55 million years: Australia’s oldest crocodile eggshells found in Queensland

Sydney, Nov 12 (The Conversation) In southeast Queensland, roughly 250 kilometres from Brisbane, lies the tiny town of Murgon. Located on Wakka Wakka Country, it’s home to about 2,000 people – and one of the most important fossil sites in the world.

From the 55 million-year-old clays there, palaeontologists have unearthed a range of precious fossils over several decades. These include the world’s oldest fossil songbirds, the only known fossils of salamanders in Australia and the oldest fossil marsupial remains in Australia.

And the site continues to serve up ancient treasures. In a new study, published today in the Journal of Vertebrate Paleontology, my colleagues and I report the discovery of the oldest crocodilian eggshells ever found in Australia.

These eggshells now serve as the basis for a new eggshell type, Wakkaoolithus godthelpi. They belong to the oldest known member of a now extinct group of crocodiles known as mekosuchines. And they offer a new look into not just their evolution – but also the forest-lined wetlands they lived in.

Tree-climbing crocodiles -------------------------- Mekosuchines were Australia’s own unique, local branch of the crocodile family. They dominated inland waters of the continent 55 million years ago. They were part of the group of species that includes alligators, true crocodiles, gharials and caiman.

But they represent a much older branch than the saltwater and freshwater crocs found in Australia today. These modern species made their way to the continent much later, as it came into contact with southeast Asia about five million years ago.

In the 1980s, fossil finds began to take off at sites such as Murgon and the Riversleigh World Heritage Area in Queensland, and Alcoota in the Northern Territory.

Since then, palaeontologists have been able to piece together a good picture of the diversity of mekosuchines. This was much higher than what we see today, with ten extinct genera now accounted for.

These fossil finds preserve a vigorous evolutionary history, and in particular a trend towards land-based hunting by about 33 million years ago.

This diversity includes the likes of Quinkana, a large land-based crocodile with narrow jaws, and dwarf species such as Trilophosuchus which may have been in a position to fill the niche of a tree-climbing hunter – a kind of “drop croc”.

The latter case was originally suggested by palaeontologist Paul Willis on the basis of the unusual musculature at the back of Trilophosuchus’s skull. Although hotly debated, Willis suggested this would allow Trilophosuchus to hold its head aloft and scan through the three dimensional environment of the canopy.

Reading eggshells ------------------- The new study, however, takes a look at one of the oldest genera of Mekosuchines, Kambara. It grew up to two metres long and is thought to have fed on fish and softshell turtles.

At Murgon, University of New South Wales researchers led by Henk Godthelp and Michael Archer uncovered fragments of eggshell, a comparatively rare find. Surprisingly little work has been done in the area of analysing crocodile eggshells.

Xavier Panadès I Blas from the Institut Català de Paleontologia at the University of Barcelona took up the challenge to explore the shell’s preserved microstructure with high-resolution microscopy.

What he found was intriguing. The eggshells of Kambara have their own unique micro-structural features, still preserved after 55 million years.

These features differ from what we know of the microstructures that have evolved among modern crocodiles and alligators. However, it will take a lot more work to put things formally into context.

Still, these eggshells may provide a valuable new avenue for understanding how mekosuchines fit into the big picture of evolution that spans Australia, Asia, Africa and the Americas.

A window into an ancient ecosystem -------------------------------------- Beyond evolution, these eggshells are an insight into the environment that existed at Murgon 55 million years ago.

Crocodile reproduction is intimately tied to their local environment and modern species display a complex mix of nesting strategies in response.

In the case of the eggshell of Kambara, there is little sign of degradation from bacteria.

This suggests the nest may have experienced periods of dryness due to the ephemeral nature of the surrounding wetlands at Murgon.

While mekosuchines enjoyed a much larger range of territories compared to our modern crocodiles, they eventually experienced a considerable contraction as the continent became increasingly arid.

Between this and the decline of large prey, mekosuchines eventually went extinct on the Australian continent. (The Conversation) GSP