Les Faunes mammaliennes du Miocène supérieur du Tchad (structure des communautés et implications paléoenvironnementales)

La connaissance des environnements passés représente un aspect fondamental dans la compréhension de l'histoire évolutive des êtres vivants. Ainsi, l'étude du(des) paléoenvironnement(s) associé(s) aux premiers hominidés permet de définir les contraintes environnementales dans lesquelles l'émergence de ce groupe s'est effectuée. L'objectif de ce travail était de reconstruire le(s) paléoenvironnement(s) associé(s) aux hominidés anciens : Sahelanthropus tchadensis (7 Ma, Toros-Menalla, Tchad), Orrorin tugenensis (6 Ma, Lukeino, Kenya) et Ardipithecus kadabba (5,77-5,54 Ma, Asa Koma, Ethiopie). Pour ce faire, l'étude de la structure faunique (structures taxinomique et écologique) des assemblages fossiles de mammifères associés à ces hominidés et d'autres assemblages africains du Miocène supérieur a été entreprise. Au préalable, il était indispensable de s'intéresser à la représentativité de ces assemblages fossiles par rapport à la/les communauté(s) dont ils sont issus. Ainsi, l'étude de la structure faunique des assemblages de Toros-Menalla a révélé qu'ils provenaient d'une même paléocommunauté, les variations fauniques observées d'un assemblage à l'autre étant probablement liées à des différences d'environnement de dépôt. Un modèle a également été proposé afin d'expliquer, par une sous-représentation des petites espèces, les différences observées entre les structures écologiques des faunes fossiles et celles des faunes modernes. Ces prérequis obtenus, l'analyse de la structure faunique a ensuite permis de reconstruire, à Lukeino et à Toros-Menalla, un mélange de zones arbustives et de zones boisées plus ouvertes. Asa Koma présenterait un environnement plus fermé, étant principalement constitué de formations arbustives. Cette dernière composante végétale semblait donc constituer un élément indispensable à la présence des hominidés anciens. Ces derniers étaient par ailleurs systématiquement associés à une combinaison de taxons (Orycteropus, Giraffa, Hystrix, Aepyceros et Deinotherium), absente des autres gisements africains contemporains.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Evaluation of the Fossil Fish-Specific Diversity in a Chadian Continental Assemblage: Exploration of Morphological Continuous Variation in Synodontis (Ostariophysi, Siluriformes): PAST DIVERSITY IN THE FISH FOSSIL RECORD

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Effect of simulated faunal impoverishment and mixture on the ecological structure of modern mammal faunas: implications for the reconstruction of Mio-Pliocene African palaeoenvironments.

15 pagesInternational audienceThe strong link between environment and the ecological diversity of communities is often used for drawing palaeoenvironmental inferences from fossil assemblages. Here we focus on the reliability of fossil samples in comparison to original communities when inferring palaeoenvironments from the ecological diversity of fossil mammal faunas. Taphonomic processes and sampling techniques generally introduce two kinds of biases in fossil samples: 1) the directional impoverishment of communities, i.e. the absence of some specific categories of bones, individuals or species; and 2) the mixture of several communities, temporally (timeaveraging) and/or spatially (space-averaging). We explore the impact of such alterations on ecological diversity and taxonomic richness by simulating an impoverishment in small species (1-45 kg) and a mixture of communities on an original dataset of 50 modern African communities. The progressive impoverishment in small species in a mammal community induces a relative enrichment in terrestrial and grazing species and a depletion (or even the disappearance) of several ecological categories (e.g., fossorial, arboreal, sub-arboreal, frugivorous, omnivorous and insectivorous species), thus leading to the ecological homogenisation of the altered communities. Other categories (carnivorous species, browsers, mixed feeders and sub-aquatic species) prove stable and seem to be good estimates for the relative abundances in the parent communities. Ideally, palaeoenvironmental inferences should be drawn from those ecological categories. For strong degrees of impoverishment, the discriminating power of the ecological diversity is reduced and other proxies should be used. Once the degree of impoverishment is assessed, it becomes possible to consider the condensation of several communities into a single fossil sample by comparing its taxonomic richness with those of modern communities, first impoverished to the same level. The mixture of communities tends to increase the taxonomic richness but does not significantly modify the overall ecological diversity. As a case study, we applied this approach to 15 African mammal samples from the late Miocene-earliest Pliocene, a period of particular interest, as it saw the emergence of hominins, together with faunal turnovers, and major climatic and environmental changes. Most of our fossil samples, especially those that yielded early hominins, appear altered to different degrees by taphonomic processes and sampling techniques, meaning that their faunal structures may not be comparable and that their inferred palaeoenvironmental differences may not be genuine

Toros-Menalla (Chad, 7 Ma), the earliest hominin-bearing area: how many mammal paleocommunities?

12 pagesInternational audienceThe fossiliferous area of Toros-Menalla (TM) (Djurab Desert, northern Chad) has yielded one of the richest African mammal faunas of the late Miocene. It is also the place where the earliest known hominin, Sahelanthropus tchadensis, was found. Although more than 300 localities are recorded in that area, previous paleoecological studies focused only on the largest and richest one. The integration of the material from other TM localities, and thus of a significant number of mammal taxa, is crucial to improve the corresponding paleoenvironmental reconstructions. Before such inferences can be drawn, it is necessary to test for the ecological integrity of these mammal assemblages: how many paleocommunities do they represent? The faunal structures of several assemblages selected for their apparent resilience to sampling biases are compared here. The criteria used in the inter-assemblage comparison are ecological diversity, taxonomic structure (taxonomic rank of abundance) and taxonomic composition. Based on multivariate analyses, two groups of TM assemblages can be distinguished. One of them contains the hominin-bearing assemblages. It is taxonomically richer and shows a wider ecological spectrum than its counterpart. The degree of taphonomic alteration undergone by the TM assemblages, as well as the distribution of amphibious mammals among them, suggest different depositional settings for these two groups of assemblages, the richest of which was probably associated with lower hydraulic energy. Overall, it seems that the TM assemblages recorded the same mammal paleocommunity preserved in two contrasted depositional settings. Moreover, the spatial overlap of these assemblages provides further evidence for the mosaic character of the landscape associated with S. tchadensis

The mammal assemblage of the hominid site TM266 (Late Miocene, Chad Basin): ecological structure and paleoenvironmental implications.

10 pagesInternational audienceCharacterizing the paleoenvironmental context of the first hominids is a key issue for understanding their behavioral and morphological evolution. The present study aims at reconstructing the paleoenvironment of the TM266 vertebrate assemblage (Toros-Menalla, Northern Chad) that yielded the earliest known hominid Sahelanthropus tchadensis (7 Ma). For the first time, a quantitative analysis is carried out on the fossil mammal assemblage associated with that hominid. Two complementary approaches were applied: (1) the analysis of the relative abundances of taxa and their habitat preferences; and (2) the study of the distribution of taxa within three meaningful ecovariables: locomotion, feeding preferences, and body mass. The resulting taxonomic and paleoecological structures are used to reconstruct the diversity and the relative extent of the habitats in that part of northern Chad seven million years ago. The paleoenvironment was composed of open areas with dry and humid grasslands, prevailing over wooded habitats. Water was also widely available as freshwater bodies and certainly swamps. It appears that the high habitat diversity of the landscape is a common feature among paleoenvironments associated with early hominids