A team of researchers recently discovered a well-preserved fish fossil from up to 16 million years ago in central New South Wales.

Freshwater Fish Fossil

15 million year old fish has shown researchers a lot about its ecology, just from its last meal. Image credit: © Salty Dingo - Australian Museum
15 million-year-old fish has shown researchers a lot about its ecology, just from its last meal; Photo: © Salty Dingo – Australian Museum

The fossils of a small freshwater fish were embedded in an iron-rich mineral called goethite at the McGraths Flat fossil site near Gulgong. The fish species, called Ferruaspis brocksi, have maintained their microscopic structural features, such as the outlines of cells that determine color, stomach contents, and shape of the fish.

The fossil has also retained remnants of color cells called melanophores, which include tiny melanin-containing granules inside called melanosomes. These details allowed the authors to determine that the fish were “counter-shaded”—darker at the top and lighter at the belly—with two stripes along their sides.

“In paleontology, there’s often so many gaps. Normally we just find isolated bones of a particular species,” said the lead author, Dr. Matthew McCurry, curator of paleontology at the Australian Museum. “We can’t often see the whole animal, and we rarely see things like soft tissues preserved.”

The fish’s stomach contents revealed that their bellies were full of antennae of phantom midge larvae (a type of insect), bits and pieces of half-digested wings, and even a small mussel or bivalve. This observation in particular paints a picture of what the final days were like for the fish, according to researchers.

Though the fossil was buried at the bottom of the lake, a second tiny bivalve attached to a fish fin may suggest they came from a nearby river that flooded or otherwise spilled over into the lake.

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.

Published in Vertebrate Palaeontology, the discovery offers the first detailed evidence in Australia for a group of fish called the Osmeriformes, which today include graylings and smelts.

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 provide 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,” said Prof Long.