What looked like a normal wall hid a secret from 240 million years ago

A pile of garden stones on Australia’s Central Coast held more than landscaping material. Inside one slab lay an ancient Arenaerpeton fossil, also known as a Chinese Giant Salamander, that dates back 240 million years.

This amphibian fossil, very well-preserved with skin outlines and an almost complete skeleton, was tucked away for decades before anyone realized what it was.

Researchers have now given it a name: Arenaerpeton supinatus, Latin for supine sand creeper. The fossil captures a moment from the Early Triassic, only a few million years after Earth’s worst mass extinction.

Lachlan J. Hart, a PhD candidate in the School of Biological, Earth and Environmental Sciences at UNSW Sydney and the Australian Museum led the research that brought this animal out of obscurity and into the scientific record.

From garden stones to science

In 2023, the formal description of the specimen appeared in a peer reviewed journal, establishing Arenaerpeton supinatus as a new member of a Gondwanan amphibian family.

The paper documents an articulated skeleton preserved on its back with traces of soft tissue around the body.

“This fossil is a unique example of a group of extinct animals known as temnospondyls, which lived before and during the time of the dinosaurs,” Hart explained.

“We do not often find skeletons with the head and body still attached, and the soft tissue preservation is an even rarer occurrence.”

Decoding the Arenaerpeton fossil

Arenaerpeton had a broad, low skull and a sturdy frame. The rock holds impressions of the body outline that suggest a heavy build, plus long ribs and stout limb bones.

The teeth tell another story. Along the jaws run many small marginal teeth, and on the roof of the mouth sit a pair of tusk like fangs. Those features point to a predator that seized slippery prey and did not let go.

Estimates put the total length around 4 feet. The UNSW team adds a precise figure from the fossil evidence, noting it was “estimated to be about 1.2m from head to tail,” said Hart.

The comparison highlights head shape, not close kinship, since the fossil lineage followed its own path long before modern salamanders appeared.

Where and when Arenaerpeton lived

The animal came from the Sydney Basin, a stack of sandstones and shales that record rivers, swamps, and lakes in what is now New South Wales. Its host rock belongs to the Terrigal Formation, a unit laid down during the Early to Middle Triassic.

High precision U Pb zircon ages from nearby volcanic ash layers in the Garie Formation cluster near 248 million years.

Those tuffs bracket the time when these river sands accumulated and help anchor Arenaerpeton in Earth’s timeline.

Waters in this basin carried fish, including species such as Cleithrolepis, and the skull and teeth of Arenaerpeton fit a life spent ambushing prey in slow channels.

Fine sands and plant fragments surrounding the skeleton point to burial in a quiet setting, not a raging flood.

The preservation includes a broad halo of soft tissue around the ribs. That band shows the body outline without mineralized skin armor and hints at how the carcass settled before final burial.

Family with staying power

Arenaerpeton belongs to chigutisaurids, a branch of temnospondyls, the diverse early amphibian grade tetrapods that flourished before and during the age of dinosaurs. These animals radiated across southern continents when they were joined in Gondwana.

Their history in Australia stretches across much of the Mesozoic.

An annotated checklist of Australian Mesozoic tetrapods notes that chigutisaurids persisted into the Early Cretaceous, with Koolasuchus from Victoria representing the youngest known temnospondyl anywhere.

Arenaerpeton sits earlier in that story, close to the recovery interval after the end Permian crisis.

The size of this animal, coupled with features shared with later giants, hints at a long lived strategy that worked in varied climates and ecosystems.

“The fossil record of temnospondyls spans across two mass extinction events, so perhaps this evolution of increased size aided in their longevity,” said Hart. He points to body size as a possible advantage over deep time. 

Placing Arenaerpeton on the tree

The team assessed traits across the skull, palate, lower jaw, shoulder girdle, and forelimb, then compared them with other Triassic and Jurassic relatives.

Those comparisons place Arenaerpeton within Chigutisauridae alongside taxa from India and South America.

The analysis used a matrix of skeletal characters tested with parsimony methods. Results group Arenaerpeton with derived chigutisaurids, not with brachyopids that shared the basin at times.

Several features support that placement. The animal has many small marginal teeth, a narrow flat cultriform process on the palate, and adjacent intercentra in the vertebral column, a suite consistent with chigutisaurid identity.

Why Arenaerpeton matters now

Beyond taxonomic placement, this fossil fills a geographic and temporal gap in Australia’s record. It is the first chigutisaurid named from New South Wales and shows that large bodied forms were already present early in the Triassic.

It also shows how science advances through community care as much as field campaigns. A homeowner saved a strange slab, museum staff preserved it, and researchers returned to it with new questions.

“This is one of the most important fossils found in New South Wales in the past 30 years, so it is exciting to formally describe it,” said Dr. Matthew McCurry, Senior Lecturer at UNSW and Curator of Palaeontology at the Australian Museum. The fossil is now part of the public story of the basin that buried it.

Arenaerpeton brings texture to a time often sketched in broad strokes. It offers a body outline, not just a list of bones, and it anchors a local river system to the global narrative of recovery after catastrophe.

The study is published in the Journal of Vertebrate Paleontology.

Photo credit: UNSW Sydney/Richard Freeman.

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