Ace History Desk – A 15-million-year-old fossil fish discovered in central NSW reveals stomach contents and colour

The discovery provides evolutionary clues for the family of fish that includes modern species, and the rock type in which it was preserved might hold other fossils at the site and elsewhere in the world.

About 15 million years ago, a small fish ventured too deep into a lake.

The fish species — a relative of the Australian grayling — would have been abundant in the tropical environment where it lived, in what is now dry NSW farmland.

But in the freshwater lake all those years ago, oxygen was scarce. The fish asphyxiated and sank to the bottom, where it was covered by layers of iron-rich material.

Millions of years later, the fish’s fossilised body was stumbled upon by Jochen Brocks, a researcher at the Australian National University. He and some colleagues had examined the site on a whim, after a dig at a nearby fossil site had proved fruitless.

“I split the rock and thought, ‘holy shit, this is a fish,’” Professor Brocks said.Loading

The fossil was discovered at McGraths Flat, near Gulgong in the NSW Central Tablelands.

The site is near another that contains 200-million-year-old Jurassic fossils.

“The probability of having two such incredible sites just next to each other … It’s almost zero,” he said.

“It was a complete fluke. Just because we had nothing else to do, we looked at those rocks.”

The fish ended up being one of a few dozen of its kind found at the site, so exquisitely preserved that researchers can tell what colour the fish originally was, and its last meal.

“It’s so well preserved that you can look into its gut and see what it has eaten,”

Professor Brocks said.

The discovery is surprising because the fish have been entombed in the iron-rich mineral goethite — something researchers didn’t previously realise could preserve fossils, according to Professor Brocks.

“No-one knew that you could preserve a fossil with iron so well.”

Midge larvae were on the menu for this little fish. (Supplied: © Salty Dingo 2020)normal

The tiny fish has now been described as a new species, and named Ferruaspis brocksi — “ferru” because of the iron-rich rocks in which it was found, and “brocksi” after Professor Brocks.

“I didn’t think it would be so much fun, having a species named after you,” he said.

“You put an ‘i’ at the end to make it a Latin species name, but ‘Brocksie’ was also my nickname in school.”

F. brocksi was described in a study published in the Journal of Vertebrate Palaeontology.

Soft tissues, stomach contents, and colour

Lead author Matthew McCurry, a palaeontologist at the Australian Museum and the University of New South Wales, said it was exciting to see a fossil site where soft tissues were preserved.

“Normally when we find fossils, we find bits and pieces of bone.

“But in this case, the skeleton’s articulated, so all the bones are in their position as they would be in life. But also, we can see the soft tissues around them.”

Australian Museum Head of Palaeontology Matt McCurry examines the fossil. (Supplied: Laura Martin)normal

They could see the fish had eaten mostly larvae from a specific type of midge, alongside other insects and bivalves, and it had a few parasites attached to its tail.

Scanning electron microscope imaging of the soft tissue revealed the fish had a light-coloured belly and a stripe on its side. 

Study co-author Michael Frese, a researcher at the CSIRO and the University of Canberra, said the key to revealing the colour was finding remnants of cells called melanosomes.

These cells produce the dark pigment melanin. In humans, this pigment causes freckles, dark skin, and dark hair.

“If we image these bodies, we know which part of the skin was dark and which was not,” Dr Frese said.

“The bones are gone, the melanin is gone. But what you have are little moulds where the melanin used to be.”

The fossil was found at McGrath’s Flat near Gulgong in central NSW. (Supplied: © Salty Dingo 2020)normal

Kate Trinajstic, a palaeontologist at Curtin University who wasn’t involved in the research, says the discovery might mean there are other iron-rich rocks containing troves of fossils.

“We tend not to think of iron-rich rocks as containing great fossils, so the fact that they’ve recovered it opens up a new area for people to investigate,” Professor Trinajstic said.

Other objects preserved at the site — such as plants and insects — also yield much more direct evidence of an ecosystem than palaeontologists can normally glean.

“It literally preserves a whole ecosystem frozen in time,” she said.Loading

The fish is the first fossil freshwater fish in the Osmeriformes group, which also includes the Australian smelt (Retropinna semoni) and Australian grayling (Prototroctes maraena), to be found in Australia. 

Dr McCurry 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.

Professor Trinajstic said the discovery could also help scientists learn more about modern Australian freshwater fish.

The fossil species has similar patterning to the still living, but vulnerable to extinction, Australian grayling (Prototroctes maraena). (iNaturalist: laurentd81, Australian grayling, CC BY-NC 4.0)normal

“The Australian freshwater fish population is actually not very diverse compared to the rest of the world,” she said.

“Understanding how it evolved and where it came from could perhaps help us understand why we don’t have the type of diversity.”

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