A 380-million-year-old fish skull from Antarctica has revealed that some prehistoric fish were already equipped with features that helped them live near the water's surface, long before any animal took its first step on land.
Scientists at Flinders University in Australia used advanced neutron imaging to look inside the only known fossil of Koharalepis jarviki, a large predatory fish from the Devonian Period. The fossil was found in Antarctica's Lashly Mountains. The scans uncovered hidden structures in the skull that had remained sealed for hundreds of millions of years.
Air-gulping holes and a light-sensing brain organ
The neutron scans showed that Koharalepis had openings in the top of its skull. Researchers believe these holes allowed the fish to gulp air from above the water. The scans also revealed an organ inside the brain that detects light and helps regulate day-night rhythms, known as circadian rhythms.
These features suggest the fish spent time near the water's surface, possibly in shallow environments where oxygen levels were low. The ability to breathe air and sense light would have been useful for survival in such conditions.
A close relative of the first land animals
Koharalepis belongs to a family of fish called Canowindridae. These fish lived across East Gondwana, an ancient supercontinent that included what is now Antarctica and Australia. Scientists consider them close relatives of the first four-limbed vertebrates, or tetrapods, that eventually evolved into land animals.
The fossil is especially valuable because it preserves the internal bones of the skull, a rare feature among Canowindridae. Lead author Corinne Mensforth, a PhD candidate at Flinders University, said this allowed the team to study the fish's braincase and neuroanatomy in detail.
What the brain reveals about the water-to-land shift
The scans showed that the brain of Koharalepis was similar to those of other fish that straddle the evolutionary transition from water to land. This suggests that some of the neurological changes needed for life on land may have already been present in fish that never left the water.
Dr. Alice Clement, a research fellow at Flinders University and coauthor of the study published in Frontiers in Ecology and Evolution, said the fish highlights the ancient links between Australia and Antarctica. She noted that the Devonian Period, often called the Age of Fishes, was a time when waters were full of predatory lobe-finned fish closely related to land animals.
The findings offer a rare look inside the head of a fish that lived millions of years before the first land animals appeared. They show that the move onto land did not happen all at once. Instead, the fish that eventually made that leap were already carrying the tools for it in their skulls.
