380 million year old fish with teeth found in one of the oldest lakes in the world

One of the most remote fossil sites in all of Australia just yielded a whopper of a new species: a predatory lobe-finned fish that was armed with large canines and bony scales. It lived 380 million years ago at a time when the Middle Devonian had plunged the planet into a period of reduced atmospheric oxygen, which could also explain why this remarkable fish could both breathe air and use gill breathing.

The species, new to science, was found in Alice Spring’s Finke River (Larapinta), considered one of the oldest rivers in the world. Although it is one of the more difficult fossil sites on the continent, it has already been proven that it was once home to a host of bizarre ancient animals, and now another has joined the team.

Called Harajicadectes zhuminithe predatory fish has been described by an international team of researchers led by palaeontologist Dr Brian Choo of Flinders University. The genus name is derived from where the crucial fossils were found in the Harajica Sandstone Member, and the Ancient Greek dēktēs (biter) as a tip of the hat to the ancient predator’s intimidating dental anatomy.

A nearly complete specimen of Harajicadectes as found in the field in 2016.

Image credit: Dr Brian Choo, Flinders University

“We found this new form of lobe-finned fish at one of the most remote fossil sites in all of Australia, the Harajica Sandstone Member in the Northern Territory, almost 200km away. [124 miles] west of Alice Springs, dating to the Middle Late Devonian, approximately 380 million years old,” study co-author and Flinders Professor John Long, a leading Australian expert on fossil fish, said in a statement.

The species name is in honor of Professor Min Zhu of the Chinese Academy of Sciences, Beijing, who is credited by the institution with having made “unique and outstanding contributions to the studies on the morphology, histology, phylogeny, biogeography and evolutionary history of many early vertebrate groups”.

Beyond its fearsome fangs and armored scales, Harajicadas is notable for its unusual combination of breathing equipment. Not content with the gill breathing we typically associate with fish, it was also revealed that it had large openings at the top of its skull through which it would also have taken air breaths.

The spiracle anatomy of Harajicadectes

The spiracle anatomy of Harajicadectes.

Image credit: Dr Brian Choo, Flinders University

“These spiral structures are thought to facilitate breathing of air at the surface, while modern African bichir fish have similar structures for taking in air at the water’s surface,” Choo explains. “This feature appears in multiple Tetrapomodorph lineages at approximately the same time during the Middle-Late Devonian.

“In addition to Harajicadas large spiracles also appeared from central Australia Gogonasus from Western Australia and elpistostegalians such as Tiktaalik (the closest relatives of tetrapods with limbs). Moreover, it also appears in the unrelated Pickeringius, a ray-finned fish from Western Australia, first described in 2018.”

It is thought that a dip in atmospheric oxygen during the Middle Devonian could explain why some fish evolved to supplement gill breathing with air breathing. That several fish from widely separated branches of tetrapodomorph fishes exhibit this double-breathing trait is therefore an example of convergent evolution, in which distantly related species independently develop the same adaptations.

It makes it difficult to determine exactly where this new species is in the ichthyological Tree of Life, but a great find nonetheless.

The study was published in the Journal of Vertebrate Paleontology.

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