Tetrapodomorpha

• More than 380 million years ago, a sleek, air-breathing predatory fish patrolled the rivers of central Australia.
• Known from at least 17 fossil specimens, Harajicadectes is the first reasonably complete bony fish found from Devonian rocks in central Australia.

Meet the biter

• This group had strongly built paired fins and usually only a single pair of external nostrils.
• Tetrapodomorph fish from the Devonian period (359–419 million years ago) have long been of great interest to science.
• They include the forerunners of modern tetrapods – animals with backbones and limbs such as amphibians, reptiles, birds and mammals.

A long road to discovery

• Packed within red sandstone blocks on a remote hilltop were hundreds of fossil fishes.
• The vast majority of them were small Bothriolepis – a type of widespread prehistoric fish known as a placoderm, covered in box-like armour.
• These included a lungfish known as Harajicadipterus youngi, named in honour of Gavin Young and his years of work on material from Harajica.

• There were early attempts at figuring out the species, but this proved troublesome.
• Then, our Flinders University expedition to the site in 2016 yielded the first almost complete fossil of this animal.

A strange apex predator

• Likely the top predator of those ancient rivers, its big mouth was lined with closely-packed sharp teeth alongside larger, widely spaced triangular fangs.
• It seems to have combined anatomical traits from different tetrapodomorph lineages via convergent evolution (when different creatures evolve similar features independently).

• Similar giant spiracles also appear in Gogonasus, a marine tetrapodomorph from the famous Late Devonian Gogo Formation of Western Australia.
• They are also seen in the unrelated Pickeringius, an early ray-finned fish that was also at Gogo.

The earliest air-breathers?

Other Devonian animals that sported such spiracles were the famous elpistostegalians – freshwater tetrapodomorphs from the Northern Hemisphere such as Elpistostege and Tiktaalik. These animals were extremely close to the ancestry of limbed vertebrates. So, enlarged spiracles seem to have arisen independently in at least four separate lineages of Devonian fishes.

• The only living fishes with similar structures are bichirs, African ray-finned fishes that live in shallow floodplains and estuaries.
• It was recently confirmed they draw surface air through their spiracles to aid survival in oxygen-poor waters.

Brian Choo receives funding from the Australian Research Council and is employed by Flinders University. Alice Clement receives funding from the Australian Research Council and is employed by Flinders University. John Long receives funding from The Australian Research Council.