Kimberley fossils reveal ancient marine amphibians

Fossils unearthed in north-western Australia reveal that, 250 million years ago, the red, dusty Kimberley region looked utterly different. Rather than desert, the area formed a shallow coastal bay, where tropical water washed across the landscape and crocodile-like amphibians swam offshore.

These animals inhabited a planet still recovering from the end-Permian extinction, when global warming had thrown the oceans into upheaval.

As the seas started to recover at the beginning of the Age of Dinosaurs, marine tetrapods – including amphibians and reptiles – evolved into the leading predators.

For decades, most fossils of these early marine hunters came from the Northern Hemisphere, while the southern continents featured scarcely at all in the account. A new examination of old Kimberley fossils is now beginning to change that.

Museum drawers reveal fossil secrets

The fossils at the heart of this account were originally discovered on scientific expeditions during the early 1960s and 1970s.

Scientists from the Swedish Museum of Natural History collected remains of ancient marine amphibians from a rock exposure at Noonkanbah cattle station, east of the isolated town of Derby. The specimens were divided between museum collections in Australia and the United States.

In 1972, researchers published a paper that identified one species from several skull fragments, naming it Erythrobatrachus noonkanbahensis.

Something unusual then occurred: the original Erythrobatrachus fossils disappeared at some point over the following 50 years.

That disappearance could have brought the story to a close. Instead, it prompted a search across museum collections around the world.

In 2024, researchers eventually located and re-evaluated the ancient marine amphibian remains. Their findings altered the picture.

Two predators in one bay

Erythrobatrachus was part of the trematosaurid group: crocodile-shaped relatives of modern salamanders and frogs that could reach roughly 2 metres in length.

Their fossils are particularly valuable because they occur in coastal rocks laid down less than a million years after the end-Permian mass extinction. This places them among the earliest marine tetrapods of the Mesozoic Era.

Initially, the skull pieces from Kimberley appeared to represent a single species. Further examination revealed otherwise.

The detailed reassessment found that the Erythrobatrachus skull fragments did not all come from one species. Instead, they represented at least two separate trematosaurid forms: Erythrobatrachus and another species assigned to the well-known genus Aphaneramma.

High-resolution 3D imaging helped resolve the question. When complete, the Erythrobatrachus skull measured around 40 cm long. It belonged to a large-bodied apex predator with a broader head. Aphaneramma was similar in overall size, but possessed a long, slender snout suited to catching small fish.

Hunting alongside one another

Both trematosaurids moved through the water column of the shallow bay, but they were not competing for precisely the same food.

One probably pursued larger prey using its broad head and more powerful bite. The other likely snapped up smaller fish with its narrow snout. They occupied the same habitat while performing different ecological roles.

This is significant because ecosystems do not recover instantly after a mass extinction. Recovery begins with a small number of resilient species, while complex food webs take longer to re-form.

The presence of two distinct marine amphibians living alongside each other so soon after the end-Permian crisis therefore indicates that ocean ecosystems were already becoming more established. Predators were diversifying, and ecological niches were being occupied.

The global connection

One of the most striking findings concerns geography. Erythrobatrachus is known only from Australia. Aphaneramma, by contrast, has been recorded in deposits of a comparable age on Svalbard in the Scandinavian Arctic, in the Russian Far East, Pakistan and Madagascar.

This distribution is remarkable. In the early Mesozoic, Earth’s landmasses remained joined as the supercontinent Pangaea, with extensive coastlines running around interconnected shores.

The Australian trematosaurid fossils indicate that these early marine tetrapods did not simply diversify into separate roles. They also dispersed worldwide, probably travelling along the coastlines of connected supercontinents during the first two million years of the Age of Dinosaurs.

Ancient sea beneath desert sands

The Kimberley today is hot, dry and remote. Cattle stations now stand where waves once broke, making it difficult to imagine a shallow bay populated by crocodile-like amphibians.

But the rocks preserve the evidence. Fossils stored in drawers for decades have restored this vanished world to view. They suggest that life recovered more rapidly and spread more widely than previously believed after the most severe mass extinction in Earth’s history.

The early Age of Dinosaurs was not solely an era of giant land reptiles. It was also a period in which limbed vertebrates returned to the sea and created new marine ecosystems from the ground up.

In what is now desert, ancient hunters once ranged through warm coastal waters. Their almost-forgotten bones are helping scientists reconstruct how life renewed itself after catastrophe.

The full study appeared in Journal of Vertebrate Paleontology.

Photo credit: Pollyanna von Knorring/Swedish Museum of Natural History