What did the brains of the first land vertebrates look like?
By Mischa Dijkstra, Frontiers science writer / Dr Alice M Clement, Flinders University
What did the brain of the early tetrapodomorphs, the first fish to develop limbs and walk on land, look like? Preserved brains are very rare in fossils, and even when preserved they were typically shrunken and deformed before becoming fossilized.
For this reason, researchers mostly rely on casts of the cranial vault of fossils to study the early evolution of the brain. The closest living relatives of tetrapodomorphs, coelacanths, are known to have brains that are tiny compared to the braincase (1% of volume), so for them endocasts don’t give much information about brain morphology. But are coelacanths representative of extinct tetrapodomorphs in that regard? In a new study in Frontiers in Ecology and Evolution, Clement et al. show that this is likely not the case: among living amphibia – together with lungfish, the next closest living relatives of tetrapodomorphs – 4 basal species of frogs and caecilians have brains with a volume of 49-78% of the braincase.
Their brains are somewhat larger relative to the braincase than those of lungfish, newts, and salamanders (38-47%), and the authors suggest that the evolution of larger brains may have been promoted by the specialized ecology of frogs, toads, and caecilians as burrowers, jumpers, and vocalizers. The new results help to set the minimum (considerably larger than the 1% of coelacanths) and maximum estimate for the relative brain size of extinct tetrapodomorphs.
The study’s first author, Dr Alice M Clement, is a paleontologist who obtained her PhD in 2012 from the Australian National University in Canberra. Subsequently, she first worked as a researcher at the University of Uppsala, and then moved to Flinders University, where she is currently a postdoctoral research associate. She is the leader of VAMP, an initiative to set up a Virtual Australian Museum of Paleontology, program secretary of the Royal Society of South Australia, and an advocate for Women in STEM. Here, we interview her about her career trajectory, her current research, and her views on open access.
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What inspired you to become a researcher? Do you have any specific memories that set off a spark?
I’d always loved animals and being outdoors, but didn’t consider myself good at science while I was at high school. It wasn’t until I had one particular teacher who was very encouraging and really inspired me to consider doing science in my final years of school and later at university. It was then that I realized that I truly loved it!
Can you tell us about the research you’re currently working on?
I am a paleontologist most interested in fish and the first ’tetrapods’ – that is, terrestrial animals with a backbone, and 4 limbs bearing fingers and toes. This group includes all amphibians, reptiles, birds and mammals today and comprises half of all vertebrate diversity.
I’m interested to see how their bodies changed over the major ecological transition from water onto land. This transition occurred more than 350 million years ago in a time period called the Devonian – when the world looked very different to what it does today.
In particular, I work in a discipline known as paleoneurology, or the science of fossil brains. I want to understand how the brains changed between fish living in water and the tetrapods who first forayed onto land. I came to this field of research via a fortuitous find.
I was working on fossil lungfish (the closest living fishy relatives to the first tetrapods) and happened to be describing a new species of exquisitely preserved specimen from the ancient Devonian reef known as the Gogo Formation in Australia. More and more, paleontologists are using modern scanning techniques, such as CT-scanning, to examine their specimens. It allows us to create digital 3D models without damage to the fossil, but also to look inside structures that are usually obscured.
From my scan data, I noticed that the internal part of the skull, the braincase, was well ossified (bony), and thus I could create a mold of the hollow space via 3D modelling on the computer. This is something known as an ’endocast’ and these are used in studies of brain evolution. It wasn’t until this point when I went to check the literature that I realized how little is known about the brains and endocasts of lungfish, or indeed, most animals (with the exception of primates and humans). And so began my paleoneurological journey.
I’m trying to trace the changes in the brains over time in the lungfishes, as well as their closely related ’tetrapod-like’ fishes that first gave rise to our earliest terrestrial ancestors. I use 2 approaches to try and answer this question: firstly, I scan and model perfectly 3D-preserved fossils from the Devonian Period (the ’Age of Fishes’, 359-419 million years ago); and I also look at the brains in animals living today to better interpret the fossils I study.
It is important to understand that the brains of many animals don’t completely fill their brain cavities and so I’m trying to tease out the specific spatial relationships between the 2. This goes in part to trying to understand our very own evolutionary neural history.
In your opinion, why is your research important?
The geological record is a pertinent and constant reminder that we are only temporary inhabitants of this Earth. We can not fully understand the diversity and breadth of life on this planet today, without understanding its evolutionary history and knowing what came before. Paleontology is vital to help us care for the planet at present, to try to prepare for its future, and appreciate our place in this world.
Are there any common misconceptions about this area of research? How would you address them?
Many paleoneurologists have dismissed the study of fish (and some other animals) as having small and uninteresting brains, and that the study of their endocasts is not informative. Obviously I disagree and believe that we simply need to take a more nuanced approach to understand early neural diversity in these animals.
By studying the specific spatial relationship between the brain and endocasts in these animals, I expect that they will yield exciting new insights into the early brain evolution of our distant tetrapod ancestors from hundreds of millions of years ago. I believe that it was some of these changes that led to the great success of the tetrapods.
How has open science benefited the reach and impact of your research?
I think open science is so important to reach the widest audience possible. Scientific literacy isn’t just for academics, but so important for everyone in society. To comprehend science is to understand and appreciate the world around you, and I don’t think there is anything more universal than that.
Read Dr Clement’s study in Frontiers in Ecology and Evolution: Brain Reconstruction Across the Fish-Tetrapod Transition: Insights From Modern Amphibians
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