title: Skull and brain of a 300-million-year-old chimaeroid fish revealed by synchrotron holotomography.
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 approximately 300-Myr-old concretions from Oklahoma and Kansas. The study was performed by using conventional X-ray microtomography (muCT), as well as absorption-based synchrotron microtomography (SR-muCT) [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-muCT 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.
authors.affiliation: Muséum 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.
authors.forename: Alan
authors.initials: A
authors.lastname: Pradel
authors.forename: Max
authors.initials: M
authors.lastname: Langer
authors.forename: John G
authors.initials: JG
authors.lastname: Maisey
authors.forename: Didier
authors.initials: D
authors.lastname: Geffard-Kuriyama
authors.forename: Peter
authors.initials: P
authors.lastname: Cloetens
authors.forename: Philippe
authors.initials: P
authors.lastname: Janvier
authors.forename: Paul
authors.initials: P
authors.lastname: Tafforeau
mesh_terms: D000818:Animals; D001921:Brain / Q000033:anatomy & histology*; D005399:Fishes / Q000033:anatomy & histology*; D005580:Fossils; D007615:Kansas; D009825:Oklahoma; D010802:Phylogeny; D012886:Skull / Q000033:anatomy & histology*; D055114:X-Ray Microtomography / Q000379:methods*