Lost Australian Fossils Reveal Two Sea-Going Salamander Relatives Lived Side by Side
The fossils had been sitting in museum collections for five decades. Collected from the red rock country of the Kimberley in far northwestern Australia during expeditions in the early 1960s and 1970s, the skull fragments belonged to ancient marine amphibians - crocodile-shaped relatives of modern frogs and salamanders that swam the shallow coastal seas of the Early Triassic period. They were studied briefly, assigned to a single species in a 1972 paper, and then, somewhere over the following half-century, the original specimens were lost.
A search through international museum collections eventually located them. Researchers at the Swedish Museum of Natural History and their collaborators have now completed a full reassessment, published in the Journal of Vertebrate Paleontology, and the results have rewritten what was known about Australia's earliest marine vertebrate communities. The story those bones tell is one of rapid ecological diversification and global animal movement at the very beginning of the Age of Dinosaurs.
The End-Permian Backdrop
About 252 million years ago, the largest mass extinction in Earth's history wiped out an estimated 80 to 95 percent of marine species. The recovery that followed, during the Early Triassic period, is one of the most consequential evolutionary episodes in the fossil record: the foundations of modern marine ecosystems were established, and limbed vertebrates - tetrapods - moved into aquatic environments in earnest for the first time.
Trematosaurids were among the earliest of these sea-going tetrapods. They were temnospondyls, a broad group of amphibian-like vertebrates now extinct, and they superficially resembled crocodiles: long bodies, four limbs, and elongated heads adapted for aquatic predation. Adults reached up to 2 meters in length. Their fossils appear in coastal and estuarine deposits laid down within the first million years after the end-Permian extinction - which makes them, as the researchers put it, "the geologically oldest currently recognisable group of Mesozoic marine tetrapods." Almost all known trematosaurid fossils come from the Northern Hemisphere.
One Collection, Two Species
The Australian material had been assigned to a single species: Erythrobatrachus noonkanbahensis, named after the Noonkanbah cattle station where the specimens were found. When the newly located specimens were examined using high-resolution 3D imaging, however, the skull fragments did not all fit the same anatomical template.
The collection contained at least two distinct types of trematosaurids. One was Erythrobatrachus itself - with a skull estimated at about 40 centimeters in length when complete, a broad head, and a body plan consistent with a top predator that likely hunted larger prey. The other belonged to Aphaneramma, a genus known from other sites but not previously documented from Australia. Aphaneramma was similar in overall body size but had a long, narrow snout suited to catching small fish - a different foraging strategy occupying a different ecological niche in the same coastal habitat.
This means the ancient shoreline of what is now the Kimberley hosted at least two coexisting trematosaurid species partitioning resources between them: a broad-headed ambush predator and a narrow-snouted fish specialist. Finding this level of ecological diversity in Early Triassic Australia revises the picture of how quickly marine vertebrate communities organized themselves after the end-Permian collapse.
The Global Connection
The wider significance of the discovery lies in where else Aphaneramma fossils have been found. Specimens attributable to the same genus have been recovered from deposits of similar age on Svalbard in the Arctic, in the Russian Far East, in Pakistan, and in Madagascar. The presence of the same genus in both Australia and the high Arctic - on opposite sides of the ancient supercontinent Pangaea - indicates that these early marine tetrapods managed to disperse across oceanic barriers within roughly two million years of the end-Permian extinction.
The mechanism of that dispersal remains open to interpretation. During the Early Triassic, the world's landmasses were assembled into a single supercontinent surrounded by the Panthalassic and Tethys oceans. Coastal routes connecting the ancient southern landmass of Gondwana to the northern lands of Laurasia may have allowed animals to move along shorelines without crossing deep ocean basins. Whether trematosaurids crossed open water or tracked coastlines, the speed and geographic breadth of their radiation is striking for animals only beginning to adapt to marine life.
The rediscovered specimens of Erythrobatrachus are in the process of being repatriated to Australia. Other trematosaurid fossils from the collection are currently on public display at the Swedish Museum of Natural History in Stockholm. The original 1972 publication that missed the second species had only skull fragments to work from; the 3D imaging techniques now available made the distinctions between the two taxa visible in ways that optical examination alone could not have revealed in the 1970s.