An article published in the journal “Science Advances” reports the results of a thorough examination of the cranial musculature of the African coelacanth (Latimeria chalumnae) (Bottom photo ©Alberto Fernandez Fernandez), a so-called living fossil because it has changed very little over tens of millions of years. Professor Aléssio Datovo of the University of São Paulo, Brazil, and G. David Johnson of the Smithsonian Institution dissected two specimens of this fish to gain new insights into the evolution of gnathostomes, a chordate infraphylum that includes jawed vertebrates. As a result, the two researchers discovered that much anatomical information about coelacanths was incorrect.
The top image (Courtesy Aléssio Datovo, G. David Johnson / Nature. All rights reserved) illustrates the musculoskeletal system of the African coelacanth (Latimeria chalumnae).
The coelacanth is a focus of scientific interest because similar fish date back to the early Devonian period, which began approximately 410 million years ago. The problem with anatomical studies of coelacanths is that in-depth examinations of fossils are only possible today thanks to modern technology, and these fish are rare today, making it difficult to have a chance to dissect a specimen.
Some museums possess specimens of the African coelacanth, one of two extant coelacanth species, but are understandably reluctant to provide them to researchers for invasive examination. Professor Aléssio Datovo explained that proper dissection doesn’t destroy the specimen. This fact was likely crucial in securing two specimens on loan from the Field Museum in Chicago and the Virginia Institute of Marine Science.
This isn’t the first time a study has been conducted on coelacanth skulls: for example, an article published in the journal “Nature” in April 2019 focused on their reconstruction and these fish’s brain. Aléssio Datovo and G. David Johnson focused on their musculature.
The two researchers realized that previous studies contained several errors in the anatomical descriptions of coelacanths. Generally, researchers assumed that certain muscles that enable the mouth moves to suck in food had evolved in an ancestor of coelacanths and that these fish also possessed them. This new study demonstrated that eleven muscles attributed to coelacanths don’t actually exist, and what was described were actually ligaments. Instead, they found three previously unseen muscle subdivisions and connections.
The information gathered was used to compare the anatomy of coelacanths with that of other groups of fish, both extinct and extant. This provided new insights into the evolution of cranial muscles in early gnathostomes.
The two researchers created 3D images of the skulls of many species using a microtomographic technique similar to the CT scan used in medicine. These images may also be useful for future studies of fish and vertebrate evolution.
Sadly, G. David Johnson passed away in November 2024, but Aléssio Datovo intends to continue the studies he began with his colleague, expanding them to include tetrapods such as amphibians and reptiles. The goal is to analyze the similarities of cranial muscles to reconstruct their evolution across various vertebrate groups.
