'Living fossil' reveals clues about how animals learned to breathe

The African coelacanth, Latimeria chalumnae, earned its “living fossil” nickname because its skeleton seems almost identical to fossils over 65 million years old. Scientists have pored over this deep‑sea relic for decades, assuming its anatomy was already well documented.

A new study, however, shows that long‑standing descriptions of the fish’s head muscles were riddled with errors.

The research suggests that correcting those mistakes reshapes how biologists think jawed vertebrates learned to bite, breathe, and swallow.

Old anatomy mostly mistaken

Re‑examining two rare museum specimens, researchers from the University of São Paulo (USP) and the Smithsonian Institution discovered that just 13 percent of the previously identified evolutionary muscle novelties for the largest vertebrate lineages were correct.

What earlier anatomists called muscles in the buccopharyngeal cavity – tissue once thought to expand the mouth to suck in prey – turned out to be ligaments. Unlike muscles, ligaments are structures incapable of contraction.

Lead author Aléssio Datovo, a professor at USP’s Museum of Zoology, said the corrected map makes the coelacanth “even more similar to cartilaginous fish [sharks, rays, and chimaeras] and tetrapods [birds, mammals, amphibians, and reptiles] than previously thought.”

Suction feeding evolved later

Bony fish split into two grand lineages roughly 420 million years ago: the ray‑fins (actinopterygians) and the lobe‑fins (sarcopterygians).

Modern ray-fins, from pet-store goldfish to tuna, use powerful mouth muscles to generate suction. This skill helped them diversify into half of all living vertebrate species. Coelacanths, lungfish, and most sharks rely on biting.

“In previous studies, it was 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, which we now show isn’t true,” Datovo explained.

“This only appeared at least 30 million years later, in the common ancestor of living ray‑finned fish.”

Securing rare deep-sea fish

Coelacanths lurk in submarine caves about 300 meters (about 984 feet) below the ocean surface. Since the first living specimen stunned scientists in 1938, fewer than a hundred have reached museum collections.

Datovo and Smithsonian co‑author G. David Johnson spent years persuading institutions to lend material. The Field Museum in Chicago and the Virginia Institute of Marine Science each entrusted them with a fish.

“Dissecting a specimen does not mean destroying it as long as it’s done properly,” Datovo noted.

Over six painstaking months, he teased apart every muscle, bone, and nerve, preserving each element so future researchers can study it individually.

Fixing 70 years of errors

“There were many contradictions in the literature,” Datovo recalled. “We detected more errors than we’d imagined. For example, 11 structures described as muscles were actually ligaments or other types of connective tissue.”

Because muscles create motion and ligaments merely transmit it, mistaking one for the other had inflated the coelacanth’s supposed ability to expand its throat for suction.

The team also identified nine completely new evolutionary transformations in cranial muscles tied to feeding and respiration across early vertebrates.

Rebuilding ancient muscle maps

With fresh data in place, the researchers turned to micro‑CT scans posted by other labs. The team used these 3D images to slide coelacanth muscle attachments onto the skulls of extinct fishes whose soft tissues are long gone.

By slotting the correct muscles into place, they traced how jaw and throat mechanics evolved in the first jawed vertebrates.

Because coelacanths sit near the base of the lobe‑fin line that gave rise to all amphibians, reptiles, birds, and mammals, the revisions ripple through the entire vertebrate family tree.

Honoring a fish anatomist

The paper doubles as an homage to Johnson, described by Datovo as “probably the greatest fish anatomist of his time.”

Johnson was born in 1945 and secured the loan of the rare specimens. He died in November 2024 while the manuscript was under review.

His meticulous approach to comparative anatomy shaped the project and ensured that each corrected label carries lasting authority.

Coelacanth holds modern insights

Coelacanths have diverged little since dinosaurs went extinct, partly because they face few predators in their deep‑reef refuges. That stability, confirmed by a 2013 genomic study, creates a living window onto ancient body plans.

Updating the blueprint clarifies which innovations fueled the explosive diversification of ray‑finned fish and which traits lobe‑fins passed on to terrestrial tetrapods.

Datovo’s team now plans to compare the revised coelacanth anatomy with muscles in amphibians and reptiles. They’re looking for shared features that might push the story closer to the water-to-land transition.

By peeling away decades of assumptions, the study reminds scientists that even icons of evolutionary stasis can still rewrite textbooks – provided someone is willing to pick up a scalpel and look again.

The study is published in the journal Science Advances.

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