PhD Defense - Thomas Laville

Les Thylacocéphales (Arthropoda: Thylacocephala) : origine, diversification et évolution d'une lignée arthropodienne disparue

Composition du jury :

• M. Sylvain CHARBONNIER, Muséum national d'Histoire naturelle, Directeur de thèse
• Mme Marie-Béatrice FOREL, Muséum national d'Histoire naturelle, Co-directrice de thèse
• M. Briggs DEREK, Yale University, Rapporteur
• Mme Catherine CRONIER, Université de Lille, Rapporteure
• Mme Carrie SCHWEITZER, Kent State University, Examinatrice
• M. Jean VANNIER, Centre national de la recherche scientifique, Examinateur
• M. Joachim T. HAUG, Ludwig-Maximilians-Universität München, Examinateur

Abstract:

Thylacocephalans are enigmatic fossil euarthropods characterised by particular body features: a folded shield enveloping most of the body, compound eyes sometimes hypertrophied, three pairs of raptorial appendages, eight pairs of gills and an eight to 22-segmented posterior trunk. They are known from at least the Silurian up to the Late Cretacous, when they probably became extinct. Their fossil record is very patchy. They mostly occur in Konservat-Lagerstätten. However, they are known from all continents except Antarctica and South America. Although usually considered as pancrustaceans, the systematic affinities of thylacocephalans are still controversial. Since their recognition, they have been related to a huge variety of pancrustaceans: thecostracans, malacostracans, and more recently remipeds. This uncertainty about their phylogenetic relationships is mainly due to a lack of knowledge on their tagmatisation. Despite an important knowledge on their anatomy, many questions remain concerning the presence of cephalic appendages and the nature of their raptorial appendages. Beyond the debate on phylogenetic affinities, the systematic of Thylacocephala is also quite controversial. Currently, only classification has been proposed by Schram (2014), which is considered by the author himself as a working hypothesis to test. Finally, various modes of life have been proposed for thylacocephalans: predatory, necrophageous, burrowing, mesopelagic… With a large array of morphologies and a wide stratigraphic range, thylacocephalans probably had various lifestyles. No synthesis exists currently to clarify these paleobiological and paleoecological parameters.

The aim of this work is therefore to study the phylogenetic affinities of thylacocephalans, but also their systematics, which will allow us to understand their evolution and diversification. Based on comparative anatomy, I studied the anatomy of thylacocephalans in detail, focusing on their tagmatisation. Using X-ray tomography, the presence of cephalic appendages, including mandibles, an anterior trunk, including the three abducting appendages, and a posterior trunk have been demonstrated. Other important anatomical details were also determined, such as the univalve nature of the carapace, which is often mistakenly considered as bivalved. This anatomical work allowed me to study the phylogenetic affinities of thylacocephalans using various phylogenetic reconstruction methods. I was thus able to confirm the position of thylacocephalans within the pancrustaceans. In addition to the phylogenetic affinities of thylacocephalans, it was also necessary to study the relationships within thylacocephalans. To this end, I undertook a taxonomic revision of species from various Mesozoic localities. This systematic work led to the description of six new species and to the definition of 49 morpho-anatomical characters. These characters allowed me to obtain a phylogeny tracing the evolutionary history of the thylacocephalans. The division between Conchyliocarida and Concavicarida proposed by Schram (1990) was confirmed by phylogenetic analysis.

Following this systematic review, a synthesis of the palaeodiversity, geographical distribution and living environments of thylacocephalans over time was carried out, highlighting in particular a change in fauna between the Paleozoic, dominated by concavicaridans, and the Mesozoic, dominated by conchyliocaridans.