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Fossilized fish brain surprises scientists

At 300 million years old, the brain is the oldest ever found. The fish skull, found in Kansas, was rare in itself; specimens of early fish are usually squashed flat.

March 07, 2009|Thomas H. Maugh II

Researchers imaging the fossil of a 300-million-year-old fish from Kansas have unexpectedly found a fossilized brain in the animal's skull -- the oldest known brain found to date.

The find was a double surprise, said John G. Maisey, a paleontologist at New York's American Museum of Natural History who co-wrote a report on the findings.

First, most specimens of the early fish -- known as an iniopterygian and distantly related to modern sharks and ratfishes -- are squashed flat and thus reveal no details of skull anatomy. And second, brain tissue, unlike that of other organs, is rarely preserved, in large part because it is mostly water.

In this case, researchers have only the fish's head. Because of the preservation, Maisey speculated that the head was bitten off by a predator, which died shortly thereafter.

The high concentration of phosphates -- which are crucial in making fossils -- in the predator's intestines probably caused the skull to fossilize quickly, before it could be crushed, and also preserved the brain, Maisey said. Paleontologist Alan Pradel of the National Museum of Natural History in Paris examined four iniopterygian skulls from Kansas and Oklahoma and found that one had an unusual "blob" inside it. He sent it to Paul Tafforeau of the European Synchrotron Radiation Facility in Grenoble, France, who used intense x-irradiation to make a three-dimensional image of the tiny brain.

The fish itself would have been small enough to fit in the palm of a human's hand. It is marked by a proportionately huge skull with two large eyes, sharklike teeth in rows, tails with clubs and pectoral fins that were nearly on its back. The brain itself is pea-sized and much smaller than the braincase, a feature that is preserved in modern descendants.

Imaging with the synchrotron, reported Tuesday in the Proceedings of the National Academy of Sciences, clearly showed the cerebellum, spinal cord and optic lobes, among other parts. The only part that appears missing is the forebrain, which was perhaps too thin to become mineralized. The brain has a large lobe for vision, and an optic nerve that stretches to the proper place on the braincase, compatible with the large eyes, which were probably used for hunting.

The auditory section of the brain is small, which is compatible with the observation that the ear canal has only horizontal loops for balance, not the three big loops present in modern species. That means the fish's brain could detect side-to-side movements, but not up and down ones.


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