A fish frozen in time: ancient fossil reveals colour, last meal, and a parasitic hitchhiker

By  Ian Dewar 27 March 2025 4 min read

A well-preserved fossil fish shows just how similar an ancient fish is to modern day equivalents.

The McGraths Flat fossil site in Central West NSW is different as the fossils aren’t layered with sand or silt. The site is made up of an iron-rich mineral called goethite. It preserved fine details of fossils.

The site is also rare because it's associated with volcanic activity. Eruptions produced basalt, the source of the iron that later preserved the fossils at McGraths Flat.

True colours shining through

Dr Michael Frese, senior author on the study, is a virologist working with our rabbit biocontrol research group and with the University of Canberra. He’s also interested in using the latest imaging techniques to investigate the soft tissues of fossils.

“It’s not necessarily the best-looking fish, there may be more spectacular species, but what is special here is we have the preservation of melanosomes in the skin,” Michael says.

Melanophores, the pigment containing cells in the skin of fish, can contain thousands of melanosomes. Melanosomes are small packages of melanin – the pigment that gives colour to eyes, hair and skin.

In the fossils these melanosomes have been preserved as moulds. Each one is roughly the size of a bacterial cell.

“We know that every hole would have been filled with pigment, so we can tell where the pigment was and we can reconstruct the colour pattern,” Michael says.

The melanosomes show us this fish was counter-shaded. So, it would have been brown or black on top, pale underneath, with two racing stripes on the side.

It’s a form of camouflage that we still see today. This makes the fish hard to see for predators, either looking from above into a dark body of water or looking from below up to the light.

An ancient river and ecosystem evolution

Dr Matt McCurry from the Australian Museum was the lead author of this research paper.

“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,” Matt says.

Dr Cameron Slatyer, a co-author of the study, is manager of National Biodiversity Data Initiatives at the Atlas of Living Australia.

“This fish was living in an ancestral version of the Murray Darling system that was feeding out into a bay that was probably sitting somewhere around where Mildura is today,” Cam says.

“McGraths Flat was an oxbow lake, or billabong, and every so often the iron content reached a concentration where it just killed everything in the water at that point.”

A mussel with hustle hitches a ride

The fossil includes a parasite that had latched onto the skin of the fish to feed and to hitch a ride.

“Normally, a mussel like this will have trouble travelling even 100 metres upstream,” Michael says.

“But with a fish for transportation it can move around. At some stage, it will let go and fall wherever the fish is at that moment.”

However, the conditions in the ancient billabong at this site wouldn’t have been suitable for the mussels to live all year round, so they wouldn’t have reached maturity there.

We think that the fish must have picked up this parasite in a river, moved into the McGraths Flat billabong and died before the mussel could release itself from the host.

Home and at bay

This fish is an ancestor of today’s southern graylings from Australia and New Zealand. Most of the modern species migrate from the ocean to rivers. So, they spawn in saltwater estuaries at the coast and then the young fish migrate upstream to live.

However, the fossil site had fish of different sizes, which suggests this ancient species wasn’t migratory.

“So, we can say that really early in this group's evolution, F. brocksi was already fully or almost fully freshwater, which is fascinating from an evolutionary perspective,” Cam says.

It’s different from a lot of fish in Australia and New Zealand which originated in saltwater and then adapted to freshwater.

A fish with a full belly

Many of the fossil fish have their stomach contents preserved. These show that phantom midges were a key food.

“These fish did not die of hunger, that's for sure,” Michael says.

The modern versions of this fish species live in rivers and streams. But when those streams flood, the fish can get into billabongs, eat whatever they can find, and retreat back to the river.

“The fossil record of what they were eating shows that they're behaving exactly the same way 15 million years later,” Cam says.

Freeze frame fossil fast forward

Sometimes with fossil ancestors, there’s a lot of guesswork and biomechanical analysis of bones to explain how the species has evolved.

But in this case, the ancient fish was clearly similar to species we see today.

“I just found it utterly fascinating that you could have this thing from so many million years ago behaving very similarly to modern species today,” Cam says.

Cam used species observation data from the Atlas of Living Australia to map where the modern-day descendants are found.

“Evolution is a lottery driven by change, and if an organism, in the various games of chance that happen, hits a niche that lasts through history then it does well,” he says.

This species has hit the jackpot with a niche in a river system that has existed in pretty much the same form for millions of years.