The iniopterygian Sibyrhynchus denisoni. Credit: P. Janvier
A 300-million-year-old brain of a relative of sharks and ratfish has been revealed using synchrotron holotomography. It is the first time that the soft tissue of such an old fossil brain has ever been found.Using absorption microtomography to study different samples scientists found a peculiar crystalline calcite structure in one specimen of an iniopterygian. The 3D reconstruction showed a tiny (about 1.5mm by 7 mm in size), symmetrical shape that sits within a large braincase.
The brain has a large lobe for vision and an optic nerve that stretches to the correct place on the braincase; both of these features correlate well with the large eye sockets. The auditory section of the brain is reduced, and this information reflects observations of the inner ear in iniopterygians. Unlike typical ear canals that regulate orientation and balance with three big loops, the ear canals in this extinct group are all pulled into a horizontal plane. This means that the fish could detect side to side movements, but not up and down. The only part the researchers couldn't spot was the forebrain, perhaps too thin to become mineralized.
The mineralization of the brain is, according to the main author of the paper, Alan Pradel, "due to the presence of bacteria that covered the brain shortly before decay and induced its phosphatization". On top of this, the environmental conditions, probably saturated with calcium phosphate, the lack of oxygen in the braincase and the presence of fatty acids in the brain may have generated a fall in pH that also shifted the appearance of calcium carbonate in favour of calcium phosphate. link
The paper will be up shortly at PNAS.org. I’ll update this posting when it appears.
