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An extensive and detailed overhaul of the present classification of the Marrolithinae is undertaken to produce a clear taxonomic framework from which to understand the phylogeny and biogeographical history of the subfamily. Historically, marrolithine species have been defined using a strongly typological approach which has increasingly proved unworkable as species show ranges of variation in morphology that have led to uncertainty in the identification of specimens and has therefore lowered their stratigraphical usefulness. The lack of appreciation of morphological variation has resulted in the naming of a great many more species than are probably actually present in the fossil record. Extensive museum collections have been analysed supplemented by field sampling within Wales and Shropshire and comparison of field locations and horizons. A large collection of over 2000 specimens of Llanvirn to Ashgill Moroccan Marrolithinae was studied for the first time. This material has been important in the reclassification of the Marrolithinae. These samples contain the earliest Onnia specimens and have therefore thrown light on the Deanaspis and Onnia specimens and have therefore thrown light on the Deanaspis and Onnia lineage as presently understood. The Moroccan samples also contain the earliest representatives of the new genus Hammannaspis. The Moroccan species played an important role in the distribution of the Marrolithinae around proto-Tethys. The re-evaluation of the classification of marrolithine genera and species has included the first cladistic analysis of a trinucleid subfamily, detailed analysis of variation in fringe pit development within samples and taxa and consideration of the palaeogeographical relationships between the marrolithine taxa in space and time. These differing approaches have led to the production of a more constrained and easier to use classification which will enhance the stratigraphical usefulness of the Marrolithinae.

L’étude des trilobites du Dévonien moyen et supérieur du Massif de l’Ardenne et du Boulonnais contribue de multiples façons à l’amélioration de notre compréhension de l’histoire de la Vie et de la Terre à cette époque. Leur biodiversité a été explorée à partir d’analyses statistiques multivariées, mettant en évidence une complexification de la structure de leurs communautés au cours du temps. En effet, dans un même contexte de plate-forme barrée, une seule association est présente dans l’ensemble des environnements du Givétien tandis que deux associations ont été identifiées au Frasnien, l’une restreinte aux constructions récifales et l’autre aux milieux périrécifaux. Une analyse des mégaguildes du Givétien souligne le caractère particulier des milieux récifaux par rapport à l’ensemble des environnements de la plateforme. La description du développement de trois espèces appartenant au genre Dechenella, via une analyse de morphométrie géométrique, a permis ensuite d’identifier une combinaison de plusieurs changements évolutifs de l’ontogenèse sur leur céphalon et pygidium. Ce travail souligne la complexité et le rôle important des modifications de l’ontogenèse dans la production de nouvelles morphologies. Enfin, l’étude phylogénétique des Asteropyginae nous permet de proposer une nouvelle vision de leur histoire évolutive : 1) traditionnellement divisée en deux clades, une évolution plus graduelle en un seul clade est ici privilégiée et 2) une nouvelle origine est suggérée à travers le genre Destombesina. A partir des résultats de cette analyse, les modèles de pygidiums et de céphalons, à la base de la systématique de ce groupe, ont également été redéfinis. / The study of trilobites of Middle and Upper Devonian from Ardenne Massif and Boulonnais contributes in multiple ways to the improvement of our understanding of the Life and Earth’s history of this period. Their biodiversity has been explored from multivariate statistical analyses. A gradually organisation of the structure of their communities has been evidenced. Indeed, in the same reefal platform context, only one association is presented in whole Givetian environments, whereas two associations have been identified at the Frasnian, the one confined to reefal build-ups and the other to perireefal facies. An analysis of Givetian megaguilds underlines the particularity of reefal environments compared to the other ones of the platform. Developmental description of three Dechenella species, through a geometric morphometry analysis, allows us to identify several evolutionary changes of ontogeny on cephala and pygidia. This work illustrates the complexity and the importance of ontogenetic changes in new morphology production. Phylogenetic study of Asteropyginae allow us to propose a new vision of their evolutive history. Traditionally splitted in two separate clades, the cladistic analysis of this subfamily shows a more progressive evolution in one clade. Moreover, a new origin has been suggested through the genus Destombesina. From these results, pygidial and cephalic patterns, at the base of the systematic of this group, have been redefined.