Dans la peau de Romundina stellina Ørvig, 1975 (Vertebrata, Placodermi, Acanthothoraci)
Anatomie crânienne d'un des premiers gnathostomes révélée par tomographie synchrotron en contraste de phase
Being Romundina stellina Ørvig, 1975 (Vertebrata, Placodermi, Acanthothoraci)
Intracranial anatomy of one of the deepest gnathostomes revealed by synchrotron tomography in phase contrast protocole
The acanthothoracid placoderms (armored fishes) are the most basal and primitive gnathostomes (jawed vertebrates; 1). However, their endocranial morphology is poorly understood, and only one genus (Brindabellaspis) has been described thoroughly (2).
Here we present the 3D reconstruction of a subcomplete skull of Romundina stellina Ørvig, 3, from the Lochkovian of Prince of Wales Island, Canadian Arctic Archipelago. The specimen was imaged in 3D with propagation phase contrast microtomography (4) on the ID19 beamline of the ESRF, using a 7.45 µm isotropic voxel size.
Most features are properly preserved and most of the missing structures can be virtually rebuilt by symmetry. Another advantage of this virtual approach is the possibility of connecting with certainty all the external foramina to the blood and nerve canals and the central/internal structures, and hence to identify accurate homologies without destroying the specimen. Ørvig’s original assumptions can now be checked with confidence.
The vasculature of the dermal bones, rendered in detail, allowed a better understanding of plate growth. It permits the visualization of dermal bone establishment over perichondral bone (5).
The high level of details of this model reveals that between the trigeminal and vagus nerve (and the inner ears), the perichondral bone wrapping the endocranial cavity shows a “lace” pattern, unknown so far in vertebrates (presumably because of the lack of data). The significance of this character is unclear, but it is definitely not an artifact of taphonomy or scanning.
1 Janvier, P. Early Vertebrates. Clarendon Press edn, Vol. 1 (Oxford Science Publications, 1996).
2 Young, G. C. A new Early Devonian placoderm from New South Wales, Australia, with a discussion of placoderm phylogeny. Palaeontographica (A) 167, 10–76 (1980).
3 Ørvig, T. Description, with special reference to the dermal skeleton, of a new Radotinid arthrodire from the Gedinnian of Arctic Canada. Extrait des Colloques internationaux du Centre National de la Recherche Scientifique - Problèmes actuels de Paléontologie - Evolution des Vertébrés 218, 41–71 (1975).
4 Tafforeau, P. et al. Applications of X-ray synchrotron microtomography for non-destructive 3D studies of paleontological specimens. Applied Physics A - Materials Science & Processing 83, 195–202 (2006).
5 Dupret, V., Sanchez, S., Goujet, D., Tafforeau, P. & Ahlberg, P. Bone vascularization and growth in placoderms (Vertebrata): the example of the premedian plate of Romundina stellina Ørvig, 1975 Comptes Rendus Palevol 9, 369–375 (2010).
Montbéliard, 2012. 19-19 p.
Quatrième Symposium "Georges Cuvier": Fossiles, Evolution, Mouvement, Montbéliard, Octobre 8-12, 2012