PhD Defense - Kevin Le Verger

Exploration of internal anatomy and morphological integration in the skull of cingulates (Xenarthra, Mammalia) and their phylogenetic implications

Thèse de doctorat sous la direction de Guillaume Billet

Composition du jury :

• Mme Isabelle Rouget, Professeur, CR2P, Paris, Examinatrice
• Mme Allowen Evin, Chargé de Recherche, ISEM, Montpellier, Examinatrice
• M. Timothy Gaudin, Professeur, University of Tennessee, Chattanooga, Rapporteur
• Mme Sophie Montuire, Professeur, Université de Bourgogne, Dijon, Rapporteure
• M. Robert J. Asher, Doctor, University of Cambridge, Cambridge, Examinateur
• M. Guillaume Billet, Maître de Conference et Chargé de Collection, CR2P, Paris, Directeur de thèse
• M. Lionel Hautier, Chargé de Recherche, ISEM, Montpellier, Invité

Abstract :

Morphological variation is a complex phenomenon whose use in phylogenetic analyses is often criticized. Many studies have called for a broader exploration of patterns of morphological variation and a better identification of the covariation among traits to improve morphological phylogenetics. Cingulates represent an ideal case study for this objective since they illustrate a typical case of conflict between morphological and molecular phylogenetic reconstructions, especially for the origins of the extinct glyptodonts. In this work, we first highlight this incongruence and point out the existing gaps concerning our knowledge of the internal cranial anatomy and the patterns of integration on the skull of cingulates. An exploration of these two aspects is relevant to the enrichment of morphological matrices and to a better understanding of the existing covariations among characters, which can mislead morphological phylogenetics. Our work starts with an in-depth study of the internal anatomy of the skull (focused on selected canals and cavities related to cranial vascularization, innervation or tooth insertion) in a diverse sample of Cingulata. We tentatively reconstruct the evolutionary scenarios of eight selected traits on these structures. These suggest a greater resemblance of glyptodonts with pampatheres, with the genus Proeutatus and/or with chlamyphorines, which is partly congruent with molecular phylogenies. Then, we explore patterns of cranial integration within several species of extant armadillos and test if the patterns found at the instrapecific level are also supported at the evolutionary level, i.e., among species, using a rich sample of extant and extinct cingulates. We first focus on one of the most powerful integration factors known in mammals - allometry - in order to target cranial covariation patterns related to size variation. Our analysis of cranial allometry enabled us to highlight several cranial allometric patterns that are widespread in cingulates. One of the strongest allometric patterns detected corresponds to the craniofacial allometry (relatively long face in large crania), but strong and widespread allometric changes were also detected for the postorbital constriction, the zygomatic arch, the nuchal crests, the mastoid process of the petrosal, the cranial roof and the foramen magnum. Second, we perform a selective exploration of pairs of strongly covarying distances in the same samples which enable us to highlight additional strong cranial correlations. The correlations supported at both the intraspecific and evolutionary levels concern in particular the anterior root of the zygomatic arch, a region particularly rich in muscular insertions. Third, we present the very first exploration of cranial modularity in cingulates at the intraspecific level that reveals a partitioning of the integration into three anteroposteriorly distributed modules on the skull. Our results are finally compared with existing morphological matrices and phylogenetic hypotheses. Although it hints at a necessary revision of several characters, the comparison of the detected patterns of integration with morphological matrices proves difficult. This highlights the necessity to further explore alternative coding strategies for a better evaluation of the covariation and allometry among scored characters and for an overall improvement of our character constructions.