Brains, brains, brains! (New paper out today)

Our latest paper was published yesterday in the journal Royal Society Open Science. This work was co-authored by myself with Tom Challands (University of Edinburgh) and Jason Pardo (University of Calgary), and has the title: “Mandibular musculature constrains brain–endocast disparity between sarcopterygians.”

In this work we applied a method developed in a previous paper to quantify and visualise the spatial relationship between the brain and its surrounding braincase in several animals. We investigated the living lobe-finned fishes (the coelacanth, Australian and African lungfish), as well as two salamanders (the axolotl and a newt).

Why did we do this? Firstly, brains are soft organs and so rarely fossilise and instead we are often only left with the hard, bony remains of an animal. This means that we must rely on the shape of the internal mould of the skull (an ‘endocast’) as a proxy for brain size and shape.

However, while birds and mammals typically have brains that nearly completely fill their brain cavities, those in fish, amphibians and reptiles are usually not so tightly constrained. This means that when we try to interpret the endocasts of fossils of extinct fish and early tetrapods (backboned animals with four limbs bearing fingers and toes) we don’t know with how much confidence we can infer how large certain brain regions were.

We found that the coelacanth is doing something very strange and has an absolutely tiny brain contained within a huge braincase (similar findings have recently been reported elsewhere). However, the lungfish and salamanders have brains that are filling their braincases to a higher degree than previously thought. We suggest that these patterns may potentially be related to the reinforcement in the skull from large jaw muscles, but we do need to examine some other basal tetrapods, such as frogs and caecilians, before we can be completely sure.

Left figures show overlap relationship between brain (grey) and endocast (red).
Right side images show the same information using a “heat-map” approach.
a-c, axolotl; d-f, newt; g-l, African lungfish; m-o, Australian lungfish; p-r, coelacanth.
(Silhouettes from http://phylopic.org/)

By investigating the brain-braincase spatial relationship in these living animals spanning the fish-tetrapod transition, we can use this information to better interpret fossils spanning the same critical juncture.

This approach is guided by something called the Principle of Proper Mass (Jerison 1973) which proposes that the larger a brain region, the more likely it is to be processing more information (remember that growing and maintaining large brains is energetically costly). Thus, if an animal has particularly large optic lobes compared to the olfactory region for example, we can reasonably assume that that animal relied more on its vision that its sense of smell.

It is my hope that this work will help us to pinpoint the origins of new behaviours as fish moved out of the water and began to colonise land over 385 million years ago.

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