A fish preserves the oldest fossilized brain of a vertebrate

Feb. 1 () –

A 319 million year old fossilized fishextracted from a coal mine in England more than a century ago, has revealed the oldest example of a well-preserved vertebrate brain.

A CT scan of the fossil, which uses X-rays to reveal internal features, showed that the creature’s skull contains a brain and cranial nerves about two centimeters long. The finding is published in Nature.

Researchers from the University of Birmingham (UK) and the University of Michigan (USA) believe the discovery opens a window into the neural anatomy and early evolution of an important group of fish living today: ray-finned fish.

Their findings shed new light on the conservation of soft parts in fossils of animals with backbones. Most animal fossils in museum collections they were formed from hard parts of the body, such as bones, teeth, and shells.

Lead author Sam Giles, from the University of Birmingham, said: “This unexpected finding of a three-dimensionally preserved vertebrate brain offers us surprising insight into the neural anatomy of ray-finned fish. It tells us about a pattern of evolution brain more complicated than living species alone suggest, allowing us to better define how and when today’s bony fishes evolved“, he recounts.

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

The CT-scanned brain that has been analyzed belongs to a ‘Coccocephalus wildi’, a primitive ray-finned fish about the size of a bream that swam in an estuary and probably fed on small crustaceans, aquatic insects and cephalopods, a group that today it includes squid, octopus and cuttlefish. Stripe-finned fish have backbones and fins supported by bony bars called rays.

Soft tissues, such as brains, decay quickly and rarely fossilize, but when this fish died, the soft tissues of its brain and cranial nerves were replaced during the fossilization process by a dense mineral that preserved, in great detail, its three-dimensional structure.

Lead author Matt Friedman of the University of Michigan notes that “an important takeaway is that these types of soft parts can be preserved, and they can be preserved in fossils that we have had for a long time: this is a fossil that has been known for a long time.” over 100 years.”

This fossil skull is the only known specimen of its species, so during the UM-led study only non-destructive techniques could be used.

Lead author Rodrigo Figueroa, also from the University of Michigan, notes that “this superficially unimpressive and small fossil not only shows us the oldest example of a fossilized vertebrate brain, but also demonstrates that much of what we thought about the evolution of the brain from living species only will have to be reworked.”

The scientists weren’t looking for a brain when they first examined the fossil skull, but instead discovered an unusual and distinct object inside. This mysterious object had several features typical of vertebrate brains: It was bilaterally symmetrical, contained hollow spaces similar in appearance to ventricles, and had multiple filaments extending into the openings of the brain, similar in appearance to cranial nerves, that travel through these channels in living species.

As he explains, the brain of ‘Coccocephalus’ folds inwards, unlike what happens in all striped-finned fish, in which the brain folds outwards.

Although preserved brain tissue has rarely been found in vertebrate fossils, scientists have had more success with invertebrates. There are about 30,000 species of ray-finned fish, representing about half of all vertebrate animal species. The other half is divided between terrestrial vertebrates -birds, mammals, reptiles and amphibians- and less diverse groups of fish, such as jawless fish and cartilaginous fish.

The Manchester Museum (United Kingdom) has lent the fossil skull of ‘Coccocephalus’ to the University of Michigan. It was recovered from the roof of the Mountain Fourfoot colliery in Lancashire and was first described scientifically in 1925. The fossil was found in a layer of soapstone adjacent to a coal seam in the mine.

The Manchester Museum (United Kingdom) has lent the fossil skull of ‘Coccocephalus’ to the University of Michigan. It was recovered from the roof of the Mountain Fourfoot coal mine, in Lancashire, and was first described scientifically in 1925. It was found in a soapstone layer adjacent to a coal mine seam.

Although only the skull was recovered, scientists believe that ‘C. wildi’ measured between 15 and 20 centimeters. Judging by the shape of its jaw and teeth, it was probably a carnivore, according to Figueroa. When the fish died, it was likely quickly buried in low-oxygen sediment. These environments can slow down the breakdown of the soft parts of the body.

The fossil captures a time before the evolution of a characteristic feature of the ray-finned fish brain, the researchers say, providing an indication of when this feature evolved.