the first specimens ever found in Central Africa (Chad) contributes to the systematic revision of the African Mio-Plio-Pleisocene Orycteropodidae. These sub-complete skeletons are key specimens for the study of the fossil forms. The description of the new specimens from Chad and Kenya leads to the determination of three new species. Their relationships are studied in the first cladistic analyse of the order. The phylogenetic study of the Tubulidentata suggests the reorganisation of the Order, with the erection of a new genera and the exclusion of the Malagasy Plesiorycteropus. On the basis of these phylogenetic results, the biochronological and palaeobiogeographical interest of the Tubulidentata is reassessed in the context of environmental studies of early hominids. Intercontinental dispersion events are evidenced for the Tubulidentata. Morphological analyses of the fossorial and myrmecophagous adaptations are performed. The present work brings up new perspectives in the study of this poorly known group of mammal.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Les préférences écologiques (paléorégimes alimentaires, paléohabitats) des grands mammifères herbivores des sites à Hominidés du miocène supérieur du Nord Tchad (reconstitution au moyen de l'analyse isotopique en carbone et oxygène du carbonate de l'émail dentaire)

POITIERS-BU Sciences (861942102) / SudocSudocFranceF