Australian-led research has provided new evidence about how the first fish to migrate to the land might have made the leap.
Flinders University led the new investigation into a 330 million-year-old fish fossil found in Queensland.
The fish, a Cladarosymblema narrienense, was an ancestor of the first land animals.
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Cladarosymblema is a type of 'megalichthyid' fish, a group which existed from the Devonian-to-Permian periods, typically living in freshwater environments, and they were large, predatory animals.
Through sophisticated CT scanning of the fossil, researchers found evidence it had a brain similar to its eventual terrestrial descendants, compared to the brains of other fishes which remained living in water.
"This fish from Queensland is one of the best preserved of its kind in the entire world, in perfect 3D shape, which is why we chose to work on it," John Long, Strategic Professor in Palaeontology at Flinders University, said.
While this fish was first described in 1995, by Professor Long and others who had earlier explored and excavated the Queensland fossil site, parts of its anatomy have remained unknown.
But some of those hidden secrets have been revealed by Australia's largest CT scanner, now at Flinders University.
New information obtained from often unseen internal bones has been revealed by the new scans – particularly in the gill arch skeleton, the shoulder girdle and the palate bones (the upper mouth roof area).
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"Additionally, a cranial endocast (mould of the internal cavity of this fish's unusually large skull) gives clues as to the shape of the brain of this animal," says Dr Alice Clement, lead author of the new paper and part of the Flinders Palaeontology Group.
"The area for the pituitary gland (so-called the 'master gland') is relatively large, suggesting a significant role in regulating various important endocrine glands."
The research has been published in PeerJ.