More than a century ago, a small fish fossil was found in a coal mine in England that didn’t look like much to the naked eye. Later studies dated it as an extinct fish the size of a bream that lived 319 million years ago at the end of the 20th century Paleozoic. But only now has a research team from the Universities of Birmingham (UK) and Michigan (USA) discovered its true value: Inside is the oldest known example of a vertebrate brain. The results have just been published in the journal Nature.
Most of the fossils recovered so far have been formed from hard body parts such as bones, teeth and shells. This is because soft tissues, such as the brain, usually decay quickly and very rarely fossilize. However, when this fish died, the soft tissues of its brain and cranial nerves were replaced in the process with a dense mineral that preserved its three-dimensional structure, which measures about an inch in detail.
“This unexpected discovery of a three-dimensional vertebrate brain gives us amazing insight into the neural anatomy of ray-finned fish. It shows us a more complicated pattern of brain evolution than is suggested from living species alone, and allows us to better define how and when modern-day bony fish evolved. Sam Gilesfrom the University of Birmingham, and one of the authors of the study.
The fish skull fossil compared to a coin Jeremy Marble, University of Michigan News
In particular, the fossil is from a Coccocephalus wildi, a primitive bream-sized fish that swam in an estuary and probably fed on small crustaceans, aquatic insects, and cephalopods, a group that now includes squid, cuttlefish, and cuttlefish. It had ray fins, a group of fish whose backbone and fins are supported by bony rods called rays. “Comparisons with living fish showed that the brains of Coccocephalus most closely resemble the brains of sturgeon and paddlefish, which are often referred to as ‘primitive’ fish because they differed from all other living ray-finned fish more than a year ago.” than 300 million years.”
The scientists weren’t looking for a brain when they first examined the fossil, but they did discover a definite and unusual object inside the skull. The mysterious object shared several features found in vertebrate brains: it was bilaterally symmetrical, contained cavities resembling ventricles in appearance, and had multiple filaments extending into openings in the braincase resembling ventricles in appearance, Cranial nerves passing through such canals in living species. Significantly, the brain of Coccocephalus folds inward, in contrast to all living ray-finned fish, in which the brain folds outward.
Although only its skull was recovered, scientists believe C. wildi would have been between 6 and 8 inches long. Judging by the shape of its jaws and teeth, it was probably a carnivore Rodrigo Figueroa, also from the University of Michigan. When the fish died, it was probably quickly buried in low-oxygen sediment. Such environments can slow down the breakdown of the body’s soft tissues.
“This small and superficially nondescript fossil shows us not only the oldest example of a fossilized vertebrate brain, but also reveals much of what we thought about the evolution of brains from living things,” says Figueroa.