May 25, 2019 | Updated: 09:32 AM EDT

New Insights into Coelacanth and the Evolution of the Vertebrate Skull

Apr 18, 2019 11:40 AM EDT

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New Insights into Coelacanth and the Evolution of the Vertebrate Skull
(Photo : Laurent Ballesta, Gombessa expeditions, Andromede Oceanology Lt.)

A new study by a team of international researchers presents the original observations of the development of the skull and brain in the living coelacanth Latimeria chalumnae. They published their research in Nature, and it offers new insights into the biology of this iconic animal and the evolution of the vertebrate skull.

As a marine fish, the coelacanth Latimeria has a connection with tetrapods, four-limbed vertebrates including mammals, amphibians, and reptiles. The thought on coelacanths was that they have been extinct for 70 million years until the accidental capture of a living specimen by a South African fisherman in 1938. About eighty years after its discovery, Latimeria remains of scientific interest for understanding the origin of tetrapods and the evolution of other closest fossil relatives, the lobe-finned fishes.

The hinged braincase is one of the unique features of Latimeria, which is otherwise only found in many fossil lobe-finned fishes from the Devonian period about 410 - 360 million years ago. An anterior and interior portion completely split the braincase of Latimeria by a joint called the 'Intracranial joint.' Also, the brain lies far at the rear of the braincase and takes up only one percent of the cavity housing it.

This mismatch between the brain and its cavity is utterly unequaled among the living vertebrates. For years, scientists are puzzled how the coelacanth skulls grow and why the brain remains so small. To get answers to their questions, scientists studied specimens at different stages of cranial development from several public natural history collections.

Even when there is the availability of several specimens of adult coelacanths in natural history collections, earlier life stages such as fetuses are extremely rare. Hence, scientists used state-of-the-art imaging technology to visualize the internal anatomy of the specimens without damaging them.

In completion, they digitalized a 5cm-long fetus, the earliest developmental stage available for Latimeria, with synchrotron X-ray microtomography at the European Synchrotron (ESRF). For more than two decades, the ESRF has developed unique expertise in designing non-invasive techniques widely used for evolutionary biology studies.

Also using a powerful Magnetic Resonance Imaging (MRI) scanner at the Brain and Spine Institute, Paris, France, the researchers were able to image other stages and a conventional X-ray micro-CT scan at the Museum national d0Histoire naturelle, Paris, France. The scientists used these data to generate detailed 3D models that allowed them to describe how the form of the skull, the brain and the notochord, a tube-extending below the spinal cord and the brain in the early stages of life changes from a fetus to an adult.

The lead author and the research associate in palaeobiology at the University of Bristol, UK, Hugo Dutel said that the observations are unique, but they represent only a tiny step forward compared to the amount they know on the development of other species. More questions still beg for answers. Still, there are still many clues on Latimeria for understating of the evolution of the vertebrate, and it is essential to protect this threatened species and its environment.

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