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The world’s largest ever salmon was a filter feeder with warthog-like tusks
An enormous extinct species of salmon wasn’t quite the fish it was thought to be.
Rather than having ferocious fangs, new research reveals that Oncorhynchus rastrosus had large, tusk-like teeth emerging from the sides of its head.
An ancient aquatic giant was more of a watery warthog than a sabre-toothed salmon.
Living over five million years ago, Oncorhynchus rastrosus has grabbed attention for its enormous size – measuring up to 2.7 metres long – and its huge pair of front teeth.
When scientists in the 1970s reconstructed what the giant salmon looked like, it seemed obvious that these enormous teeth were fangs, but new research shows this wasn’t the case.
Using new fossils and CT scans, a team of scientists have now shown they were more like tusks, sticking out from the fish’s upper jaw. The lead author of the new study, Professor Kerin Claeson, explains that these teeth, found in both males and females, would have had a variety of roles.
“We have known for decades that these extinct salmon from central Oregon were the largest to ever live,” Kerin says. “However, discoveries like ours show they probably weren’t gentle giants.”
“These massive spikes at the tip of their snouts would have been useful to defend against predators, compete against other salmon, and ultimately build the nests where they would incubate their eggs.”
The findings of the study were published in the journal PLOS One.
Like their modern relatives, it's believed Oncorhynchus rastrosus migrated from the ocean up river to spawn. Image © Tory Kallman/Shutterstock.
Reconstructing the ‘sabre-toothed salmon’
Figuring out what the animals of the past looked like isn’t easy. There’s often no skin or soft tissue to work with, while many bones could be missing. For the parts of the skeleton which are left, the process of burial can squash, bend and stretch the fossil.
These challenges were among the problems facing researchers who first attempted to reconstruct the appearance of O. rastrosus. While the discovery of a largely complete skull in Gateway, Oregon, helped to reveal what its head might have looked like, the teeth weren’t attached meaning that the scientists were unsure where to put them.
When reconstructing extinct animals, researchers will often look to living relatives of the animal but modern salmon aren’t easily comparable to O. rastrosus. For a start it was more than 70 centimetres longer than the largest living species, while its teeth were on a totally different scale.
Instead, the size of the teeth meant that researchers looked to sabre-toothed cats for inspiration. The fish was even originally named Smilodonichthys, and while later study saw the fish’s scientific name change, it’s common description as the ‘sabre-toothed salmon’ stuck.
But fossils unearthed about 10 years ago questioned this interpretation. New excavations in Gateway uncovered skulls which still had their front teeth in place. They revealed that the teeth stuck out to the side, prompting researchers to go back to the original fossils and reinvestigate the giant salmon.
Compared to the the king salmon, the largest living in the Pacific today, Oncorhynchus rastrosus was much larger. Image adapted from © Ray Troll, licensed under CC-BY 4.0 via EurekAlert!.
The life of the largest salmon
By taking CT scans of the historic and newly discovered fossils, the team were able to get a deeper insight into how these giant fish might have lived.
The researchers believe that the salmon’s enormous size meant it was a filter feeder. Many of the world’s largest living fish are also filter feeders, with animals like the whale shark using their large size to gulp down huge amounts of water to filter out the plankton.
This idea is supported by O. rastrosus having more gill rakers than any other known salmon species, living or extinct. These small structures in the mouth are used to filter out plankton. The team think that the fish may have targeted dense swarms of plankton or filtered prey out of mud on the seabed.
This means it was very unlikely that the giant salmon’s tusks were used to take down prey like the sabre-toothed tigers. As a result, researchers have dubbed O. rastrosus the ‘spike-toothed salmon’ instead, to recognise that the teeth were probably multifunctional tools rather than predatory weapons.
The team also found subtle differences in the density and shape of the bones between specimens, which they suspect could have been different sexes. In particular, they believe males developed a structure known as a kype. This is the hooked jaw that some modern male salmon grow to help to establish dominance during the breeding season.
This suggests that although O. rastrosus was radically different from modern salmon in its diet, it was, in many ways, also quite similar to its living relatives.
