Anatomie, Ökologie und Ontogenie von Mesosauriern unter Anwendung von 3D-Bildtechnologien
Zusammenfassung der Projektergebnisse
The early Permian reptiles mesosaurs represent a key step in amniote evolutionary history by being the first secondarily aquatic amniotes. With marked adaptations to an aquatic lifestyle, excellent overall preservation, complete ontogenetic series, and an abundance of specimens, they constitute a great subject for studying the links between ecology and anatomy in early amniotes. Our project intended to reveal new phylogenetic characters, resolve the phylogeny of mesosaurs, and provide new insights into their ecology based on novel CT data. Due to poor cranial preservation in the mesosaur specimens, we could not conduct the planned investigation of their skull anatomy and feeding mechanisms. To address the issue of mesosaur ecology and development, we redirected the focus of our project towards more postcranium-based aspects. One of these aspects consisted in the study of ontogeny and development in mesosaurs. Thanks to the preservation of complete ontogenetic series, we were able to study the long bone, skull, and tooth morphometrics in over 300 mesosaur specimens. Our results support the recent synonymization of all mesosaur species and suggest that “S. tumidum” and “B. sanpauloensis” represent immature stages or incomplete individuals. We also measure that mesosaurs underwent a number of morphological changes during their growth. Along with the environmental shift already described between species, these suggest that mesosaurs transitioned from an active predatory lifestyle in their youth to a more filter-feeding diet as adults. We emphasize that more detailed information on trophic structure in the Irati and Mangrullo formations would be needed to properly test this hypothesis. Another aspect of our project was the histological investigation of the degree of adaptation to an aquatic lifestyle in mesosaurs. We analyzed the microanatomy of vertebrae, ribs, haemal arch, humeri, and femora of “S. tumidum” and “B. sanpauloensis”. Histology in both taxa is consistent with an aquatic lifestyle, but they display a less advanced degree of pachyosteosclerosis than M. tenuidens, suggesting differences in lifestyles or environment. We also highlighted that sexual maturity markers are absent in the bones of “B. sanpauloensis” and “S. tumidum”. In retrospect, this reinforces the idea that they correspond to immature stages of M. tenuidens, as has been suggested since then. As a rare case in paleontology, we were able to identify the degree of ossification and fusion of skeletal elements in some exceptionally preserved juvenile M. tenuidens specimens. We gathered information on four major vertebral developmental patterns in mesosaurs. By coupling this information with a review of the published literature on the topic, we were able to reconstruct their evolutionary history as well as the ancestral condition in Amniota. The four patterns demonstrate remarkable stability over the 300 million years of amniote evolution, showing conservatism in the framework of developmental constraints in the clade. Moreover, the major lineages of amniotes show a certain homogeneity of vertebral developmental constraints within them, which provides a framework for understanding the evolution of axial development in all terrestrial vertebrates. These results pave the way for developing a new framework for understanding the evolution of development in amniotes. Our browsing of mesosaur specimens in collections worldwide led us to reevaluate a usually overlooked feature of their anatomy: the presence of fracture planes in the caudal vertebrae. Fracture planes usually reflect a potential for caudal autotomy, i.e., the ability to drop the tail. Given that mesosaurs are usually reconstructed as tail-driven swimmers, it was paradoxical that they would use this behavior. Although our examination confirms the presence of fracture planes, we show that they probably would not autotomize their tails, due to histological constraints and to the lack of predation which would justify using the behavior. Instead, fracture planes in mesosaurs likely constitute an evolutionary relic, retained but rendered useless in the clade. In sum, although our project ended up being quite different from what was initially planned, it led to many advances in the understanding of mesosaur biology. Our results confirm or challenge established hypotheses on the ecology and ontogeny of mesosaurs. More generally, our research has shed new light on several aspects of the evolutionary history of early amniotes and lays the groundwork for many future projects for our research group and others.
Projektbezogene Publikationen (Auswahl)
- 2018. Response: A reassessment of the taxonomic position of Mesosaurs, and a surprising phylogeny of early amniotes. Front Earth Sci, 6
MacDougall MJ, Modesto SP, Brocklehurst N, Verrière A, Reisz RR, & Fröbisch J
(Siehe online unter https://doi.org/10.3389/feart.2018.00099) - 2019. Microanatomy and growth of the mesosaurs Stereosternum tumidum and Brazilosaurus sanpauloensis (Reptilia, Parareptilia). Foss Rec, 22:2, 91–110
Klein N, Verrière A, Sartorelli H, Wintrich T, & Fröbisch J
(Siehe online unter https://doi.org/10.5194/fr-22-91-2019) - 2020. Conflicting evidence for the use of caudal autotomy in mesosaurs. Sci Rep, 10:1, 1–9
MacDougall MJ, Verrière A, Wintrich T, LeBlanc ARH, Fernandez V, & Fröbisch J
(Siehe online unter https://doi.org/10.1038/s41598-020-63625-0) - 2021. Regionalization, constraints, and the ancestral ossification patterns in the vertebral column of amniotes
Verrière A, Fröbisch NB, & Fröbisch J
(Siehe online unter https://doi.org/10.1101/2021.09.23.461462)
