Rare opalized plesiosaur discovery sheds light on prehistoric aquatic ecosystems

“Previous studies examined the exterior surface of Eric’s opalised skeleton to find clues,” Ph.D. researcher Joshua White from the ANU Research School of Physics and the AMRI said.

“But this approach can be difficult and limiting as fossilised stomach contents are rare to find and there can more hidden beneath the surface that would be near impossible for palaeontologists to see without destroying the fossil. We believe our study is the first in Australia to use X-rays to study the gut contents of a prehistoric marine reptile," he added.

3D model of Eric’s gut

To distinguish between what he thought to be evidence of fish bones, gastroliths, commonly known as stomach stones and other things that the reptile had digested, Mr. White combed through mounds of data and CT imagery. The data was used to create a 3D model of Eric’s gut contents. The information was utilized to build a 3D model of Eric's digestive system.

“Eric was a mid-tier predator, sort of like a sea lion equivalent, that ate small fish and was likely preyed upon by larger, apex predators,” Mr. White said.

“We are also lucky in the sense that Eric is one of the most complete opalised vertebrae skeletons in Australia. The fossil is approximately 93 per cent complete which is pretty much unheard of in any fossil record. There is practically nowhere else other than Australia that can actually get opalised vertebrae fossils.”

In addition to aiding in our understanding of how current creatures may be impacted by factors like climate change, the ANU scientists argue that learning more about the food of extinct organisms is a key step in comprehending their evolutionary past.

Eric was found for the first time in 1987 in the opal mines of Coober Pedy, South Australia. The Australian Museum in Sydney has the extinct predator on display.

The study was published in Alcheringa: An Australasian Journal of Palaeontology.

Study abstract:

The holotype of Umoonasaurus demoscyllus (AM F.99374) is one of the most complete plesiosaur skeletons ever discovered in Australia. It preserves a gastric mass in the pectoral girdle region that has not yet been documented in detail. Here, we use high resolution micro-computed tomography (micro-CT) to image the gut contents of this specimen. These comprise 17 vertebrae from an unidentified teleost, together with at least 60 rounded gastroliths averaging about 5 mm in maximum length. Our study demonstrates the potential for 3D tomographic imaging to investigate Mesozoic marine reptile diets and ecology.