Fossil fish so exquisitely preserved that scientists have been able to reconstruct their final days from up to 16m years ago have been discovered in central New South Wales.
Several fossils of small freshwater fish, embedded in an iron-rich mineral called goethite at the McGraths Flat fossil site near Gulgong, have retained microscopic structural features including their stomach contents and the outlines of cells that determine colour.
This unusual level of detail – including the slender shape of the fish and position of its bones and fins – has revealed a lot about the species, called Ferruaspis brocksi, said the lead author, Dr Matthew McCurry, curator of palaeontology at the Australian Museum.
“In palaeontology there’s often so many gaps. Normally we just find isolated bones of a particular species,” McCurry said. “We can’t often see the whole animal, and we rarely see things like soft tissues preserved.”
The find, published in Vertebrate Palaeontology, has provided the first detailed evidence in Australia for a group of fish called the Osmeriformes, which today include graylings and smelts, McCurry said.
Unusually, the fossils retained remnants of colour cells, called melanophores, including the tiny melanin-containing granules inside called melanosomes (measuring just over a thousandth of a millimetre).
Co-author Dr Michael Frese, an associate professor and virologist adept at microscopy and based at the University of Canberra, said this level of detail “pushed the boundaries” of what could be preserved.
From these microscopic details, the authors determined the fish were “counter shaded” – darker at the top and lighter at the belly – with two stripes along their sides.
Also preserved were the animals’ stomach contents, and in some cases the intestinal tract.
Frese said when viewed under a high-powered microscope, the fish bellies were full of the antennae of phantom midge larvae (a type of insect), bits and pieces of half-digested wings, and even a small mussel or bivalve.
These minute details offered a glimpse into the life of the fish in its final days, he said.
The fossils were buried at the bottom of a lake like a billabong that was separated from nearby rivers, he said. Yet a second tiny bivalve, attached to a fish fin, suggested the animals came from a nearby river.
That river might have flooded or otherwise spilled over into the lake, where the fish probably gorged themselves on phantom midge larvae. “They died with a full stomach,” Frese said.
The evolutionary biologist and palaeontologist Dr Alice Clement, who was not involved in the paper, said analysing melanophores to reconstruct colour patterns was a “big advancement in the study of fossil fishes”.
Colours and patterns were important in the animal world – used for attracting mates, warning off predators and for camouflage. Yet the study of these characteristics in fossils was still in its infancy, she said.
The species is named after Prof Jochen J Brocks, from the Australian National University, who has discovered several fossilised species at the McGraths Flat site.
As well as detailed insights into the species itself, the fossils provided an “unprecedented opportunity” to understand Australia’s ancient ecosystems and the evolution of fish during the Miocene, McCurry said.
The palaeontologist Prof John Long, an expert in ancient fishes at Flinders University who was not involved in the study, said fish fossils from the Miocene provided a “rare window” into ecosystems at a time of dramatic environmental change, when deserts expanded and forests declined.
“It helps us appreciate the diversity of Australia’s unique fauna and how it evolved to cope with changing climatic conditions.”
Frese said uncovering fossils at McGraths Flat was like flipping individual pieces of an upside-down mosaic.
The more you flipped, the more it revealed about the environment around the lake, he said. Over time, an even bigger picture emerged about how species evolved, and how continents and landscapes transformed at a critical point in history.
“At the time these fish died and were preserved, that was a transitional period for Australia,” he said. “Basically it’s a history lesson, or a geological lesson, of what happens if the climate changes fundamentally.”
• This article was amended on 19 March 2025 to replace a map that incorrectly located McGraths Flat. It is in the central west of NSW, near the town of Gulgong.
