Comparative bone histology of the turtle shell (carapace and plastron): Implications for turtle systematics, functional morphology, and turtle origins
Zusammenfassung der Projektergebnisse
Among modem vertebrates, turtles have a unique body bauplan. The turtle trunk and girdle bones are encased in a rigid shell generally composed of a dorsal, domed carapace and a rather flat ventral plastron, connected by a bony bridge. Although the turtle shell is easily identified as such, the peculiar turtle bauplan also prevents straightfonvard anatomical comparison with other vertebrate groups. The majority of bones in the shell are of dermal origin, whereas the neural series and the costal plates incorporate also endoskeletal parts, i.e., the neural arches and the ribs, respectively. We investigated the bone microstructures of the turtle shell as a composite bone structure, to see if and how the microstructures and microanatomy of the bone is influenced by, e.g., phylogenetic and functional aspects. Furthermore we tried to elucidate how the bone histology of the turtle shell and that of other vertebrate osteoderms might shed light on the still controversial question of the origin of turtles. The bone histology of the turtle shell was found to be procuring valuable data for the reconstruction of fossil integumentary (dermal) soft-tissue structures, phylogenetic hypotheses of turtle interrelationships and functional aspects acting on the turtle shell, with both carapace and plastron showing simitar results. Furthermore, the systematic position of problematic turtle taxa or poorly known shell material could be evaluated through the bone histology of the turtle shell. The bone microstructures seem to be influenced, to various degrees, by a mosaic of phylogenetic, functional, and structural factors. Groups that are well defined by bone histological characters are for example the soft-shelled turtles (Trionychidae), which exhibit a highly derived plywood-like structure, both in the soft dermis and the shell bone. Other groups do not show clear phylogenetic signals, because functional factors seem to override any existing signals of systematic value. One functional aspect that profoundly influences turtle shell bone microstructures is the adaptation to an aquatic habitat and life-style, thus all extant and fossil turtles could be grouped into categories from "terrestrial environment" to "extreme adaptation to aquatic/marine environments", based on their respective ecology/palaeoecology. Although the palaeoecology of stem turtles from the Late Triassic was classically viewed as being semi-aquatic, similar to the lifestyle of modern snapping turtles, we were able to independently show that, based on comparison with modem ten-estrial and semiaquatic/aquatic turtles, turtles originated in a terrestrial environment. Comparison of basal turtles and several outgroup taxa including osteoderm-bearing fossil parareptilian pareiasaurs, fossil mammals, and fossil placodonts, as well as fossil and extant eureptiles, the turtles are hypothesized to be diapsid eureptiles, with close relationships to archosaurs, the group that combines modem birds and crododytians, but also for example dinosaurs. In one outgroup, the placodont armor plates, a unique cartilaginous bone tissue ('postcranial fibrocartilaginous bone') was discovered. This is highly unusual, because osteoderms generally develop In the skin of an animal without any cartilage precursor as is the case in endoskeletal bones.
Projektbezogene Publikationen (Auswahl)
- Scheyer, T. M., and M. R. Sánchez-Villagra. 2005. Auswirkungen phylogenetischer und funktioneller Zwänge auf die Knochenhistologie von Schildkrötenpanzem; pp. 114-115 in B. Hubmann, and W. E. Piller (eds.), 75. Jahrestagung der Paläontologischen Gesellschaft - Beitragskurzfassungen. - Berichte des Institutes für Erdwissenschaften. Institut für Erdwissenschaften, Bereich Geologie und Paläontologie, Kari-Franzens- Universität, Graz.
- Winkler, J. D., Scheyer, T., Aguilera, O. and Sánchez-Villagra, M. R. 2005. Palaeobiology and diversity of the turtle fauna from the Late Miocene Urumaco Formation in Venezuela. 5th Worid Congress of Herpetology.
- Scheyer, T. M., and M. R. Sánchez-Villagra. 2007. Carapace bone histology in the giant pleurodiran turtle Stupendemys geographicus: phylogeny and function. Acta Palaeontologica Polonica 52:137-154.
- Scheyer, T. M., B. Brüllmann. W. Maier, and M. R. Sánchez-Villagra. 2007. The ontogeny of the shell in side-necked turtles, with emphasis on the homologies of costal and neural bones. 8th Intemational Congress of Vertebrate Morphology, Paris 16.-21. July 2007, Abstracts.:112.
- Scheyer, T. IUI. 2006. Amniote osteoderm and turtle shell bone histology: turtle origins remain enigmatic-Journal of Vertebrate Paleontology 26:121A.
- Scheyer, T. M. 2004: Biomechanische Besonderheiten im knochenhistologischen Intembau von Osteodermen. Geobiologie - 74. Jahrestagung der Paläontotogischen Gesellschaft, Göttingen, 02. - 08. Oktober 2004, Göttingen, 2004.
- Scheyer, T. M. 2006. Vergleichende Untersuchungen der Knochenmikrostruktur des Schildkrötenpanzers - Funktionelle, phylogenetische und palökologische Fragestellungen. Berichte-Reports, Institut für Geowissenschaften, Christian-Albrechts- Universitätzu Kiel 22:116-118.
- Scheyer, T. M. 2007. Comparative bone histology of the turtle shell (carapace and plastron): implications for turtle systematics. functional morphology, and turtle origins. PhD Thesis, pp. 343. Mathematisch-Natunwissenschaftllche Fakultät, University of Bonn, Bonn, Germany. (Siehe online unter: http://nbn-resolvinQ.de/urn:nbn:de:hb2:5N-12299 http://hss.ulb.uni-bonn.de/diss online/math nat fak/2007/schever torstenl )
- Scheyer, T. M. 2007. Skeletal histology of the armor of the Placodontia: the occurrence of 'postcranial fibro-cartilaginous bone' and its developmental implications. Joumal of Anatomy 211:737-753.
- Scheyer, T. M. and P. M. Sander. 2007. Terrestrial palaeoecology for basal turtles indicated by shell bone histology. Proceedings of the Royal Society of London, B 274(1620): 1885-1893.
- Scheyer, T. M., and J. Anquetin. 2008. Bone histology of the Middle Jurassic turtle shell remains from Kirtlington, Oxfordshire, England. Lethaia 41 (1):85-96.
- Scheyer, T. M., and P. M. Sander. 2007. Comparative bone histology of the turtle shell (carapace and plastron): implications for turtle systematics, functional morphology, and turtle origins. 8th Intemational Congress of Vertebrate Morphology, Paris 16.-21. July 2007,Abstracts.:112.
- Scheyer, T. M., P. M. Sander, W. G. Joyce, W. Böhme & U. Witzel. 2007. A plywood structure in the shell of fossil and living soft-shelled turtles (Trionychidae) and its evolutionary implications. Organisms, Diverstiy & Evolution 7(2):136-144.
- Scheyer, T. M., P. M. Sander, W. G. Joyce, W. Böhme, and U. Witzel. 2005. Is a 'bulletproof vest' linked to the evolutionary success of soft-shelled turtles (Trionychidae)? Joumal of Vertebrate Paleontotogy 25:110A.
