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319-million-year-old fish holds clues about brain evolution

The skull of a fish that lived around 319 million years ago has recently been analyzed with by scientists from the University of Michigan and the University of Birmingham. The analysis turned up something unexpected. Within the fossilized skull, the researchers found a discrete, 3D object that could only be the fish’s brain. 

This is unusual because soft tissues, such as brains, usually decay when an animal dies, and are seldom fossilized. It has allowed the researchers a window into the neural anatomy and early evolution of the ray-finned fishes, a major group that contains about 30,000 different species, roughly half of all known vertebrate species. 

The ancient fossil skull was collected from the roof of a coal mine in Lancashire, UK, around 100 years ago and first described in 1925. At the time, it was identified as belonging to a new species, Coccocephalus wildi, and was put away in a draw in the Manchester Museum. No other fossilized body parts were recovered from the layer of soapstone where the skull was found. 

The researchers were not looking for a brain when they used CT-scanning to visualize the structure of the skull. But the unexpected object detected within the skull displayed several features found in vertebrate brains: it was bilaterally symmetrical, it contained hollow spaces similar in appearance to ventricles, and it had multiple filaments extending toward openings in the braincase, comparable in appearance to cranial nerves. 

The analysis of the findings has been published in the journal Nature, and the researchers say their discovery sheds new light on the preservation of soft parts in fossils of backboned animals. Although most of the animal fossils in museum collections were formed from hard body parts such as bones, teeth and shells, the researchers say their work shows soft parts can become fossilized under certain circumstances and can provide valuable information. 

“An important conclusion is that these kinds of soft parts can be preserved, and they may be preserved in fossils that we’ve had for a long time – this is a fossil that’s been known for over 100 years,” said study senior author Matt Friedman.

The small skull is the only fossil of the extinct species ever to have been found. For this reason, the researchers had to use non-invasive methods to analyze it. They suggest that C. wildi was an early ray-finned fish, roughly the size of a bream (6–8 inches long), that swam in an estuary and probably preyed on small crustaceans, aquatic insects and cephalopods, a group that today includes squid, octopuses and cuttlefish. Ray-finned fishes have backbones and fins supported by bony rods called rays. 

The exceptionally preserved brain had another surprise in store for the researchers. Ray-finned fish show an unusual feature in the early formation of their brains. The embryonic neural tube grows through eversion (folding inwards) of the dorsal walls of the forebrain. This results in a forebrain formed in two solid hemispheres that do not enclose a ventricle (cavity filled with cerebrospinal fluid). 

This is different from early brain development in all other vertebrates, for example mammals, birds and reptiles. Their embryonic neural tubes grow by evagination (folding outwards) of the lateral walls of the forebrain, which forms two swellings that each enclose a large ventricle. Thus ray-finned fish show a unique pattern of early brain development. But this is not true for C. wildi

C. wildi is the only example of a ray-finned fish that does not show the brain development typical of its group. Its brain develops in the same way as in other vertebrate groups, making it different from all living ray-finned fishes. The researchers say that C. wildi thus gives an indication of what the original path of brain development could have been, and that the principle neuroanatomical feature of all modern ray-finned fish (namely an everted forebrain) originated somewhere subsequently along the evolutionary pathway of the group. 

“This unexpected find of a three-dimensionally preserved vertebrate brain gives us a startling insight into the neural anatomy of ray-finned fish. It tells us a more complicated pattern of brain evolution than suggested by living species alone, allowing us to better define how and when present day bony fishes evolved,” explained study senior author Sam Giles.

“Comparisons to living fishes showed that the brain of Coccocephalus is most similar to the brains of sturgeons and paddlefish, which are often called ‘primitive’ fishes because they diverged from all other living ray-finned fishes more than 300 million years ago.”

Thus, the exceptional small fossil gives a window into brain development at a time before the signature feature of ray-finned fish brains evolved, providing an indication of what the primitive brain anatomy could have been in the very early ray-finned fish. Since an everted forebrain is absent in C. wildi, it indicates that this feature evolved during later evolution of the group. 

“Not only does this superficially unimpressive and small fossil show us the oldest example of a fossilized vertebrate brain, but it also shows that much of what we thought about brain evolution from living species alone will need reworking,” said study lead author Rodrigo Figueroa.

The authors also highlight the importance of searching for fossilized brains within the skulls of other preserved fish and vertebrates. They state that preservation of brain tissue is likely to be more common than previously thought, and that if researchers assume brains will not have fossilized, they run the risk of overlooking potentially valuable information. 

By Alison Bosman, Staff Writer

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