15-Million-year-old fish fossil with preserved stomach contents. (Credit: Salthy Dingo)
In a nutshell
- Ferruaspis brocksi was found with its stomach contents, skin pigmentation, and even a parasitic mussel still intact, offering a rare glimpse into its life and environment 15 million years ago.
- Unlike its modern relatives, which migrate between freshwater and the ocean, this species likely lived its entire life in freshwater, rewriting part of the evolutionary history of southern osmeriform fishes.
- The fossil’s unique features were so distinct that scientists placed it in an entirely new family, Ferruaspidae, highlighting just how much remains to be discovered in Australia’s fossil-rich McGraths Flat.
SYDNEY, Australia — Most fossils tell us what ancient creatures looked like. The newly discovered Ferruaspis brocksi goes several steps further. Australian researchers uncovered its perfectly preserved remains that tell us what it ate for breakfast, which parasites it hosted, and how its skin was patterned. This previously unknown species changes what we know about the history of fish in Australia, where they lived, and how they evolved.
New research published in the Journal of Vertebrate Paleontology was centered in McGraths Flat, a fossil site in New South Wales that’s turning out to be an extraordinary repository of Miocene fossils. What makes this find particularly remarkable is how exceptionally well the specimens were preserved.
Most fossil fish leave behind only bones or impressions in rock. But these specimens retained details of their stomach contents, skin pigmentation, and even evidence of parasites. This level of preservation is exceedingly rare and gives scientists an unusually intimate look at not just what the fish looked like, but how it lived and interacted with its environment.
Breaking the Migration Pattern
The research team’s analysis places the fish in the order Osmeriformes, which includes modern-day smelts and graylings. Modern members of this fish group in Australia and New Zealand typically need both freshwater and marine environments to complete their life cycles. Many migrate between rivers and the ocean during different life stages, a behavior called diadromy.
What’s fascinating about Ferruaspis brocksi is that researchers found it far inland from what would have been the coastline during the Miocene. This location, along with the discovery of specimens of different sizes (suggesting different age groups living together), points to something unexpected: unlike many of its modern relatives, this species probably spent its entire life in freshwater. This challenges previous thinking about when and how often freshwater adaptation evolved in this group of fishes.
A Last Meal Preserved for Millions of Years
The fossils were embedded in iron-rich sediments, which created the perfect conditions for preserving even the most delicate structures. The minerals formed molds of specialized cells containing pigment, allowing researchers to determine the fish’s coloration pattern. Their analysis showed that Ferruaspis brocksi had a darker upper body and lighter underside, known as counter-shading, a common camouflage strategy in aquatic animals. They also had two distinctive stripes running along its body.
The researchers found something even more exciting when they peered into the fossilized stomachs of several specimens. The last meals of these ancient fish were still identifiable: primarily phantom midge larvae (small aquatic insects), along with some adult insects and at least one small shellfish.
In one truly unprecedented finding, a specimen showed evidence of a parasitic infection, specifically, a larval freshwater mussel attached to its tail fin. This marks the first documented case of such an infection in the fossil record.
“These juvenile mussels attach themselves to the gills or tails of fish to hitch rides up and down streams,” says lead study author Matthew McCurry from the Australian Museum Research Institute, in a statement.
Rewriting Evolutionary History
To figure out where Ferruaspis brocksi fits in the fish family tree, the research team conducted a detailed analysis of its physical characteristics. Their findings indicate it represents an early branch of the southern hemisphere radiation of Osmeriformes. The fish had such a unique set of characteristics that the researchers determined it warranted classification as an entirely new family: Ferruaspidae.
This discovery reveals that fish fully adapted to freshwater evolved multiple times and have been part of Australian ecosystems for much longer than previously thought, calling into question earlier assumptions about the group’s evolutionary timeline.
The species was named Ferruaspis brocksi because it was found encased in iron-rich rock. “Ferru,” from ferrum (Latin), meaning “iron,” and “aspis,” a Greek term for shield, commonly used when naming fossil fish. The species name honors professor Jochen J. Brocks of Canberra, who discovered several of the specimens.
“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,” says Brocks. “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.”
Sixteen million years after its death, this ancient fish has provided a rare glimpse into life in Australia’s prehistoric waterways, complete with striped camouflage, parasitic hitchhikers, and a belly full of midges.
Paper Summary
Methodology
The research team combined traditional paleontological techniques with advanced imaging technology. They photographed specimens using a specialized camera system and examined microscopic structures with a scanning electron microscope that magnifies objects thousands of times without damaging the fossils. To determine evolutionary relationships, they compared 12 different fish species across 112 anatomical features using phylogenetic software. They also quantified pigmentation patterns by analyzing the distribution of melanophores (pigment cells) across the body.
Results
The team discovered 13 specimens ranging from 53 to 77 millimeters in length, representing different age groups of the same species. The fish had 16 dorsal fin rays, 14 anal fin rays, and 19 primary caudal fin rays—a combination unlike other known osmeriforms. Analysis of four specimens’ stomach contents consistently revealed phantom midge larvae (at least 21 individuals per stomach), insect wings, and a bivalve mollusk. One specimen showed a parasitic mussel larva attached to its tail fin—the first fossil record of this relationship. The distribution of preserved pigment cells indicated counter-shading coloration with two lateral stripes.
Limitations
Despite exceptional soft tissue preservation, the hard tissues like teeth and bones were poorly preserved, limiting detailed skeletal descriptions. Otoliths (ear stones), which aid in fish classification, were absent. The phylogenetic analysis used a matrix originally developed in 1996 that required manual updating. With only 13 specimens, the sample size restricted statistical analyses. The research couldn’t definitively determine whether some associations (like a beetle found near one specimen’s mouth) represented actual feeding behavior or post-mortem positioning.
Discussion and Takeaways
The discovery of Ferruaspis brocksi offers an unprecedented opportunity to understand Australia’s ancient ecosystems and the evolution of fish species during the Miocene epoch, 11-16 million years ago. Finding this species far inland with evidence of different age groups living together suggests it completed its entire life cycle in freshwater, unlike many modern relatives that migrate between fresh and salt water. This indicates freshwater-adapted osmeriforms evolved multiple times and have existed in Australian ecosystems since at least the Neogene period. The preserved stomach contents and parasite evidence provide valuable insights into ancient ecological relationships that would be impossible to determine from skeletal remains alone.
Funding and Disclosures
The initial McGraths Flat expedition in 2017 was funded through a donation from a descendant of Robert Etheridge, an English paleontologist who came to Australia in 1866 and later became curator of the Australian Museum. Additional support came from an Australian Research Council Linkage Grant (LP210301049) and internal grants from the Australian Museum and University of Canberra. Dr. Viktor Baranov’s work was supported by the Australian Museum Foundation and a Spanish State Research agency fellowship. The team acknowledged the traditional landowners—the Wiradjuri people—where the research took place (Gulgong is a Wiradjuri word meaning “deep waterhole”).
Publication Information
The paper, “The paleobiology of a new osmeriform fish species from Australia,” was published in the Journal of Vertebrate Paleontology on March 17, 2025. The research team included Matthew R. McCurry, Anthony C. Gill, Viktor Baranov, Lachlan J. Hart, Cameron Slatyer, and Michael Frese. The article is available as open access under a Creative Commons Attribution-NonCommercial-NoDerivatives License.
