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In an Australian first, researchers have described a new species of 15-million-year-old fossilized freshwater fish, Ferruaspis brocksi, that shows preserved stomach contents as well as the pattern of coloration.

The team was led by Australian Museum and UNSW Sydney paleontologist Dr. Matthew McCurry. The work is in the Journal of Vertebrate Paleontology.

Named after Professor Jochen J. Brocks from the Australian National University, who discovered several of the fossilized species at the Australian Museum's McGraths Flat fossil site near Gulgong, NSW, Ferruaspis brocksi is the first fossil freshwater smelt (order Osmeriformes) to be found in Australia.

Dr. McCurry, the lead author of the paper, said that before this fossil discovery scientists lacked concrete evidence to pinpoint when this group of fish arrived in Australia and how they evolved over time.

"The discovery of the 15 million-year-old freshwater fish fossil offers us an unprecedented opportunity to understand Australia's ancient ecosystems and the evolution of its fish species, specifically the Osmeriformes group during the Miocene epoch, 11–15 million years ago," McCurry said.

"Osmeriformes are a diverse group of fish within Australia, which includes species like the Australian grayling and the Australian smelt. But, without fossils, it has been hard for us to tell exactly when the group arrived in Australia and whether they changed at all through time," McCurry continued.

"Not only does this fossil provide a unique snapshot into the environment in which it lived in the Central Tablelands, but also, because its stomach contents are so well preserved, it allows us a glimpse into the behavior of these ancient species. We now know that they fed on a range of invertebrates, but the most common prey was small phantom midge larvae," McCurry said.

"One of the fossils even shows a parasite attached to the tail of the fish. It's a juvenile freshwater mussel called a glochidium. These juvenile mussels attach themselves to the gills or tails of fish to hitch rides up and down streams," McCurry added.

The species was named Ferruaspis brocksi because it was found encased in iron-rich rock. "Ferru" comes from ferrum (Latin), meaning "iron," and "brocksi" comes from Professor Jochen J. Brocks of Canberra, Australia.

Acknowledging this honor, Professor Brocks said he was thrilled to have this superb specimen named after him.

"Collecting fossils at McGrath Flat is a highlight for me every year. Splitting the rust-red slabs of rock is like opening an ancient book, revealing the creatures that inhabited an Australian oxbow lake some 15 million years ago. This little fish is one of the most beautiful fossils there, and finding the first vertebrate among the abundant plant and insect fossils was a real surprise. Having F. brocksi named after me is a real joy," Professor Brocks said.

Dr. Michael Frese, a researcher from the University of Canberra and CSIRO, Australia's national science agency, said that one of the most exciting aspects of the work was that they could tell the color of the fish.

"The fish was darker on its dorsal surface, lighter in color on its belly and had two lateral stripes running along its side," Dr. Frese said.

"Using a powerful microscope, we were able to see tiny color-producing structures known as melanosomes. Fossilized melanosomes have previously enabled paleontologists to reconstruct the color of feathers, but melanosomes have never been used to reconstruct the color pattern of a long extinct fish species," Dr. Frese said.

Director and Chief Scientist of the Australian Museum Research Institute, Professor Kris Helgen, said, "Dr. Matt McCurry's work at the McGraths Flat fossil site has been outstanding. This is one of many discoveries made at this site, which continues to play a significant role in advancing our understanding of Australia's evolution during the Miocene era."

The AM's new fossil site, McGraths Flat, is located in the Central Tablelands, NSW near the town of Gulgong. It represents one of only a handful of fossil sites in Australia that can be classified as a Lagerstätte—a site that contains fossils of exceptional quality with remarkable detail.

"The fossils formed between 11 and 16 million years ago and provide a window into the past. They prove that the area was once a temperate wet rainforest and that life was rich and abundant in the Central Tablelands, NSW," McCurry explained.