MY ABSTRACTS 3
Lerosey-Aubril, R., J., Claude & G. Chatain. 2007. 5ème Symposium Morphométrie et Evolution des Formes, Paris (26-27 March)
Chez les trilobites, le régime alimentaire est généralement déterminé à partir de l’étude de deux structures céphaliques minéralisées: la glabelle sur la face dorsale et l’hypostome sur la face ventrale. Ces deux structures, vraisemblablement liées l’une à l’autre par des ligaments du vivant de l’animal, protégeaient la région antérieure du tube digestif. Afin de déterminer dans quelle mesure elles constituaient une réelle unité fonctionnelle du vivant de l’animal, les patrons de covariation ont été étudiés et comparés aux patrons de variation ontogénétique chez un proetoïdé famennien du Maroc, Cyrtosymbole rectifrons. L’analyse morphométrique, portant sur 52 cranidia et 32 hypostomes, révèle que les patrons de covariation sont très similaires aux patrons de variation associés à l’ontogenèse. Les parties antérieures de ces deux structures s’élargissent au cours du développement. En revanche, la région postérieure de l’hypostome tend à s’amincir au cours de l’ontogenèse alors que celle de la glabelle s’élargit. L’étude de la covariation révèle une très forte interdépendance de la forme de ces deux structures. Ceci semble indiquer que la glabelle et l’hypostome ne sont pas deux unités de variation distinctes mais qu’au contraire, elles font partie intégrante d’un même complexe tant fonctionnel que développemental. L’élargissement conjoint de ces structures dans la région antérieure est interprété comme le reflet du développement important de l’estomac alors que les changements morphologiques antagonistes de leurs parties postérieures pourraient être le résultat du développement de puissants appendices céphaliques. Ainsi, l’ensemble de ces modifications semble indiquer la présence d’un changement notable de régime alimentaire au cours du développement.
Lerosey-Aubril, R. 2007. First International Palaeobiogeography Symposium, Paris (10-13 July)
With only 31 genera and about 120 species described, trilobites represent a minor element of Permian benthic marine faunas. As a consequence, little attention has been paid to the last representatives of this formerly diverse and important group, in particular with reference to the end-Permian mass extinctions. Likewise, trilobites have been almost completely ignored in discussions of the evolution of Permian palaeogeography. Of course, their low diversity during this period significantly reduces their utility in constraining palaeogeographical reconstitutions. Nevertheless, significant differences in the range of geographical distribution exist among trilobite genera. While some taxa are obviously cosmopolitan, others are restricted to particular regions, enabling probable faunal provinces to be recognized.
Following the establishment of chronostratigraphical subdivisions of potential global application, Owens (2003) recently performed a comprehensive review of the stratigraphical distribution of Permian trilobites. Similarly, this contribution aims at reviewing the evolution of trilobite biogeography during the Permian. Geographical distribution of trilobites is depicted for six time slices: Asselian-Sakmarian, Artinskian, Kungurian-Roadian, Wordian, Capitanian, and Wuchiapingian-Changhsingian (i.e. Lopingian). In the early Cisuralian (Asselian-Sakmarian), three regions show a certain degree of endemism: Western Pangea (from the Western and Central areas of the USA to Bolivia), the eastern Palaeo-Tethys margin, and the Indochina Terrane. The latter two regions have three genera in common, while Western Pangea shares only one taxon with either of the other two. The distribution of Artinskian trilobites is particularly unbalanced, with most genera occurring on the North Gondwanan margin and neighbouring terranes. Two genera, however, demonstrate that relationships between the Western Pangea and the North Gondwanan margin still exist across the East European inland sea and the Arctic basin. In addition, two peculiar trilobite genera characterized high latitudinal regions, Anujaspis in the north (Eastern Siberia) and Doublatia in the south (Southeastern Australia). A rather diverse fauna occurs in Western Pangea during the Kungurian-Roadian. By contrast, trilobites of the Tethyan area are virtually unknown during this period, with Pseudophillipsia (from South China) as the only taxon described so far. The absence of relationships between the Tethyan area and the Western Pangea becomes evident in the Wordian, when diverse but distinct faunas occur in the two regions. From the Capitanian to the end-Permian, trilobites are restricted to the South Laurasia margin, the North Gondwana margin, and the Cathaysian and Cimmerian microcontinents. The end-Guadalupian (Capitanian) is characterized by the occurrence of a rich fauna in Japan, which displays a rather important degree of endemism; however, relationships with the North Gondwana margin and the Cathaysian microcontinents (e.g. South China, Indochina) definitely exist. Trilobites are more homogeneously distributed in the Lopingian. The complex relationships between the South Laurasia margin, the North Gondwana margin, and the Cathaysian and Cimmerian microcontinents are discussed.
