A predatory fish that could breathe air existed over 380 million years ago and swam in central Australia’s rivers. The sediments of those rivers have become outcrops of red sandstone in the outback. A new study, published in the “Journal of Vertebrate Paleontology”, introduces the fossils of this fish, which have been named Harajicadectes zhumini. Harajicadectes is the first bony fish from Devonian rocks in central Australia that is reasonably complete and has proved to be an unusual animal, known from at least 17 fossil specimens.

Harajicadectes belongs to the Tetrapodomorpha group, which had strongly built paired fins and typically had only a single pair of external nostrils. The Tetrapodomorph fish from the Devonian period have been of great interest to science for a long time, as they include the forerunners of modern tetrapods such as amphibians, reptiles, birds, and mammals. Recent fossil discoveries have shown that fingers and toes arose in this group. Northwestern and eastern Australia’s Devonian fossil sites have produced many spectacular discoveries of early tetrapodomorphs. The poorly sampled interior of the continent had previously only offered tantalizing fossil fragments, until the Journal’s discovery.

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Palaeontologist Gavin Young from the Australian National University made the initial discoveries in 1973 while exploring the Middle-Late Devonian Harajica Sandstone on Luritja/Arrernte country, more than 150 kilometers west of Alice Springs (Mparntwe). The fossils of hundreds of fish were packed within red sandstone blocks on a remote hilltop, with the vast majority being small Bothriolepis, a type of widespread prehistoric fish known as a placoderm, covered in box-like armor. Among them were fragments of other fishes, including a lungfish known as Harajicadipterus youngi, named in honor of Gavin Young and his years of work on material from Harajica, as well as spines from acanthodians, plates of phyllolepids, and jaw fragments of a previously unknown tetrapodomorph. Many more partial specimens of this Harajica tetrapodomorph were collected in 1991, including some by the late palaeontologist Alex Ritchie. Although early attempts were made to figure out the species, it proved troublesome. Then, the Flinders University expedition in 2016 yielded the first almost complete fossil of this animal, which is now in the collections of the Museum and Art Gallery of the Northern Territory, serving as the type specimen of Harajicadectes.

Harajicadectes, up to 40 centimeters long, is the largest fish found in the Harajica rocks and likely the top predator of those ancient rivers. It had a big mouth lined with closely-packed sharp teeth alongside larger, widely spaced triangular fangs. It appears to have combined anatomical traits from different tetrapodomorph lineages via convergent evolution. The patterns of bones in its skull and scales are an example of this. It’s difficult to determine exactly where it sits among its closest relatives. The most striking and, perhaps, most important features are the two huge openings on the top of the skull called spiracles. These typically only appear as minute slits in most early bony fishes. Similar giant spiracles also appear in Gogonasus, a marine tetrapodomorph from the famous Late Devonian Gogo Formation of Western Australia, but it doesn’t appear to be an immediate relative of Harajicadectes. They are also seen in the unrelated Pickeringius, an early ray-finned fish that was also at Gogo.

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 that they draw surface air through their spiracles to aid survival in oxygen-poor waters. That these structures appeared roughly simultaneously in four Devonian lineages provides a fossil “signal” for scientists attempting to reconstruct atmospheric conditions in the distant past. It could help us uncover the evolution.

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References:
1. Clement, A., Choo, B., & Long, J. (2024, February 6). A 380-million-year old predatory fish from Central Australia is finally named after decades of digging. The Conversation. https://theconversation.com/a-380-million-year-old-predatory-fish-from-central-australia-is-finally-named-after-decades-of-digging-219397
‌2. Ahlberg, P. E. (1991). A re-examination of sarcopterygian interrelationships, with special reference to the Porolepiformes. Zoological Journal of the Linnean Society, 103, 241–287.
3. Ahlberg, P. E. (2018). Follow the footprints and mind the gaps: a new look at the origin of tetrapods. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 109(1–2), 115–137.
4. Ahlberg, P. E., & Johanson, Z. (1997). The second tristichopterid (Sarcopterygii, Osteolepiformes) from the Upper Devonian of Canowindra, New South Wales, Australia. Journal of Vertebrate Paleontology, 17, 653–673.
5. Cloutier, R., Clement, A. M., Lee, M. S. Y., Noël, R., Béchard, I., Roy, V., & Long, J. A. (2020). Elpistostege and the origin of the vertebrate hand. Nature, 579, 549–554.

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