Sensational find in an old museum piece: CT scans have revealed an amazingly detailed brain in a skull fossil of a 319-million-year-old fish. The fossilized nerve structure now sheds new light on the early evolutionary history of the ray-finned fish - today's largest group of fish. In addition, the study makes it clear that paleontological treasures may still be waiting to be discovered in many other fossils, the researchers say.
Bones, teeth and armor - usually remains of hard structures testify to the creatures of the past. Soft tissues such as muscles, organs and nerves, on the other hand, have rarely left fossil traces. However, previous studies have already shown that this is possible in principle. For example, the remains of organs were discovered in a primeval armored fish, and brain fossils have already been reported: fossil remains of nerve structures have already been discovered in primeval arthropods and a 300-million-year-old cartilaginous fish. A particularly old and exceptionally well-preserved find is now part of this collection.
Accidental find in a museum piece
The petrified brain was discovered in a fossil that has been known for around 100 years. It is a small fish skull recovered from a Lancashire coal mine and first described in 1925. The fossil was then stored in the Manchester Museum. The skull came from a specimen of the fish Coccocephalus wildi, which is believed to be around 20 centimeters long and was estimated to be 319 million years old. According to the morphological characteristics, Coccocephalus was an early representative of the ray-finned fish. This is the largest group of fish today and due to their diversity in species, the ray-finned fish even make up about half of all vertebrates.
As the researchers led by senior author Matt Friedman from the University of Michigan in Ann Arbor report, the spectacular find in the fish skull fossil was a coincidence. Because they didn't expect to find a brain, but actually only hoped to gain insights into the internal bony structures through the examination using computed tomography (CT). "I scanned the fossil and then noticed in the visualization of the data that an unusual object was emerging in the skull," reports Friedman. "This entity seemed to have a well-defined structure," says the paleontologist. Subsequent investigations made it increasingly clear that this was a brain fossil that had been preserved in astonishing detail.
Surprising features
Analysis of its complex structures revealed that the Coccocephalus brain had a raisin-sized central body with three main regions corresponding to the forebrain, midbrain and hindbrain of modern fish. However, the structures also reflected something very surprising: "The brain of Coccocephalus apparently folded inward during embryonic development," says Friedman. In contrast, he explains, the brains of all modern-day ray-finned fish develop by folding tissue outwards from the inside of the embryo. In all other vertebrates, neuronal tissue is folded inward during brain development. "So this fossil represents a phase before this distinctive feature of ray-finned fish brains evolved," says Friedman. His colleague Rodrigo Figueroa comments: "The brain fossil is not just a curiosity, but has considerable value for understanding patterns of brain evolution," emphasizes the first author of the study.
As far as the astonishingly good preservation is concerned, the researchers suspect that when the fish died, it was probably buried very quickly in sediments with little oxygen, which slowed decomposition. In addition, the chemical microenvironment in the fish's skull may have helped preserve the delicate brain tissue. So it was apparently eventually replaced by a dense mineral, the researchers say.
As they point out, the study now also has a fundamental message: "It shows how soft structures can have been preserved, and they can apparently lie dormant in fossils that we have known for a long time - as in this case for over 100 years ,” says Friedman. Figueroa continues: "Given the widespread availability of modern imaging techniques, I wouldn't be surprised if we began to find that fossil brains and other soft parts were preserved much more frequently than we previously thought."
Source: University of Michigan, professional article: Nature, doi: 10.1038/s41586-022-05666-1