Skull and brain of a 300-million-year-old chimaeroid fish revealed by synchrotron holotomography

doi:10.1073/pnas.0807047106

Skull and brain of a 300-million-year-old chimaeroid fish revealed by synchrotron holotomography

^aMuséum National d'Histoire Naturelle, Département Histoire de la Terre, Unité Mixte de Recherche 7207 du Centre National de la Recherche Scientifique, 47 Rue Cuvier 75231 Paris Cedex 05, France;
^bEuropean Synchrotron Radiation Facility, BP 220, 6 Rue Jules Horowitz, 38043 Grenoble Cedex, France;
^dDepartment of Vertebrate Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024;
^eDepartment of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom; and
^cCentre de Recherche et d'Applications en Traitement de l'Image et du Signal-Lyon Résonance Magnétique Nucleaire, Institut National de la Santé et de la Recherche Médicale U630, Centre National de la Recherche Scientifique 5220, Institut National des Sciences Appliquées-Lyon, Université de Lyon, F-69621, Lyon, France

Edited by David B. Wake, University of California, Berkeley, CA, and approved January 21, 2009 (received for review July 21, 2008)

Abstract

Living cartilaginous fishes, or chondrichthyans, include numerous elasmobranch (sharks and rays) species but only few chimaeroid (ratfish) species. The early history of chimaeroids, or holocephalans, and the modalities of their divergence from elasmobranchs are much debated. During Carboniferous times, 358–300 million years (Myr) ago, they underwent a remarkable evolutionary radiation, with some odd and poorly understood forms, including the enigmatic iniopterygians that were known until now from poorly informative flattened impressions. Here, we report iniopterygian skulls found preserved in 3 dimensions in ≈300-Myr-old concretions from Oklahoma and Kansas. The study was performed by using conventional X-ray microtomography (μCT), as well as absorption-based synchrotron microtomography (SR-μCT) [Tafforeau P, et al. (2006) Applications of X-ray synchrotron microtomography for non-destructive 3D studies of paleontological specimens. Appl Phys A 83:95–202] and a new holotomographic approach [Guigay P, Langer M, Boistel R, Cloetens P (2007) Mixed transfer function and transport of intensity approach for phase retrieval in the Fresnel region. Opt Lett 32:1617–1619], which revealed their peculiar anatomy. Iniopterygians also share unique characters with living chimaeroids, suggesting that the key chimaeroid skull features were already established 300 Myr ago. Moreover, SR-μCT of an articulated skull revealed a strikingly brain-shaped structure inside the endocranial cavity, which seems to be an exceptional case of soft-tissue mineralization of the brain, presumably as a result of microbially induced postmortem phosphatization. This was imaged with exceptional accuracy by using holotomography, which demonstrates its great potential to image preserved soft parts in dense fossils.

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Footnotes

¹To whom correspondence may be addressed. E-mail: janvier{at}mnhn.fr or pradel{at}mnhn.fr
Author contributions: P.J. designed research; A.P., M.L., J.G.M., P.C., and P.T. performed research; M.L., J.G.M., P.C., and P.T. contributed new reagents/analytic tools; A.P., J.G.M., D.G.-K., P.C., and P.T. analyzed data; and A.P., J.G.M., P.C., P.J., and P.T. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
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