Fossil brain undoes 350 million years of scientific understanding
'Stunning' scans reveal how ray-finned fish diverged from other vertebrates
A fish fossil first described in northwest England more than 100 years ago is shedding light on the evolution of the brain with the help of X-ray scanning and 3D reconstruction.
Many vertebrates' forebrains are formed from the neural tube in the embryo by two cerebral hemispheres that enclose a hollow space called a ventricle; it is called an evaginated forebrain. In ray-finned fishes, which make up about half of living vertebrates, the forebrain is everted, which means it consists of solid cerebral hemispheres that are separated by a narrow ventricle.
It was assumed ray-finned fishes, such as tuna and monkfish, have possessed this feature of brain anatomy and development since they first evolved about 350 million years ago.
However, a Nature paper by Matt Friedman, associate professor of evolutionary biology at the University of Michigan, shows there is much more to it.
His team of researchers succeeded in examining a 319-million-year-old fossil specimen of the extinct ray-finned fish Coccocephalichthys wildi using X-ray scanning. Although fossils usually only reveal evidence of hard tissue such as bones, the resulting 3D images showed the ancient creature's brain in "stunning" detail.
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"When studying its internal anatomy with X-ray scanning and 3D reconstruction techniques, [the researchers] found not only that the main regions of the brain and cranial nerves were fossilized in stunning detail, but also that the forebrain is evaginated – a feature previously unknown in ray-finned fishes. Fossilized soft tissues in such ancient vertebrates are unusual, and this fossil is the oldest known fossilized vertebrate brain," wrote Bristol University's Hugo Dutelis and Matteo Fabbri of Chicago's Field Museum of Natural History in an accompanying paper.
The pair said the finding and the position of Coccocephalichthys in the vertebrate family tree had "crucial implications for our understanding of brain evolution." Also, it casts doubt over using such brain features as a means of classifying ancient ray-finned fishes.
"This example of a well-preserved vertebrate brain provides a window into neural anatomy deep within ray-finned fish phylogeny [evolutionary relationships]," the paper said.
"Coccocephalus indicates a more complicated pattern of brain evolution than suggested by living species alone… Our findings, along with a growing set of studies in other animal groups, point to the importance of ancient soft tissue preservation in understanding the deep evolutionary assembly of major anatomical systems outside of the narrow subset of skeletal tissues." ®