The Mystery of the Sabertoothed Salmon

Image Credit: Jacob Biewer, Sankey et al., 2015

The charisma of enormous, slashing teeth is undeniable. Despite the fact that there are a myriad of fascinating prehistoric carnivores, the big mammals that the documentaries, big-budget films and kid’s shows seem to come back to are the sabre-toothed carnivores. Massive slashing teeth are actually a trait that has popped up a lot over the course of the Earth’s history, with at least three different groups of cat or cat-like mammal evolving them as a hunting mechanism. As well as a fish.

Now for you, that last sentence may not have been as mind blowing as it was for me. Who cares if a fish had big teeth you may say. Nimravids had furry teeth pockets to slide their sabres into (gross), and you’re concerned with some weird salmon?

For starters, look at these creeps.

Oncorhynchus_rastrosus

Image Credit: Apokryltaros, CC BY-SA 3.0

This is the first image I ever saw of them, while watching PBS Eons video on sabertooth cats (great watch by the way, here’s the link). They were estimated to be about two metres long, so you can imagine how big that shark lurking in the background is.

They’re called Oncorhynchus rastrosus, but they’re commonly known as sabertoothed salmon. Oncorhynchus are a fairly common genus, there are plenty of examples alive today, including the sockeye, pink and Chinook salmon, and the rainbow trout. Apart from those strange ‘teeth’, the salmon don’t look that different from a sockeye salmon (estimated to be their closest living relative), albeit one that is the length of your average basketball player.

All of those examples I mentioned do fine without the use of saberteeth. And we’re talking about a salmon after all. So why the hell were they needed? What was the advantage conferred in evolving saberteeth?

Sparse Records

Ok, so for starters there is almost no information on these guys in the literature. Typing in Oncorhynchus rastrosus on the Web of Science will get you next to no results. It surprised me, given that a) salmon are a popular fish that turn up everywhere and b) these things apparently had SABERTEETH. Why don’t these things have obscure cult following, like alligator gars or sawfish?

What managed to find was that the teeth were found next to a salmon fossil in Oregon, USA, back in 1972 by Ted Cavender and Robert Miller. It was dated in the late Miocene, an age that ran from 23 to 5 million years ago. There was also a landlocked dwarf form of the fish The assumption was made back then that the teeth stuck straight down, much like they do on the big cats (hence the depiction above).

But even a two metre salmon doesn’t need tusks to feed. This was confirmed back in 2007, when Thomas Eiting and Gerald Smith had a look at the gill raker structure of our new favourite fish. Gill rakers are bony structures which fish use to filter food from the water. If you’ve got lots of long gill rakers, you’re well-equipped to filter in lots of smaller prey, like planton. A few short gill rakers mean you’re probably after small fish or amphibians. The Sabertoothed Salmon was that first one. A planktivore, which probably went extinct as the abundance of zooplankton in oceans dropped. So what gives.

Gill-rakers on a Bighead Carp (Image Credit: Asian Carp Regional Coordinating Committee, CC BY 2.0)

Gill-rakers on a Bighead Carp (Image Credit: Asian Carp Regional Coordinating Committee, CC BY 2.0)

The Let-Down

In 2016 a well-preserved fossil turned up, again in Oregon, with the teeth attached. They weren’t saberteeth, which is disappointing. Karen Claeson and her team stated that they were in fact tusks, which is… weirder? I can’t reproduce the image here, but check out this article by Franz Anthony for a great depiction of what they might have looked like.

In any case, they didn’t look like moustachioed creeps all their life. Many anadromous salmonids (fish that spend time in the ocean feeding before moving back into freshwater to spawn) undergo stark physiological changes when they head back to their spawning grounds. When they’re in the ocean, they look like pleasant, very edible fish. Right before they begin their journey upstream is when male salmon often make their transformation into meth-addicted horrors (a purely subjective take). The bright red sockeye or the humpbacked pink salmon are the river forms of their respective species, both of which also grow a kype; that’s that hook on either the upper and lower jaws. The pink salmon’s transformation is particularly weird – they practically rot, to the point that if you catch one you can just about stick a finger through it. There hasn’t been any evidence of this transformation in the sabretoothed (or as they’re now referred to, spike-toothed) salmon, with no signs of a kype being found in fossil records, but that doesn’t mean it didn’t occur.

So that brilliant depiction above MAY be somewhat wrong in more than just its approximation of the tusks orientation, unless that shark has found itself quite far upstream (not impossible, bull sharks have been found over 1000 kilometres up the length of the Mississippi river before). Additionally, Julia Sankey’s 2016 paper (from which that awesome cover image originates) stated that as the fish travelled upstream, the few regular teeth they had were probably reabsorbed. The tusks stayed, however, and were perhaps used to move gravel around in their spawning grounds, or for fighting with other fish.

In any case, this was a wild ride for a freshwater ecologist. I hope you enjoyed it.

Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who is posting this while he should be helping his Master’s student out. You can read more about his research on his Ecology for the Masses profile here, and follow him on Twitter here.

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