Fossil fish brain studied

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French and American scientists have described the first images of the world's oldest fossil brain in a recent issue of the Proceedings of the National Academy of Sciences.

Alan Pradel and coauthors were studying some remarkably well-preserved skulls of Sibyrhynchus denisoni, an iniopterygian fish that lived about 300 milion years ago, from Oklahoma and Kansas when they made this discovery.

Iniopterygians are cartilaginous fishes distantly related to chimaeras or ratfishes that grew no bigger than 50 cm.

They possess an unusual morphology characterized by enlarged pectoral fins mounted high on their bodies, with numerous denticles (teeth) on the front edge of the fins and club-like tails.

The team of scientists from the National Natural History Museum in Paris, the European Synchrotron Radiation Facility in Grenoble and the American Museum of Natural History in New York were examining one of the few skulls of an iniopterygian fish that has maintained its three dimensions (most of the fossil skulls tend to be squashed) using absorption microtomography when they noticed one sample from Kansas possessing an unusual structure in the head that was denser than the surrounding matrix filling the braincase.

A reanalysis of the fossil using X-ray holotomography revealed a paired, symmetrical and elongated object in the same position where a brain would have been.

A three-dimensional reconstruction of the object revealed a brain and some cranial nerves, with different components such as the cerebellum, spinal cord or optic lobes and tracts, among others) readily discernible.

The forebrain could not be made out, however, perhaps too thin to become mineralized. The soft-tissue mineralization of the brain was thought to be a result of microbially induced postmortem phosphatization.

According to the authors, such a presence of a fossilized brain allows a tentative paleoneuroanatomical study of a fossil vertebrate based on the actual brain and not merely the endocranial cavity. It also points to similar findings in other vertebrates preserved under comparable conditions.

For more information, see the paper: Pradel, A, M Langer, JG Maisey, D Geffard-Kuriyama, P Cloetens, P Janvier, and P Tafforeau (2009) Skull and brain of a 300-million-year-old chimaeroid fish revealed by synchrotron holotomography. Proceedings of the National Academy of Sciences 106, pp. 5224"5228.