Bones preserved in underwater caves offer a rare and powerful window into the past – but until now, researchers have had limited tools to understand how the remains of extinct megafauna and other animals came to be found in their underwater graveyards.
New research led by Griffith University changes that.
"By analysing animal bones from two underwater cave systems in South Australia, we have revealed how different cave environments leave distinct preservation 'fingerprints' on skeletal remains," said Meg Walker, a PhD Candidate supervised by Australian Research Centre for Human Evolution Director Professor Julien Louys .
"Backed by radiocarbon-dated bones, we tracked how skeletons accumulated and were modified over decades and centuries in underwater caves, then compared them to those buried in dry caves.
"Using a range of methods, from the macro to the micro, we looked at features associated with wet and dry caves.
"Things like spatial distributions of the bones and their surfaces, down to elemental compositions and proteins trapped in ancient cells."
The results were clear: underwater caves often preserved bones remarkably well, maintaining their structure and surfaces in exquisite detail.
Yet these same environments also left unique chemical and biological traces, shaped by light levels and aquatic life.
Different types of algae and plants grew in cave waters near bright entrances, often on bone surfaces where they left unique signatures.
But in the midnight regions of the caves, where light never reaches and no plants grow, bones stayed pristine.
Bones left in dry caves, by contrast, did not show these signatures, and were instead eaten by land-based bacteria, and marked by plant roots in the form of long grooves.
Ms Walker and the team, including specialist cave divers from the Cave Divers Association of Australia, collected historical animal bones from two underwater caves, Green Waterhole and Gouldens Sinkhole, near Mount Gambier, South Australia.
Native and non-native animals included cows, kangaroos, emus, sheep, pigs, dingoes, rabbits, possums, quolls and swamp rats – some of which may date to the very first European arrival and establishment of the city in the 1840s.
By studying these bones, the researchers hoped to better understand how the fossil remains of extinct megafauna also ended up in these underwater caves, and under what sort of environmental conditions.
"This study has delivered the first framework for interpreting how megafauna fossils formed, survived, and changed in underwater caves," Miss Walker said.
"It will provide archaeologists and palaeontologists worldwide with a powerful new tool for reconstructing past environments and histories in these challenging conditions."
The study 'Neotaphonomic characteristics of vertebrate site formation in underwater caves' has been published in PLOS One.
