Feature key to evolution found in fossil in Canadian Rockies

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A fictional ancestor has been found to be real and has been located in the Canadian Rockies.

Scientists studying fossils have, for decades, used a fictional creature to fill the gap in hard evidence when explaining how animals evolved. They were guessing how jaws came to be, and now fossils found in British Columbia prove their imaginary specimen was real, according to a new paper published Wednesday.

"Nobody ever expected, I think, to find such an animal," said Simon Conway Morris from the University of Cambridge and one of the paper's authors. "Evolving a jaw is really just one of the really great breakthroughs in the history of life."

Researchers believe fish fossils collected near Marble Canyon in Kootenay National Park show a feature key to the evolution of jaws in vertebrates – the first time the characteristic has been found in the early fossil record. The discovery is crucial to solving the evolutionary puzzle because jaws would allow fish slurping up plankton to transform into predators, capable of catching and consuming larger chunks of food – and therefore energy.

The fossilized fish, known as Metaspriggina, dates back to the Cambrian period, making it about 505-million years old. The Royal Ontario Museum in 2012 collected 44 Metaspriggina fossils from the Marble Canyon site, which is similar to the famous Burgess Shale fossil bed in B.C.

The animal had seven sets of gill bars on each side of its body, researchers found. Gill bars gave the animal structure, and were probably similar to cartilage. Bones did not exist then. The rods closest to the head, however, were not associated with gills, and were slightly larger than its six counterparts. The spare set may have been used to allow more water, and therefore food, into the mouth, Jean-Bernard Caron, curator of invertebrate paleontology at the ROM and a professor at the University of Toronto, said.

But most importantly, these rogue gill bars led to the evolution of jaws in vertebrates, Mr. Caron and Mr. Morris argue in a paper in Nature.

Modern fish have five sets of gill bars, Mr. Caron said, so it is possible another set of these arches found in the Metaspriggina freed itself of its duties associated with gills and joined the other gill-free gill bars to further strengthen the jaw.

The specimen, Mr. Morris says, is strikingly similar to the "hypothetical ancestor" usually in the form of a fish scientists have long relied upon. That animal was a "convenient fiction" and a "useful construct," he said.

Two Metaspriggina fossils existed prior to the 2012 expedition to Marble Canyon, but those specimens, collected from the Burgess Shale by Charles Walcott and housed at the Smithsonian Institution, are poorly preserved. The details that allowed Mr. Caron and Mr. Morris make their hypothesis on gill bars and jaws are found only on the specimens collected from the Marble Canyon site, about 40 kilometres southeast of the main Burgess Shale site.

About 10 researchers will use a mix of brute force and delicate nudging on Marble Canyon's mountain rocks in July and August, Mr. Caron said. They will haul jackhammers to the site, as well as hammers, chisels, and other tools of the trade. Their search will be precise, covering about 15 square metres by 2 metres deep, documenting their findings in the rock layers at intervals 5 centimetres apart.

Fossils in the area are exceptionally well-preserved, and scientists believe this may be owing to rapid burial, critically low levels of oxygen, or clay minerals in the area. The fossils capture soft tissue such as eyes and guts – a rarity in the fossil record. Mr. Caron, alongside another researcher, discovered about 10 creatures no one had ever seen before near Stanley Glacier in Kootenay National Park a few years ago. The Stanley Glacier fossil bed, like Marble Canyon, is similar to the Burgess Shale.

"This area of Kootenay National Park is going to be a hotbed for science," Mr. Caron said.