PhD Defense - Marjorie Roscian

Morphologie fonctionnelle et adaptations des mâchoires de céphalopodes actuels : implications chez les fossiles

Composition du jury :

• Theodore Uyeno, Professeur, Valdosta State University, USA - Rapporteur
• Allowen Evin - DR CNRS, ISEM, France, - Rapportrice
• Laure Bonnaud-Ponticelli , Professeure, BOREA, MNHN, France - Examinatrice
• Christian Klug - Professeur, University of Zurich, Suisse - Examinateur
• Anne-Claire Fabre - Post doctorante, Museum fur Naturkunde, Allemagne - Examinatrice
• Janet Voight – Curator, Field Museum – Examinatrice
• Yves Cherel - DR CNRS, CEBC, France, - Invité
• Isabelle Rouget - Professeur, CR2P, MNHN - Directrice de thèse
• Anthony Herrel – DR CNRS, Mecadev, MNHN - Co-directeur de thèse

Absract:

Adaptations of feeding structures to different diets are commonly found in different animal groups. In cephalopods, the jaws, also called beaks, are composed of two chitinous parts, the shapes of which are specific.The specific shape and composition make them very good indicators of trophic interactions allowing to replace these animals in their food web. The buccal mass of cephalopods is made up of an unusual joint since it is entirely muscular and does not have a contact point between the two beaks. Suprisingly, this muscular articulation and its function remain poorly studied. However, a better understanding of the trophic ecology, prey manipulation and biting capabilities of the different species would allow a better correlation of form and function and the evolution of this unique biting system. The first objective of this dissertation is to characterize diversity of cephalopod beak morphology in 3D and to test whether there is a link between the shape of the beaks and ecological parameters, such as diet, using a 3D geometric morphometric approach. The second objective is to study the buccal musculature in order to estimate theoretical bite forces for the different groups and to identify functions linking muscles and beak shapes. Finally, the third objective is to test the suitability of using the results obtained on extant forms to interpret fossil beaks.Seventy-five species of coleoid cephalopod were imaged in 3D using underwater photogrammetry and X-ray tomography. Morphometric analyses reveal that the shape of the beaks carry a moderate phylogenetic signal. The lower beaks also show a link with the habitat (pelagic, benthic, demersal) and trophic level ; this relationship does not hold for the upper beak. The variability of the latter isrelated to the shape of the rostrum and appears to allow two distinct biting functions: chopping or stabbing. The study of the muscular anatomy reveals significant differences in the musculature between Octopodiformes and Decapodiformes as well as possible differences in bite forces in relation to variation in the shape of the beaks. Benthic octopods have a relatively high bite force compared to cuttlefish or some squid. However, there is a wide range of variation in force among squid related to the wide diversity in beak shapes and diets. Finally,I analyzed the morphology of a fossilized fragment of jaw from an unknown cephalopod and found its morphology to be comparable to an extant group. Using shape-based classification methods this fossil was inferred to be an upper beak of a pelagic octopodiform.This same analysis, based on a robust study of the diversity of extant cephalopods, may be used in the future to interpret the form and function of other fossils.