The oldest fossils of Oncorhynchus rastrosus date from around 12 million years ago, along the coast of California. This salmon lived along the Pacific coasts of North America and Japan, reaching sizes of up to 2.4 meters (7 ft 10 in) and weighing 200 kg (440 lb).

Previous studies of the fossils showed that the Pacific salmon had strange, tusk-like teeth. Now, a new analysis suggests they were pointed sideways — and likely used for fighting.

Not a saber-toothed salmon

Salmon are pretty interesting creatures. They’re powerful swimmers that migrate over thousands of miles, and they grow to pretty impressive sizes. But none were as big as O. rastrosus.

This gargantuan species was the largest salmon to ever live (as far as we know, at least). From the very first fossils, it was obvious that it had some oversized teeth. Initially, researchers thought these teeth pointed backwards, like fangs. They assumed this because the fangs were found disassociated from the rest of the skull, which is more likely to be the case if they face inwards. This would have made the salmon a “saber-toothed salmon”, similar to the famous “saber-toothed cat.”

In the new study, researchers performed Computed Tomography (CT) scans to get a new view of these fossils. CT is a non-invasive imaging technique that enables researchers to visualize structures not visible to the naked eye, without damaging the fossil.

CT scans can expose the most minute details of an organism’s anatomy, including tiny bones, soft tissues, and even remnants of last meals providing unprecedented insight into the biology, behavior, and evolution of extinct species. In this case, the CT scan showed that the teeth actually pointed sideways, much like a warthog. So, it’s not a “saber-toothed salmon”, it’s a “spike-toothed salmon”, researchers say.

Feisty salmon

It’s not entirely clear what the teeth were for, although they were probably not used for peaceful purposes or display. Researchers suspect they would have been used for fighting, either amongst themselves or against other species.

“We have known for decades that these extinct salmon from Central Oregon were the largest to ever live. 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,” says Kerin Claeson, lead author and professor of anatomy at Philadelphia College of Osteopathic Medicine.

The team suspects the teeth weren’t used for display because both the males and the females seemed to have them — so there must have been a practical purpose.

“We also stress that females and males alike possessed the enormous, tusk-like teeth. Therefore, the sexes were equally fearsome,” adds Brian Sidlauskas, professor in the Department of Fisheries, Wildlife and Conservation Sciences at Oregon State University.

The teeth could still be used as a tool, possibly for digging out nests. But one thing that’s clear is that they weren’t used for hunting. This giant salmon was a filter feeder that fed on plankton, the team notes.

This latest research underscores the importance of continually reevaluating our assumptions in the scientific narrative. The revelation that this gargantuan salmon wielded its spike-like teeth sideways opens new avenues for understanding the ecological dynamics of ancient marine environments.

Claeson KM, Sidlauskas BL, Troll R, Prescott ZM, Davis EB (2024) From sabers to spikes: A newfangled reconstruction of the ancient, giant, sexually dimorphic Pacific salmon, †Oncorhynchus rastrosus (SALMONINAE: SALMONINI). PLoS ONE 19(4): e0300252. https://doi.org/10.1371/journal.pone.0300252