Largest ever salmon had tusks, not sabres

Reanalysis of the largest ever salmon, which lived several million years ago, shows that it sported tusk-like teeth which might have been used for fighting.

Oncorhynchus rastrosus was first described in the 1970s. It’s the largest species of salmon, reaching up to 2.7 metres in length and weighing between 200 and 400kg, roughly the size of Atlantic bluefin tuna.

Salmon size comparison with fisherman
Comparative size of the spike-toothed salmon to the largest living salmon and a 1.8m-tall angler. Credit: Ray Troll.

O. rastrosus fossils have been found along the US west coast and Japan.

It swam the North Pacific during the late Miocene (23–5.3 million years ago) and early Pliocene (5.3–2.6 million years ago). It was a period of global cooling and drying leading up to the last Ice Age.

Fossils of O. rastrosus include oversized front teeth measuring 2–3cm in length.

Because these teeth have been found away from skull fossils, their orientation has been subject to speculation until now. It was previously believed the front teeth pointed straight down, leading to the ancient fish to be dubbed the “sabre-toothed salmon.”

New research published in the open-access journal PLOS ONE confirms these teeth actually pointed to the side, more like the tusks of a warthog than the sabre teeth of Smilodon. The researchers say that the species should be renamed the “spike-toothed salmon.”

Previous research led by the same scientists has hinted at the front teeth pointing to the sides.

Now, CT scans of fossils found over the years “leave no doubt that the teeth of Oncorhynchus rastrosus projected laterally from the skull,” the authors write.

They’re not certain, but the researchers have some ideas about how the ancient salmon might have used their spikes.

“Discoveries like ours show they probably weren’t gentle giants,” says lead author Kerin Claeson, a professor of anatomy at Philadelphia College of Osteopathic Medicine in the US. “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 authors also found that, while there are slight differences in the anatomy of males and females of the ancient fish, both sexes had enormous, tusk-like teeth.

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