Tuesday, 17 March 2009 15:14
A remarkably well-preserved brain has been discovered in a 300-million-year-old fossil of a fish from Kansas. Philippe Janvier of the National Museum of Natural History in Paris and his colleagues used X-rays from the European Synchrotron Radiation Facility in Grenoble, France, to peer inside the skulls of iniopterygians, extinct relations of modern sharks and ratfish.
In one skull (pictured), they imaged a dense calcium phosphate structure. Its shape and relationship to nerve locations suggest that it is a brain, mineralized by phosphate-fixing microbes before the soft tissue could decay. The researchers hope that other vertebrates fossilized in similar conditions might yield further preserved organs, potentially throwing light on brain evolution.
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.
Article Link: http://dx.doi.org/10.1073/pnas.0807047106