380-Million-Year-Old Fossils of Air-Breathing Tetrapod Fish Found in Australia

Flinders University paleontologist Brian Choo and his colleagues have described a new genus and species of Devonian tetrapodomorph fish based on several nearly complete skulls and postcranial skeletons.

“Tetrapodomorpha comprises the limbed tetrapods and their closest fish relatives, whose earliest record is from the Pragian of China,” Dr. Choo and co-authors said.

“The group diversified greatly in both marine and freshwater habitats during the Middle-to-Late Devonian while giving rise to several distinct lineages, including the earliest limbed tetrapods.”

“Whereas the tetrapods flourished after the Devonian, limbless fish-grade tetrapodomorphs underwent a marked reduction in diversity during the Carboniferous, with only a handful of representatives persisting into the early Permian before vanishing from the fossil record.”

The new tetrapodomorph species lived around 380 million years ago and was up to 45-50 cm in length.

Scientifically named Harajicadectes zhumini, the fish is particularly distinctive for its large openings on the top of their skull.

“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,” Dr. Choo said.

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

“In addition to Harajicadectes zhumini from central Australia, large spiracles also appeared in Gogonasus from Western Australia and elpistostegalians like Tiktaalik — the closest relatives to limbed tetrapods.”

“Plus it also appears in the unrelated Pickeringius a ray-finned fish from Western Australia, first described in 2018.”

“The synchronized appearance of this air-breathing adaptation may have coincided with a time of decreased atmospheric oxygen during the mid-Devonian,” said Flinders University’s Professor John Long.

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

“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 200 km west of Alice Springs, dating from the Middle-Late Devonian roughly 380 million years old.”

“It is difficult to pinpoint where Harajicadectes zhumini sits in this group of fish as it appears to have convergently acquired a mosaic of specialized features characteristic of widely separate branches of the tetrapodomorph radiation.”

The findings appear in the Journal of Vertebrate Paleontology.

_____

Brian Choo et al. A new stem-tetrapod fish from the Middle–Late Devonian of central Australia. Journal of Vertebrate Paleontology, published online February 5, 2024; doi: 10.1080/02724634.2023.2285000