The specimen that included the brain was imaged as a holtomography by Paul Tafforeau and colleagues at the European Synchrotron Radiation Facility. This more powerful scan brought the brain to light in great detail. It is a tiny (about 1.5mm by 7 mm in size), symmetrical shape that sits within a large braincase; as in many lower vertebrates, the brains of these fish ceased to grow as their skulls continued to expand. 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.
"There is nothing like this known today; it is really bizarre," says Maisey. "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."
Pradel agrees and will next look for possible brains of spiny rayed fish found in the same fossil beds from Kansas, Oklahoma, and Texas. "This fossilized brain allows for real paleo-neuroanatomical studies of fossil vertebrates," he says. "Now that we have fossilized soft tissue in addition to bone, we can see that there is no general correspondence between the morphology of the brain and that of the endocranial cavity and that past paleo-neuroanatomical studies must be taken with caution."
Source: American Museum of Natural History
3-D reconstruction of the braincase in ventral view (green=braincase; red=endocranial cavity; orange=brain).
(Photo Credit: Alan Pradel)