Ancient lungfish fossils have provided a missing link in the evolutionary chain that led to the rise of land-dwelling, four-legged creatures on Earth, allowing scientists to trace the development of the animal brain and nervous system back more than 400 million years.

An international team lead by researchers at Australia’s Flinders University has created 3D cranial endocasts of six Paleozoic lungfish (Dipnoi) fossils and compared them to the brain spaces of the living sister group of terrestrial vertebrates to better understand the evolution of lungfish brains.

According to lead author Dr. Alice Clement of Flinders University, this in turn can help interpret the earliest tetrapods, which later transitioned from the ocean to land on four legs.

This finding, published in the peer-reviewed international journal eLife, sheds light on the evolutionary trajectory of lobe-finned fish (Sarcopterygii) by demonstrating how the olfactory region seems to to be more highly plastic than the hindbrain, and develops significant elongation in a several taxa.

The findings of the study indicate, according to Dr. Clement, “that the brains of lungfish have been evolving constantly throughout their 400-million-year history, but it suggests they have likely always relied on their sense of smell rather than vision to navigate their environments. This is quite unlike other fish which use sight much more powerfully.”

According to her, knowing how the brains of lungfish have evolved over time can help us visualize what the first tetrapods, or our land-based ancestors, would have looked like. This can help us determine which senses were more important than others (such as vision vs. olfaction).

The Australian researchers, together with co-authors from the UK, Canada, and Sweden, used powerful imaging techniques to virtually rebuild these brain models for this study.

The ongoing research, according to senior author Dr. Tom Challands of the University of Edinburgh in Scotland, is important for the field of general evolutionary and palaeontological study.

“This paper effectively doubles the number of lungfish endocasts known,” he adds, “as their preservation quality is often damaged by a fossil being crushed or broken, and the brain itself has very poor preservation potential and is not currently known in any fossil lungfish.”

From the Devonian Period to the present, lungfish have survived for more than 400 million years. They offer special insights on the state of the first tetrapods as well as their own evolutionary history.

The cranial endocasts of six Palaeozoic lungfish (Iowadipterus halli, Gogodipterus paddyensis, Pillararhynchus longi, Griphognathus whitei, Orlovichthys limnatis, and Rhinodipterus ulrichi) could be investigated non-destructively using X-ray tomography as a palaeontological method. The fossils are from Germany, Russia, Australia, and the United States.

Six fossil taxa and two living taxa were used in a multivariate morphometric analysis with 17 different variables.

Lungfish research continues to advance our knowledge of how fish first emerged from the water 350 million years ago and began to evolve into tetrapods, which subsequently gave rise to humans. Perhaps some of their neurological system characteristics are still present in us, speculates Dr. Clements.

Image Credit: Courtesy A Clement, Flinders University

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