How did fish evolve into four-legged beasts that roam the land? A key part of that mystery has been solved by fossils found on a Nova Scotia beach.

Tetrapods, named for their four limbs, are fish-like amphibians that made the transition from the sea to the land about 370 million years ago. A 30-million-year gap in the fossil record of tetrapod evolution has puzzled scientists since the 1950s, says Jason Anderson, associate professor of veterinary anatomy at the University of Calgary.

But tetrapod fossils found at Blue Beach, located on the Avon River estuary on the Bay of Fundy near Wolfville, N.S., suggest the gap may have been an illusion. The findings were published in a new paper published in PLOS ONE by Anderson and Canadian and British colleagues.

Chris Mansky and Jenny Clack

Chris Mansky of the Blue Beach Fossil Museum and Jenny Clack of the University of Cambridge, co-authors of the new paper, walk along the intertidal beach at the base of a cliff at Blue Beach, N.S. The site is a trove of fossils from about 350 million years ago, when vertebrates first started emerging onto land. (Jason Anderson)

Tetrapods first developed flipper-like limbs and started crawling out of the water about 370 million years ago, Anderson told CBC's Quirks & Quarks. At that time, they were quite large, about the size of a person, and very fish-like, with tail fins and internal gills. They also had variable numbers of fingers and toes — "up to eight," Anderson said in an interview that airs Saturday.

Femur of early tetrapod

Pictured is a femur of an early tetrapod found at Blue Beach that was very similar to Acanthostega. (Jason Anderson)

Thirty-million years later, tetrapods had become well-adapted to moving around on land and had the five fingers and toes that most amphibians, reptiles, birds and mammals have today. They had lost their gills and tail fins, and developed the muscles for holding their bodies up from the ground, as shown by their footprints.

"There are no trail marks for either tails or stomachs," Anderson said.

The apparent gap in the fossil record between the fish-like and more advanced tetrapods was first written about by U.S. paleontologist Al Romer and has since been named "Romer's gap." To explain the gap, some paleontologists proposed that low oxygen during that period prevented tetrapods from evolving.

Now, Anderson, Chris Mansky of the Blue Beach Fossil Museum in Nova Scotia, and their collaborators have found and described fossils from at least four different kinds of tetrapods that lived during the early part of Romer's gap.

The fossils show that both fish-like and advanced tetrapods lived alongside one another at that time.

Tetrapod pelvis fossil

This is a pelvis of a more advanced tetrapod that likely had five fingers and toes. (Jason Anderson)

That means the advanced tetrapods didn't just appear at the end of Romer's gap.

"They were present much earlier than we gave them credit for," Anderson said. It also means that Romer's gap may be a misinterpretation of the fossil data.

"This gap itself isn't really a thing," he said. "It's just the result of a poor fossil record and the difficulty of finding rocks from this particular time."