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A 319-million-year-old fossilised fish, pulled from a coal mine in England more than a century ago, has revealed the oldest example of a well-preserved vertebrate brain.
CT-scanning, where X-rays are used to reveal internal features, shows the skull of the creature contains a brain and cranial nerves that are roughly an inch long.
Researchers at the University of Birmingham (UK) and the University of Michigan (USA) believe that the discovery opens a window into the neural anatomy and early evolution of a major group of fishes alive today - ray-finned fishes.
Their findings, published today (1 Feb) in Nature, shed new light into the preservation of soft parts in fossils of backboned animals. Most of the animal fossils in museum collections were formed from hard body parts such as bones, teeth and shells.
Senior author Sam Giles, of the University of Birmingham, commented: "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.
"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."
The CT-scanned brain analysed belongs to Coccocephalus wildi, an early ray-finned fish roughly the size of a bream that swam in an estuary and likely dined 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.
Soft tissues such as the brain normally decay quickly and very rarely fossilise. But when this fish died, the soft tissues of its brain and cranial nerves were replaced during the fossilization process with a dense mineral that preserved, in exquisite detail, their three-dimensional structure.
Senior author Matt Friedman, from the University of Michigan, commented: "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."
The skull fossil from England is the only known specimen of its species, so only non-destructive techniques could be used during the U-M-led study.
Lead author Rodrigo Figueroa, also from the University of Michigan, commented: "Not only does this superficially unimpressive and small fossil show us the oldest example of a fossilised vertebrate brain, but it also shows that much of what we thought about brain evolution from living species alone will need reworking.
Scientists were not looking for a brain when they examined the skull fossil for the first time, but discovered an unusual, distinct object inside the skull. The mystery object displayed several features found in vertebrate brains: Iit was bilaterally symmetrical, it contained hollow spaces similar in appearance to ventricles, and it had multiple filaments extending toward openings in the braincase, similar in appearance to cranial nerves, which travel through such canals in living species. Significantly, the brain of Coccocephalus folds inward, unlike in all living ray-finned fishes, in which the brain folds outward.
Though preserved brain tissue has rarely been found in vertebrate fossils, scientists have had better success with invertebrates. There are roughly 30,000 ray-finned fish species, and they account for about half of all backboned animal species. The other half is split between land vertebrates—birds, mammals, reptiles and amphibians—and less diverse fish groups like jawless fishes and cartilaginous fishes.
The Coccocephalus skull fossil is on loan to the University of Michigan from Manchester Museum, in the UK. It was recovered from the roof of the Mountain Fourfoot coal mine in Lancashire and was first scientifically described in 1925. The fossil was found in a layer of soapstone adjacent to a coal seam in the mine.
Though only its skull was recovered, scientists believe that C. wildi would have been 6 to 8 inches long. Judging from its jaw shape and its teeth, it was probably a carnivore, according to Figueroa. When the fish died it was probably quickly buried in sediments with little oxygen present. Such environments can slow the decomposition of soft body parts.
The fossil captures a time before a signature feature of ray-finned fish brains evolved, providing an indication of when this trait evolved.
