Scientists on Monday revealed the 300-million-year-old brain of an extinct relative of today's sharks, the first time that soft tissue from such an ancient fossil has ever been seen. French and US scientists obtained three-dimensional pictures of the mineralised brain using computerised axial tomography, also known as a CAT scan, and carried out micro-imaging using an atomic particle accelerator called a synchrotron.
The brain was found in the skull of a fish called an iniopterygian, an relative of sharks and another latter-day species called the ratfish. Iniopterygians were once commonplace in the world's oceans, living in shallow and muddy marine waters. They measured 50 centimetres (20 inches) at most.
The imaging achievement, reported in the US journal Proceedings of the National Academy of Sciences, is important because soft tissue in fossils is very rarely preserved, except for muscle and organs such as kidneys. The sample that was scanned was unearthed from a deposit of shale in Kansas, and itself was special as its skull had not been squashed. Fossils are inevitably flattened because of the weight of layers of soil or rock that accumulate on the dead animal over millions of years.
The tiny brain, measuring 1.5 millimetres by seven millimetres (0.06 by 0.27 of an inch), offers intriguing insights into the evolution of fishes, the authors said. The brain has a large lobe for vision but a reduced area for auditory input. This, together with the position of the ear canals, suggests the iniopterygian could hear side-to-side movements but not up-and-down ones.