‘Living fossils’ reveal how skulls evolved in vertebrates

A study into one of the world’s oldest types of fish is giving experts new clues about how lobe-finned fishes and land animals evolved. The analysis of the coelacanth, Latimeria chalumnae, is also answering questions about the skull and brain development of vertebrates.

The coelacanth was thought to have vanished with the dinosaurs over 65 million years ago, until a living specimen was found off the coast of South Africa in 1938. The discovery prompted debate about how this fish fits into our understanding of the evolution of land animals.

The skull of this rare fish was found to be completely split in half a by special “intracranial joint,” and its brain is only one percent the size of the skull. An international team of researchers led by Dr. Hugo Dutel at the University of Bristol has looked into the biology of this unique skull at different stages of development to determine when it divides and forms a hinge.

“We think that formation of this special joint is probably caused by the unique development of the notochord (a tube extending below the brain and the spinal cord in the early stages of life),” explained study co-author Professor John Long.

“It usually degenerates into a small rod below the brain in some fishes. However, the notochord for Coelacanth expands dramatically to become 50 times bigger than the brain in the adult fish.”

“This process of brain growth is very unusual, especially compared to primates like us in which the brain expands dramatically. A mismatch between the brain and its cavity also exists in some other living and fossil fishes, but what is observed here is totally unequalled among vertebrates.”

Today, the two species of coelacanth that remain are referred to as “living fossils” because they are physically unchanged. To visualize the internal anatomy of the fish, the scientists used advanced imaging techniques. The data was used to create detailed 3D models, which allowed the team to analyze how the shape of the skull, the brain. and the notochord changes from a fetus to an adult.

“Coelacanths are iconic animals thought to be on the line to the first land animals or tetrapods, because of their strange hinged head,” said Professor Long. “This new research shows the peculiar hinge in the skull was caused by persistence of the large cartilaginous rod, or notochord, preventing the skull form ossifying as one solid unit.”

The team noted how these structures are positioned relative to each other throughout development, and compared the observations to the formation of the skull in other vertebrates.

“There are still more questions than answers! Latimeria still holds many clues for our understanding of vertebrate evolution, and it is important to protect this threatened species and its environment,” said Dr. Hutel.

The study is published in the journal Nature.

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By Chrissy Sexton, Earth.com Staff Writer

Image Credit: Laurent Ballista, Gombessa expeditions, Andromede Oceanology Ltd