The next time you open wide to bite into a triple-decker sandwich, you'll be able to trace the origin of that ability back 505 million years — to a minnow-sized fish that once lived in a sea in what is now the Canadian Rockies.

Paleontologists have discovered fossils of an ancient fish named Metaspriggina that fill a missing link in the evolution of vertebrates and reveal where a key feature of vertebrates – our jaws – came from.  

The discovery is described in a paper published in Nature Wednesday by University of Cambridge earth sciences professor Simon Conway Morris and Jean-Bernard Caron, a paleontologist at the Royal Ontario Museum and the University of Toronto.

Caron called the fossils a "stunning find."

Metaspriggina reconstruction

Metaspriggina, seen in an artist's reconstruction, had big eyes on the top of its head and swam with a side-to-side motion like modern-day fish. (Marianne Collins, Simon Conway Morris and Jean-Bernard Caron)

"We consider this fossil among the oldest fish known anywhere," he added.

Even more amazing was how well the fossils are preserved. As he studied them, Caron realized he could see big eyes on the top of Metaspriggina's head and a pair of nostrils behind its eyes. Its muscles showed it swam with the sinuous, side-to-side motion of a fish, and it had a notochord – a structure that gives rise to the backbone in vertebrates.

"In some ways, you can recognize yourself in there," Caron said in an interview with CBCNews.ca. "He has all the features that all vertebrates will eventually share."

No bones, no jaws

In some ways, Metaspriggina was unlike the fish of today that swim in our aquariums and appear on our dinner plates – it had no bones and it didn't yet have jaws, so it couldn't open and close its mouth.

But along its throat, it had seven pairs of rod-like structures, likely made of cartilage, called gill bars or pharyngeal bars that got Caron and his colleagues very excited.

In modern fish, the front-most of those bars form the jaws – a feature that allowed them to move beyond filter feeding and grasp and swallow much bigger prey.

"The evolution of these bars in the first place had a profound impact into the evolution of vertebrates," Caron said. "As soon as you evolved jaws, vertebrates were able to explore new niches and diversify considerably."

Jaws gave animals such a competitive edge that they are now found in almost all vertebrates, including frogs, lizards, dinosaurs, birds and mammals like ourselves.

Morris and Caron's findings come from an examination of 44 fossils collected in 2012 at the recently discovered Marble Canyon fossil site in Kootenay National Park.

The site is part of a famous deposit called the Burgess Shale, first known from a site at nearby Yoho National Park. The Burgess Shale dates back to a time when the Canadian Rockies were part of a sea near the equator. Back then, the creatures that teemed in those waters were mostly arthropods — the group of invertebrates that includes crabs, spiders and insects.

The first two partial fossils of Metaspriggina were found in the Burgess Shale in Yoho National Park, where vertebrates made up only a tiny handful of the 200,000 fossils dug up at the site. When the fossils were first described, in 1993, researchers thought they were two different animals, neither of which was a vertebrate, Caron said.

Based on the examination of the 44 new fossils, the researchers estimate Metaspriggina grew no longer than 10 centimetres – about the size of a minnow – although that was fairly large compared to many of the other creatures of its time. Because its eyes were on the top of its head and designed to see above itself, the researchers think it probably lived on the sea floor. And since it had no jaws, it was probably a filter feeder.

Caron thinks its gill bars were probably used as "sort of a suction device" to increase water flow and help it filter food more efficiently.

As in modern day fish, Metaspriggina's gill bars come in seven pairs, with one member of each pair across its back and the other across its belly.

The front-most gill bars later gave rise to the upper and lower jaws of fish, which use the second pair to support their jaws and the last five to support their gills. In humans, some of the gill bars form our jaws, Caron said, while others form the bones inside our ears that allow us to hear.

Caron is currently preparing his team to return to Marble Canyon for 10 weeks this summer, where they hope to find more vertebrates.

"We're quite thrilled and we anticipate many new important specimens for understanding our roots."