Most fossil specimens can only wish they had a brain. But paleontologists recently discovered the oldest known example nestled within a 300-million-year-old fish fossil from Kansas.
The rare find provides an unusually detailed view of brain structure in prehistoric life. It similarly sheds light on the extinct relatives of modern ratfishes, also known as "ghost sharks" or chimaeras.
"Soft tissue has fossilized in the past, but it is usually muscle and organs like kidneys because of phosphate bacteria from the gut that permeates into tissue and preserves its features," said John Maisey, paleontologist at the American Museum of Natural History in New York City. "Fossilized brains are unusual, and this is by far the oldest known example."
These chimaera relatives, called iniopterygians, represented bizarre beasts that sported massive skulls with huge eye sockets, shark-like teeth in rows, tails with clubs, huge pectoral fins that were almost placed on their backs, and bone-like spikes or hooks tipping the fins. They typically averaged 6 inches in length.
The remarkably preserved fossil brain shows details such as a large vision lobe and optic nerve stretching to the proper place on the braincase, which fits with the fish's large eye sockets. But unlike typical ear canals that have three big loops to regulate orientation and balance, the ear canals of the extinct fish only exist on a horizontal plane. That meant the fish could detect only side to side movements, and not up or down.
"There is nothing like this known today; it is really bizarre,” Maisey said. "But now that we know that brains might be preserved in such ancient fossils, we can start looking for others. We are limited in information about early vertebrate brains, and the evolution of the brain lies at the core of vertebrate history."
The fossilized brain shows little connection with the shape of the braincase, which may force researchers to rethink earlier assumptions about the missing brains of previous specimens. The team will next look for possible brains of spiny rayed fish found in the same fossil beds from Kansas, Oklahoma and Texas.
"For a long time, paleontologists have used the shape of the cranial cavity to research the general morphology of the brain—because soft tissue was not available until today," said Alan Pradel, a paleontologist at the Museum of Natural History in Paris.
Researchers used CAT scans and an X-ray synchrotron in France to examine several other braincases, which represent the first 3-D fossils from this group of extinct fishes. The full discovery is detailed in the early online edition of the journal Proceedings of the National Academy of Sciences.
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