An extraordinary fossil bed in the arid grasslands of the Australian continent, called McGraths Flat, really is the Lagerstätte that keeps on giving.
Just a few years after uncovering a trove of exceptionally preserved fossils, paleontologists have now described a brand new fish species that lived and died during the Miocene, 15 million years ago.
So perfectly intact is this animal that a team of paleontologists led by Matthew McCurry of the Australian Museum Research Institute could determine its coloration. They could even see what at least one specimen devoured for its last meal – the contents of which were still in its stomach after millions of years trapped in the iron-rich rocks of the fossil bed.
The fish has been named Ferruaspis brocksi, after paleontologist Jochen Brocks of the Australian National University, who discovered several specimens of the fossilized fish at McGraths Flat.
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“This little fish is one of the most beautiful fossils I’ve found at McGraths Flat, and finding the first vertebrate among the abundant plant and insect fossils was a real surprise,” Brocks says. “This discovery opens new avenues for understanding the evolutionary history of Australia’s freshwater fish species and ancient ecosystems.”
Fossilization is an intense process that often involves pressure and heat – it’s not kind on bone, never mind soft tissue. For an organism to be fossilized after it dies is rare. For soft tissue to survive is rarer still. Fossil beds in which the preservation level is so exquisite that soft tissue and fine details remain are known as Lagerstätten.
McGraths Flat is one such Lagerstätte, a formation of an iron-rich rock called goethite in which fossils were so intricately captured that structures smaller than a cell can be discerned. It was in this bed that Brocks found several beautiful fish of a species never seen before, and a family never found fossilized in Australia, freshwater smelt.
“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,” McCurry explains.
“This fossil is part of the Osmeriforms fish family – a diverse group of fish species within Australia that includes species like the Australian grayling and the Australian smelt. But, without fossils it has been hard for us to tell exactly when the group arrived in Australia and whether they changed at all through time.”
Because the fish specimens were so well preserved, the researchers were able to make observations about their lifestyle. F. brocksi was an opportunistic feeder that mostly dined on invertebrates; stomach contents included insect wings and a partial bivalve shell.
However, the most abundant ingredient was the larvae of midges – tiny flying insects that lay their eggs in water, where the larvae grow until they are ready to enter their adult life stage.
“One of the fossils even shows a parasite attached to the tail of the fish,” McCurry says. “It’s a juvenile freshwater mussel called a glochidium. These juvenile mussels attach themselves to the gills or tails of fish to hitch rides up and down streams.”
Using a powerful microscope, the researchers were even able to make out tiny, subcellular structures in the skin of the fish, called melanosomes, which give tissues their pigment. These revealed that the fish were darker on their backs, or dorsal sides, and paler on their tummies, or ventral sides. They even had two dark stripes running down the length of their bodies, near the spinal column.
“Fossilized melanosomes have previously enabled paleontologists to reconstruct the color of feathers,” marvels paleontologist Michael Frese of the University of Canberra and CSIRO, “but melanosomes have never been used to reconstruct the color pattern of a long extinct fish species.”
The McGraths Flat fossils have much to offer us yet. The researchers have described a wonderful ‘giant’ trapdoor spider discovered therein, but there are multitudinous other fossils from the site, including plants, insects, and even a bird feather that has not yet been formally described.
“The fossils found at this site formed between 11 and 16 million years ago and provide a window into the past,” McCurry says. “They prove that the area was once a temperate, wet rainforest and that life was rich and abundant in the Central Tablelands.”
The research has been published in the Journal of Vertebrate Paleontology.
