Scientists Unlock Mystery of Coelacanth's Hinged Skull and Tiny Brain

In 1938, a South African fisherman pulled something impossible from the ocean: a living coelacanth, a fish that scientists had believed vanished seventy million years ago. Eighty years later, researchers were still asking fundamental questions about this creature—not where it came from, but how it was built. In 2019, a team led by Hugo Dutel at the University of Bristol published findings that answered one of the coelacanth's strangest riddles: why its brain occupies only one percent of the skull cavity that contains it, and how a hinged braincase—a feature shared with ancient lobe-finned fishes from the Devonian period—actually develops.

The coelacanth matters to evolutionary biologists because it is a living window into the deep past. These deep-sea fish are closely related to tetrapods, the four-limbed vertebrates that gave rise to amphibians, mammals, and reptiles. Understanding how the coelacanth's body plan works offers clues to how our own distant ancestors were organized. But the coelacanth's anatomy presents a puzzle unlike anything seen in modern vertebrates. Its braincase is split completely by a joint called the intracranial joint, dividing it into front and rear sections. The brain sits far back in the skull, leaving an enormous empty space around it—a mismatch so extreme that it has no parallel among any living vertebrate species.

To understand how this unusual arrangement comes to be, the researchers needed to examine the coelacanth at different stages of life, from fetus to adult. Adult specimens exist in museum collections around the world, but fetal coelacanths are extraordinarily rare. The team, working with specimens from the National Museum of Natural History in Paris and other institutions, turned to technology that could reveal internal structure without damage. They used synchrotron X-ray microtomography—a technique that fires intense beams of X-rays through an object to create detailed cross-sectional images—on a five-centimeter fetus, the earliest developmental stage available for the species Latimeria chalumnae. They combined this data with magnetic resonance imaging and micro-computed tomography scans to construct precise three-dimensional models showing how the skull, brain, and notochord transformed from fetus to adult.

What emerged from these models pointed to an unexpected culprit: the notochord, a tube-like structure that runs beneath the brain and spinal cord in early vertebrate life. In most vertebrates, the notochord is replaced by the vertebral column early in embryonic development and then disappears. In the coelacanth, something different happens. The notochord expands considerably as the animal develops. The researchers propose that this enlargement shapes how the braincase is patterned and likely drives the formation of the intracranial joint itself. More strikingly, the expanding notochord may also explain why the brain's relative size shrinks so dramatically during development—the growing structure around it leaves proportionally less room for neural tissue to occupy.

The 2019 study, published in the journal Nature under the title "Neurocranial development of the coelacanth and the evolution of the sarcopterygian head," opened new pathways for understanding how vertebrate skulls evolved. The coelacanth is not simply a living fossil frozen in time; it is an active laboratory for studying how developmental processes shape anatomy across millions of years. The findings suggest that the hinged braincase and tiny brain are not separate oddities but linked consequences of a single developmental process—one that has persisted in this species since the Devonian period, when such features were more common among lobe-finned fishes. As climate change and overfishing threaten these critically endangered deep-sea creatures, the knowledge locked inside their bodies becomes ever more precious.