What if one of the most iconic “living fossils” has been misleading evolutionary biology for decades? The African coelacanth, for a long time considered a key to the distant past, is now in the middle of a scientific storm that upends hypotheses about the origin and diversification of vertebrate skull muscles.
A broad anatomical reconsideration, performed by University of São Paulo’s Aléssio Datovo and published in Science Advances, revealed that only 13% of the previously identified evolutionary muscle novelties for major vertebrate lineages in coelacanths were accurate. Textbooks of anatomy and evolution models have perpetuated mistakes for over seven decades mainly the misidentification of putative cranial muscles in coelacanths that were actually ligaments. There were many contradictions in the literature. “When we finally got to examine the specimens, we detected more errors than we’d imagined. For example, 11 structures described as muscles were actually ligaments or other types of connective tissue. This has a drastic consequence for the functioning of the mouth and breathing, because muscles perform movement, while ligaments only transmit it,” Datovo explained.
This discovery upends the understanding of feeding and breathing mechanics in ancient vertebrates. Muscles previously thought to enable active enlargement of the buccopharyngeal cavity necessary for suction feeding are absent in coelacanths. Instead, these features are passive ligaments that cannot produce the intense suction seen in modern ray-finned fishes. “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. This only appeared at least 30 million years later, in the common ancestor of living ray-finned fish,” Datovo explained.
The timing of suction-feeding musculature’s evolutionary origin now appears much later, specific to the actinopterygian lineage. This is no minor realignment: the emergence of specialized suction feeding musculature in ray-finned fishes occurred at least 30 million years after the divergence from their lobe-finned relatives, which conferred a significant adaptive benefit and likely helped drive their explosive radiation. In contrast, biting is a primary mechanism of feeding in sharks and coelacanths, and this points to an underlying biomechanical dichotomy within these groups.
The meticulous study design six months of dissection and saving of all muscles and skull bones allowed the scientists to correct errors that had been perpetuated for centuries. The state of availability of coelacanth specimens was so rare that it would be possible only because of collaboration among major institutions, such as the Field Museum and the Virginia Institute of Marine Science. Due to this, the anatomical material is now conserved for future scientists, reducing further destructive sampling.
Crucially, the new data show that the cranial musculature of coelacanths is more similar to that of cartilaginous fishes and tetrapods than to ray-finned fishes. “Ultimately, it’s 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,” Datovo said. This is supported by comparative analysis using three-dimensional microtomography, a high-resolution, non-invasive imaging technique for musculoskeletal morphology in living and fossil animals. Micro-CT technology, capable of defining fine anatomy in infrequently occurring or brittle specimens, is now part of mainstream comparative morphology to enable accurate quantification of muscle fiber numbers, attachment areas, and bone shapes key requirements for functional and evolutionary modeling.
The findings extend far beyond the coelacanth. Because of its pivotal phylogenetic placement, solving these anatomical myths informs jawed vertebrates’ cranial muscle evolution reconstruction. The study found nine new evolutionary changes in feeding and respiratory musculature, providing a revised framework on how major vertebrate innovations arose. For instance, the division of the levator palatoquadrati muscle into the dilatator operculi and levator arcus palatini is currently regarded as an evolutionary innovation occurring in crown actinopterygians alone but not in their ancestors or in coelacanths. Anatomical innovation enables the high-performance suction feeding observed in most recent ray-finned fishes where axial musculature, not cranial muscles, supplies most of the power for buccal expansion.
This paradigm has been reshaping the emphasis of the field on vertebrate functional morphology. Rather than looking at the cranial apparatus as a sole independent force in feeding mechanics, researchers are now bringing in the role of axial musculature and skeletal elements to feeding strategy evolution. The marriage of micro-CT imaging with traditional dissection and comparative anatomy is providing unprecedented vistas, allowing researchers to scrutinize long-held evolutionary suppositions with new eyes.
The coelacanth, a once-fossilized symbol of evolutionary stasis, has emerged as a living source of scientific reformation, compelling scientists to consider again the origin of one of the most fundamental vertebrate mechanisms: the skull and its muscles.
