When dinosaurs roamed, salmon swarmed in Arctic rivers

When most people think of the Cretaceous Period, salmon probably aren’t the first thing that comes to mind. Towering dinosaurs and steamy tropical landscapes usually steal the spotlight.

But in what is now northern Alaska, the picture was very different – and surprisingly familiar. Freshwater rivers and streams, instead of being dominated by dinosaurs or reptiles, teemed with ancient versions of fish we still see today, including early forms of salmon and pike.

In a recent study from the University of Alaska Fairbanks, researchers identified three new species of fish from 73 million years ago.

One of them, Sivulliusalmo alaskensis, has taken the spotlight as the oldest known member of the salmon family ever found in the fossil record.

A new first for salmon

“This is not only a new species; it’s the oldest salmonid in the fossil record,” said Patrick Druckenmiller, director of the University of Alaska Museum of the North.

The name Sivulliusalmo alaskensis combines the Inupiaq word for “to be first” with the Latin word for salmon.

Before this, the earliest known salmonid fossils came from British Columbia and Washington. This new find pushes the lineage back by around 20 million years.

The researchers also found fossils of two new species of pike, as well as the earliest Arctic example of the fish group that includes carp and minnows.

“Many of the fish groups that we think of as being distinctive today in the high-latitude environment in Alaska were already in place at the same time as dinosaurs,” said Druckenmiller.

Warm world, cold-loving fish

The presence of salmonids in such ancient, northern waters may seem surprising. After all, salmon today are known for thriving in cold environments.

But even during the Cretaceous – a time known for its warm global temperatures – the Arctic still experienced sharp seasonal changes.

“Salmon were already the kind of fish that do well in a place where those dramatic shifts were happening,” said Andrés López, curator of fish at the UA Museum of the North.

“Despite all of the changes that the planet has gone through, all of the changes in the geography and the climate, you still had the ancestors of the same groups of species that dominate the fresh waters of the region today.”

The researchers believe these fish adapted well to fluctuations in temperature and daylight. This resilience could explain how their descendants still thrive in the far north today.

Salmon and dinosaurs shared rivers

The fossilized fish remains came from the Prince Creek Formation, a site in northern Alaska known for its rich collection of dinosaur bones.

But while dinosaurs steal the headlines, it’s the tiny fossils that often hold the most detailed stories of the ecosystem.

“These types of fossils are often overlooked,” said Druckenmiller. “You couldn’t begin to understand a modern Arctic ecosystem without understanding the smallest animals that live there.”

That same principle, he argued, applies to ancient ecosystems. To better understand Cretaceous life, the team studied even the tiniest vertebrate fossils – especially those of ancient fish.

Finding the oldest salmon fossil

Finding those fossils wasn’t easy. Fish fossils are common in the Prince Creek Formation, but they’re often too small to spot during fieldwork. So the researchers hauled back buckets of sediment to their lab, where they sifted through them under microscopes.

The key fossils that revealed the new salmon species were tiny jaws, some small enough to rest on a pencil eraser.

To see them in detail, the research team – including scientists from Western University in Ontario and the University of Colorado Boulder – used micro-computed tomography. This technology let them digitally reconstruct the fossils in 3D.

“We found a really distinct jaw and other parts that we recognized as a member of the salmon family,” Druckenmiller said.

Born in dinosaur-era rivers

The team’s findings suggest that salmon may not have migrated north, as some previously thought. Instead, their evolutionary roots may lie in the North from the very beginning.

“Northern high latitude regions were probably the crucible of their evolutionary history,” said Druckenmiller.

That idea fits with the absence of lower-latitude fish species in the same rock layers. For now, the story of salmon seems to have started in the shadow of Arctic dinosaurs – in rivers that once flowed through a polar world that was cold, warm, and ever-changing all at once.

The full study was published in the journal Papers in Palaeontology.

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