Recent research into the cranial musculature of the African coelacanth (Latimeria chalumnae) has revealed significant inaccuracies in the understanding of its evolutionary adaptations. The study, led by Professor Aléssio Datovo from the Museum of Zoology (MZ) at USP and supported by FAPESP, found that only 13% of previously identified evolutionary muscle novelties in the largest vertebrate lineages were correct. In addition, the researchers identified nine new evolutionary transformations related to feeding and respiration in these vertebrate groups, suggesting a closer relationship between coelacanths and both cartilaginous fish, like sharks and rays, as well as tetrapods including birds, mammals, amphibians, and reptiles.
The study highlighted that many evolutionary novelties previously thought to be present in coelacanths, such as muscles that actively expand the buccopharyngeal cavity, were actually misidentified. These muscles, crucial for food capture and respiration, were proven to be ligaments instead—structures that do not possess the ability to contract. This significant finding challenges earlier assumptions that these features were shared with the common ancestor of bony vertebrates and indicates that such adaptations only appeared at least 30 million years later in the lineage of living ray-finned fish.
The divergence between ray-finned fish (actinopterygii) and lobe-finned fish (sarcopterygii) occurred approximately 420 million years ago. The sarcopterygii group includes coelacanths, lungfish, and all tetrapods, which evolved from a shared aquatic ancestor. In contrast, ray-finned fish, such as aquarium carp, display a mouth movement that allows them to suck in food, providing them a significant evolutionary advantage. This ability has led ray-finned fish to dominate the vertebrate population, comprising about half of all living vertebrates today.
Coelacanths are intriguing creatures that inhabit depths of about 300 meters and often reside in underwater caves. Their minimal changes since the extinction of the dinosaurs can be attributed to their protected environment and low predation rates. A 2013 study published in the journal Nature further confirmed the slow genetic changes in coelacanths. Initially known only from fossils dating back approximately 400 million years, the first living coelacanth was discovered in 1938, surprising the scientific community, with another species identified in 1999.
Due to the rarity of coelacanth specimens, researchers from USP and the Smithsonian Institution's National Museum of Natural History faced challenges in obtaining animals for dissection. Fortunately, the Field Museum in Chicago and the Virginia Institute of Marine Science lent them specimens, enabling crucial insights into this ancient species. Professor Datovo credits G. David Johnson, co-author of the study, for his instrumental role in securing these loans. Johnson, recognized as one of the greatest fish anatomists, unfortunately passed away in November 2024, shortly after the study was submitted for review.
Dissecting a specimen does not equate to destruction, as emphasized by Datovo. Over six months, he meticulously separated the muscles and skull bones of the coelacanth, preserving them for future studies. This approach allowed researchers to accurately identify previously undocumented structures and correct misconceptions that had persisted in scientific literature for over 70 years. Notably, the study found that 11 structures previously classified as muscles were, in fact, ligaments or other connective tissues, significantly impacting our understanding of the mouth's functionality and breathing in coelacanths.
The findings from this research have broader implications for our understanding of cranial evolution across large vertebrate groups. By utilizing three-dimensional microtomography images of skulls from both extinct and extant species, Datovo and Johnson were able to infer the evolutionary progression of muscles in early jawed vertebrates. Future research will aim to explore the similarities between coelacanth muscles and those of tetrapods, such as amphibians and reptiles, further illuminating the complex evolutionary history of vertebrates.
