380 million year old fossil fish breathed through holes on top of head

An ancient, fossilised fish that probably breathed through holes on the top of its head has been discovered in remote Australia.

The species was sleek and predatory with large fangs and bony scales and would have grown to about 40 cm, according to a paper describing the finding in the Journal of Vertebrate Paleontology.

“We found this new form of lobe-finned fish in one of the most remote fossil sites in all of Australia, the Harajica Sandstone Member in the Northern Territory, almost 200km west of Alice Springs, dating from the Middle-Late Devonian roughly 380 million years old,” says co-author John Long, a leading expert in fossil fish from Flinders University, Australia.

The species has been named Harajicadectes zhumini after its location of discovery and the ancient Greek word dēktēs which means “biter”. It’s name also pays homage to Professor Min Zhu of the Chinese Academy of Sciences, who has contributed significantly to the research of early backboned animals (vertebrates).

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The type specimen of Harajicadectes as found in the field in 2016 (an almost complete fish seen in dorsal view), a latex peel of the fossil and an interpretative diagram. Credit: Brian Choo, Flinders University

Despite sounding bizarre by modern fish standards the large openings on the top of its skull, known as spiracles, aren’t actually that unheard of in fish.

“These spiracular structures are thought to facilitate surface air-breathing, with modern-day African bichir fish having similar structures for taking in air at the water’s surface,” says first author Dr Brian Choo of the Flinders Palaeontology Lab.

“This feature appears in multiple Tetrapomodorph lineages at about the same time during the Middle-Late Devonian.”

Tetrapodomorphs are a clade of vertebrate animals made up of the limbed tetrapods – animals with four limbs – and their closest fish relatives.

Photograph of the fossilised ancient fish in a large, reddish rock
Fossilised Harajicadectes zhumini. Credit: Flinders University

“In addition to Harajicadectes from central Australia, large spiracles also appeared in Gogonasus from Western Australia and elpistostegalians like Tiktaalik (the closest relatives to limbed tetrapods). It also appears in the unrelated Pickeringius a ray-finned fish from Western Australia, first described in 2018,” adds Choo.

According to Long, the synchronised appearance of this air-breathing adaptation may have coincided with a time of decreased atmospheric oxygen during the mid-Devonian.

“The ability to supplement gill respiration with aerial oxygen likely afforded an adaptive advantage,” he says.

It is difficult to pinpoint exactly where Harajicadectes sits in this group of fish, as it appears to have acquired a variety of specialised features found in widely separate branches of Tetropodomorpha.

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