The discovery of Australia's oldest known crocodile eggshells is helping UNSW scientists unlock clues to animals and ecosystems that thrived millions of years ago, before Australia became an island continent.
In a local grazier's backyard in the small southeast QLD town of Murgon, scientists have been digging for decades in what looks like an unremarkable clay pit. But within the clay lies one of Australia's oldest fossil sites - a window back in time to when the continent was still connected to Antarctica and South America.
Now, an international team led by the Institut Català de Paleontologia Miquel Crusafont (ICP), including researchers from UNSW Sydney, has uncovered the oldest crocodilian eggshells ever found in Australia.
The fragments, named Wakkaoolithus godthelpi, once belonged to mekosuchine crocodiles. This group of now extinct crocs dominated inland waters 55 million years ago. Modern saltwater and freshwater crocs only arrived later in Australia, around 3.8 million years ago.
"These eggshells have given us a glimpse of the intimate life history of mekosuchines," says the study's lead author Xavier Panadès i Blas.
"We can now investigate not only the strange anatomy of these crocs, but also how they reproduced and adapted to changing environments."
From swimmers to tree-climbing hunters
Unlike today's saltwater and freshwater crocodiles, mekosuchines filled strange ecological niches.
"It's a bizarre idea," says UNSW palaeontologist Professor Michael Archer. "But some of them appear to have been terrestrial hunters in the forests."
He says there are suggestions of this throughout a wide range of younger mekosuchine fossils that were discovered earlier within 25-million-year-old deposits from another region: the Riversleigh World Heritage Area, Boodjamulla National Park on Waanyi country in northwestern QLD.
Prof. Archer says some riverine species there grew to at least five metres long.
"Some were also apparently at least partly semi-arboreal 'drop crocs'," he says.
"They were perhaps hunting like leopards - dropping out of trees on any unsuspecting thing they fancied for dinner."
A delicate time capsule
Panadès i Blas says eggshells are an underused resource in vertebrate palaeontology.
"They preserve microstructural and geochemical signals that tell us not only what kinds of animals laid them, but also where they nested and how they bred," he says.
"Our study shows just how powerful these fragments can be. Eggshells should be a routine, standard component of palaeontological research - collected, curated and analysed alongside bones and teeth."
The Murgon shell fragments were examined under optical and electron microscopes. Their microstructure suggests the crocs laid eggs on the margins of a lake, with their reproductive strategy adapting to fluctuating conditions.
Coauthor Dr Michael Stein says mekosuchine crocs may have lost much of their inland territory because of encroaching dryland - eventually having to compete in the shrinking waterways not only with new arrivals to Australia, but dwindling numbers of their megafaunal-sized prey as well.
Dr Stein says the Murgon lake was surrounded by a lush forest.
"This forest was also home to the world's oldest-known songbirds, Australia's earliest frogs and snakes, a wide range of small mammals with South American links, as well as one of the world's oldest known bats," he says.
A story with teeth
Prof. Archer says the discovery within the Tingamarra deposit at Murgon is part of a much bigger story - one that enriches the understanding of ancient ecosystems before Australia became an independent continent.
He recalls finding a bizarre crocodilian jaw fragment in 1975 in the Texas Caves of southeastern Queensland.
While this has since been confirmed as a mekosuchine croc, at the time it seemed so strange to Prof. Archer that he didn't suspect it was a crocodile. It seemed to be a reptile - but one with dinosaur-like teeth. He was so baffled that he decided to consult Professor Max Hecht - a reptile specialist at the American Museum of Natural History.
"When he saw it, Max nearly dropped his coffee cup," Prof. Archer says.
"It closely resembled another kind of extinct croc with dinosaur-type teeth that had been found in South America.
"That was the first realisation that crocodiles with teeth like this were also part of the older record in Australia."
Prof. Archer says he and his colleagues have been excavating in the Murgon area since 1983, literally in the backyard of excited local residents.
"That year, UNSW colleague Henk Godhelp and I drove to Murgon, parked the car on the side of the road, grabbed our shovels, knocked on the door and asked if we could dig up their backyard," Prof. Archer says.
"After explaining the prehistoric treasures that might lay under their sheep paddock and that fossil turtle shells had already been found in the area, they grinned and said 'of course!'.
"From subsequent excavations, that's where the eggshell pieces came from.
"And, quite clearly, from the many fascinating animals that we've already found in this deposit since 1983, we know that with more digging there will be a lot more surprises to come."
They were perhaps hunting like leopards - dropping out of trees on any unsuspecting thing they fancied for dinner.
Informing conservation
For Prof. Archer, discoveries like this are more than just glimpses of a vanished past - they're sometimes reminders that Australia's fossil record can reveal important clues about how to save some of today's threatened species.
He has been working with a multi-institutional team on the 'Burramys Project' to bring the Mountain Pygmy-possum - Burramys parvus - back from the edge of extinction.
This species, native to eastern Australia's alpine regions, is critically threatened as pressure from climate change mounts. However, Prof. Archer's team discovered that its prehistoric relatives, evolving over the past 25 million years, have always thrived in temperate lowland rainforests - including those that covered the Riversleigh area between 25 to 12 million years ago.
This led to the theory that the immediate ancestors of today's Mountain Pygmy-possum likely followed rainforests as they moved up into alpine areas during a warm, wet interval during the Pleistocene Epoch. But when the climate in the alpine zone changed and cooled, they had to develop evasive behaviours like hibernation to survive increasingly inhospitable conditions.
A few years ago, Prof. Archer's team - working with Trevor Evans, the manager of Secret Creek Sanctuary, and with donations from many organisations including the Prague Zoo in the Czech Republic - built a breeding facility in an area of non-alpine rainforest near the town of Lithgow.
Today, Mountain Pygmy-possums are flourishing within that non-alpine sanctuary - just as Prof. Archer says the fossil record predicted they would.
As climate change increasingly threatens the existence of more and more species, he says not all stories need be about the gloom and doom of disasters triggered by climate change.
"The Burramys Project is a demonstration that, at least in some cases, we can develop strategies to save endangered species," Prof. Archer says.
"Clues from fossil records matter," he says.
"Not just to understand the past, but also to help secure the future."
The new croc species' name, Wakkaoolithus godthelpi, acknowledges, with permission, the Wakka Wakka First Nations people, on whose Country the fossils were found. The second part of the name recognises Henk Godthelp, who was part of the UNSW palaeontologist crew along with Prof. Archer, Professor Sue Hand and many other research staff and students.
