Discovery of Harajicadectes zhumini: Unraveling the Enigmatic Past of Central Australian Rivers

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The artist's reenactment of the Harajicadectes, which threatens a pair of armored Bothriolepis. (Brian Choo)

Over 380 million years ago, a sleek, air-breathing predatory fish roamed the rivers of central Australia. Today, the remnants of those waterways are visible as red sandstone formations in the remote outback.

A recent publication in the Journal of Vertebrate Paleontology unveils the fossils of this remarkable fish, now named Harajicadectes zhumini.

Harajicadectes, identified from at least 17 fossil specimens, represents the first relatively complete bony fish discovered in central Australia’s Devonian rock formations. Its characteristics mark it as an extraordinary creature.

Introducing the “Harajica-biter”

The name “Harajicadectes zhumini” pays homage to its discovery site, presumed predatory behavior, and the esteemed Chinese paleontologist Min Zhu, known for significant contributions to early vertebrate research.

Belonging to the Tetrapodomorpha group, Harajicadectes featured robust paired fins and typically a lone pair of external nostrils.

Tetrapodomorph fishes from the Devonian era (359–419 million years ago) have long captivated scientific interest, representing the predecessors of modern tetrapods – vertebrates with limbs and backbones, including amphibians, reptiles, birds, and mammals.

For instance, recent fossil findings have shed light on the emergence of digits in this group.

While northwestern and eastern Australian Devonian fossil sites have yielded remarkable discoveries of early tetrapodomorphs, the interior of the continent, until this recent finding, had only produced fragmentary fossils, teasing researchers with incomplete glimpses.

"Discovery of Harajicadectes zhumini: Unraveling the Enigmatic Past of Central Australian Rivers"
Harajicadectes travels through the ancient rivers of central Australia about 385 million years ago. (Brian Choo)

A Journey of Exploration

The description of Harajicadectes zhumini culminates five decades of dedicated exploration and research.

"Discovery of Harajicadectes zhumini: Unraveling the Enigmatic Past of Central Australian Rivers"
The moment of discovery in 2016 when we found the entire Harajicadectes fossil. Flinders University paleontologists John Long (centre), Brian Choo (right) and Alice Clement (left) and ANU paleontologist Gavin Young (top left). (Author supplied)

Palaeontologist Gavin Young from the Australian National University made initial discoveries in 1973 while surveying the Middle-Late Devonian Harajica Sandstone in Luritja/Arrernte territory, over 150 kilometers west of Alice Springs (Mparntwe).

Amidst red sandstone blocks atop a remote hill, Young unearthed hundreds of fossil fishes, predominantly Bothriolepis, a type of widespread placoderm covered in armor-like plates.

Among the trove were fragments of other fishes, including Harajicadipterus youngi, a lungfish named in honor of Young’s extensive work on Harajica material.

Additionally, there were spines from acanthodians, phyllolepids’ flat plates, and notably, jaw fragments belonging to an unknown tetrapodomorph.

Type specimen of Harajicadectes discovered in 2016. (Author supplied)

Despite early attempts to classify the species, definitive identification proved challenging until a 2016 expedition by Flinders University yielded the first almost complete fossil of the animal.

This pristine specimen confirmed that all previously collected fragments belonged to a single new fish species, now housed at the Museum and Art Gallery of the Northern Territory as the type specimen of Harajicadectes.

A Peculiar Apex Predator

Measuring up to 40 centimeters long, Harajicadectes stands as the largest fish discovered in the Harajica rock layers. Presumably, it occupied the apex predator niche in ancient river ecosystems, featuring a large mouth lined with densely packed sharp teeth and prominent triangular fangs.

Harajicadectes displays a mix of anatomical traits from various tetrapodomorph lineages, suggestive of convergent evolution. Notably, its skull and scale patterns exhibit such convergence, complicating its precise placement among relatives.

Of particular significance are the two large openings atop its skull, known as spiracles, typically minute in most early bony fishes.

Similar giant spiracles are observed in Gogonasus, a marine tetrapodomorph from Western Australia’s Late Devonian Gogo Formation, and the unrelated Pickeringius, an early ray-finned fish also found at Gogo.

These structures were also present in elpistostegalians, freshwater tetrapodomorphs like Elpistostege and Tiktaalik, which are closely related to limbed vertebrates. Their emergence across different Devonian lineages offers insights into atmospheric conditions of the past and the evolution of air-breathing in vertebrates.

Source: Read the original article.


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