Going into the field isn’t strictly necessary to make new discoveries in paleontology. It can also be useful to view old collections and reanalyze known fossils.
This 319-million-year-old fish-fish fossil, recovered from a coal mine in England 100 years ago, has indeed revealed new surprises. By passing his skull to the scanner, the researchers accidentally identified what appears to be a fossilized brain.
A detailed mineral cast
Although originally soft tissue that is generally difficult to fossilize, this cerebral brain appears intact and in excellent condition. The nerve network in the skull has also been preserved, allowing the study of the neural anatomy and early evolutionary stages of ray-finned fish (Actinopterygiansactinopterygians) still found in our oceans today. The fish in question, of the species Coccocephalus wildi, which lived in the oceans at the end of the Carboniferous, actually belongs to this large group of vertebrates.
A 319-million-year-old fossilized fish sheds light on vertebrate brain development. © Senses of the Mind
Of course, the brain no longer exists in its original form. Soft tissues were slowly degraded in low-oxygen conditions after rapid burial and then gradually replaced by mineral minerals (presumably pyritic pyrite). The shape of the organ has therefore been preserved, giving a relatively detailed cast of the inside of the fish skull. The skull scan thus made it possible to reproduce the brain digitally and precisely in 3D without destroying the precious fossil. The results are published in the journal Nature.
The oldest fossil brain of a vertebrate
This makes it the oldest fossilized brain that has ever belonged to a vertebrate. Other older brain traces have been discovered, but these are invertebrates. Studying its structure has shown that the evolution of the vertebrate brain would be more complicated than previously thought. This discovery shows the importance of conserving fossilized specimens and re-examining them regularly thanks to new technologies that are being developed over time.
In the fossil remains of animals, soft tissues are extremely rare. Those that come from the brain are even more so. As for the find in fossils hundreds of millions of years old, the hope seemed in vain. And yet…
Article by Jean-Luc GoudetJean-Luc Goudet, published March 5, 2009
It all began with the rediscovery of four remarkably well-preserved, 300-million-year-old fossil fish skulls from Kansas, part of the collections of the National Museum of Natural History, Paris (MNHNMNHN). These remains belong to a long-vanished species, Sibyrhynchus denisoni, a roughly a foot fish from the group of iniopterygians, cousins of modern-day chimeras and relatives of sharks and rays. Iniopterygians, like them, are elasmobranchs, fish characterized by the lack of hard tissue, which makes their fossils just fragile.
The fossil discovered in Kansas. Tomography enables non-destructive analysis. © PNAS
Alain Pradel of the MNHN and his collaborators decided to examine these skulls using X-ray X-ray microtomography, similar to that used by medical CT scanners (for computed tomography) and able to reconstruct a three-dimensional image from a series of measures. In one of the skulls, the researchers discovered a structure of a special kind, denser than that of the fossilized skull case, consisting of crystalline calcite.
This skull then made its way to Grenoble to join the ESRF (European Synchrotron Radiation Facility). This particle accelerator is used as a synchrotron and made available to the international scientific community for a variety of analyses. In fact, a synchrotron generates very fine and extremely strong X-rays, which can be used to study matter using various methods.
The ESRF synchrotron near Grenoble, a kind of giant microscope… © ESRF
The researchers used holotomography. This newer method also uses X-rays, but requires phase contrast (a technique also known in optical microscopy), which highlights otherwise indistinguishable shapes. The strange internal structure was thus shown more clearly. Its shape and location corresponded fairly closely to that of this animal’s brain.
A very nice study that gives others hope
The scientists were then able to research this nervous system with unprecedented precision and have just described their results in the Pnas Review. Three hundred million years later, the images clearly show the cerebellum, cerebellum, spinal cord, optic nerves, and even several nerves, including the optic nerve. The front part of the brain (called the telencephalon) remains invisible. The researchers conclude that this part may have been very small in the living animal, as it is in today’s chimeras.
An iniopterygian, cousin of the current chimeras. © PNAS
The optic lobes are of good size, of a size that agrees well with that of the eyes. On the other hand, the inner ear inner ear appears very small, as previous observations had shown. It appears that this fish had only semi-circular canals grouped in a single plane, horizontal in this case. This arrangement probably only enabled it to perceive lateral, not vertical, water movement, suggesting that Sibyrhynchus denisoni must have lived on the bottom rather than in open water.
Finally, this analysis reveals a small brain, 7 millimeters long and 1.5 millimeters wide, a very modest size compared to the skull that houses it. The team believes this small size is not due to shrinkage from fossilization or fossilization, as the nerve attachments appear to be in their places.
The researchers explain why this specimen has been so well preserved. Chemical analyzes have shown that the surface of the brain is covered with calcium calcium phosphate. This layer could have been deposited by a bacterial film or by a chemical process, thus petrifying the brain.
This remarkable study also offers hope for paleontology. It shows that new analytical techniques can take us deeper into the history of very ancient vertebrates. Other well-preserved fossils may hold many mysteries just waiting for a well-done tomography to reveal themselves…