A study of coelacanth skulls offers clues to the evolution of vertebrates

An article published in the journal “Nature” reports the results of a study of the skull of Latimeria chalumnae (Photo ©Alberto Fernandez Fernandez), one of the two species of coelacanth existing today. A team of researchers led by Hugo Dutel of the British University of Bristol subjected several specimens to very sophisticated tests to create reconstructions of their skulls and brains from the fetal state to the adult state. The aim is to study these fish considered a living fossil to better understand the evolution of the skull in vertebrates and also the reason why the coelacanth brain occupies only 1% of the cranial cavity.

Coelacanths – order Coelacanthiformes – are among the oldest jawed vertebrates. The remains found of fish similar to them date back to the beginning of the Devonian period, which started about 410 million years ago. They were considered extinct but a species was discovered in 1938 in the Mozambique channel, which was given the scientific name Latimeria chalumnae while it’s commonly known as the West Indian Ocean coelacanth or African coelacanth. In 1999 another species was discovered near the coasts of Indonesia whose scientific name is Latimeria menadoensis while it’s commonly known as Indonesian coelacanth.

The fact of being a species that had minimal anatomical changes over the course of 400 million years, a so-called living fossil, made the coelacanth an interesting research subject despite the difficulty in finding specimens to study. In April 2013 the DNA sequencing of the Latimeria chalumnae was announced but the studies also concern its anatomy.

Among the coelacanth’s anatomical features There’s the intracranial joint, which splits the skull into an anterior and posterior parts. It’s the only living vertebrate with this characteristic and the mechanisms that led to anatomical changes in the skull of tetrapods with the fusion of the two halves of the skull are still poorly understood. That’s why various specimens from various collections were studied in depth using modern technologies.

A 5 cm long coelacanth fetus, the earliest stage of development available, was examined using an X-ray microtomography at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Other specimens in other development stages up to the adults were examined using versions of Magnetic Resonance Imaging (MRI) and micro-CT scans that allowed to reconstruct 3D models of their skulls and their brains, including the notochord, an embryonic tubular structure that in vertebrates forms the spinal cord.

The image (courtesy Dutel et al / The Conversation) shows the 3D reconstructions of various organs of Latimeria chalumnae specimens at different development stages including the skull in blue and their brain in yellow.

These examinations and the 3D models obtained allowed to observe the growth of the various anatomical structures and therefore to compare that process with the analogous what occurs in other vertebrates. In particular, in the coelacanth the notochord isn’t developed into the spinal cord but greatly expands to reach a size 50 times greater than the brain. According to the researchers, that’s a key process in the formation of the intracranial joint.

The remarkable development of the notochord could also influence the brain, determining its size really tiny compared to those of the joint and the skull. A similar phenomenon was discovered in some living fish and fossils but never at those levels. On the contrary, for example in primates the brain grows filling the whole skull.

Hugo Dutel pointed out that the observations made by his team are only a small step forward compared to what we know about the development of other species. There’s still a lot to be discovered about the evolution of vertebrates and for this reason it’s also important to protect the environment in which coelacants live as they survived for 400 million years but today are among the threatened species.

A speciman of Latimeria Chalumnae, a modern Coelacanth
A speciman of Latimeria Chalumnae, a modern Coelacanth

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