A new study has rewritten a key chapter in the story of vertebrate evolution. Researchers from the University of São Paulo (USP) in Brazil and the Smithsonian Institution in the United States have revealed that many of the muscles once believed to define the anatomy of the coelacanth, a fish often described as a “living fossil,” were in fact misidentified ligaments.
A Living Fossil With Hidden Secrets
The coelacanth (Latimeria chalumnae) has changed little over the past 65 million years, yet it continues to astonish researchers. By carefully dissecting the head of this deep-sea species, scientists discovered that only 13% of the previously proposed muscle innovations for the largest vertebrate lineages were accurate. Even more striking, the team identified nine previously unknown evolutionary transformations related to feeding and respiration.
According to Aléssio Datovo, professor at USP’s Museum of Zoology, the coelacanth turned out to be “even more similar to cartilaginous fish [sharks, rays, and chimaeras] and tetrapods [birds, mammals, amphibians, and reptiles] than previously thought. And even more distinct from ray-finned fish, which make up about half of living vertebrates.”
This discovery overturns a long-held belief that the coelacanth had a specialized set of muscles for expanding the buccopharyngeal cavity, a structure essential for suction feeding and respiration. Instead, the supposed muscles were revealed to be ligaments, incapable of contraction, which means the coelacanth relies primarily on biting rather than suction feeding, much like sharks.
A Window Into Deep Evolutionary History
The evolutionary divide between ray-finned fish (actinopterygii) and lobe-finned fish (sarcopterygii) dates back around 420 million years. The sarcopterygii include coelacanths, lungfish, and every tetrapod species — mammals, birds, reptiles, and amphibians. In contrast, ray-finned fish, which rely on powerful suction feeding, gained a significant evolutionary advantage, allowing them to dominate today’s aquatic ecosystems and account for roughly half of all living vertebrate species.
“Previous studies assumed that this set of muscles that would give greater suction capacity was also present in coelacanths and, therefore, would have evolved in the common ancestor of bony vertebrates,” Datovo explained. “We now show this isn’t true. This only appeared at least 30 million years later, in the common ancestor of living ray-finned fish.”
Unlocking The Past Through Rare Specimens
Coelacanths are elusive creatures that live about 300 meters below the ocean’s surface, spending their days hidden in underwater caves. Their isolated habitat and lack of predators have kept their genome remarkably stable for millions of years. They were once known only from fossils until a living specimen shocked the scientific community in 1938. Another species was later discovered in Asian waters in 1999.
Due to their rarity, obtaining specimens for study was a major challenge. Researchers from USP and the Smithsonian had to secure loans from two U.S. institutions — the Field Museum in Chicago and the Virginia Institute of Marine Science. G. David Johnson, a leading fish anatomist and co-author of the study, was instrumental in this effort before his passing in November 2024 while the paper was under review.
Datovo, who spent six months meticulously separating every muscle and bone of the coelacanth’s skull, emphasized that proper dissection preserves specimens for future research. “Seeing each muscle and nerve firsthand allowed us to identify what was actually in the coelacanth’s head with certainty,” he said. The team found that 11 structures previously described as muscles were in fact ligaments or connective tissue, a discovery that fundamentally changes how scientists interpret the fish’s feeding and breathing mechanisms.
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