Scientists in Nova Scotia hope a huge fossil find will reveal more about the evolution of plants.

The 300-million-year-old rock may contain one of the largest ever tree fern fossils found in Canada, geologists say. It differs from other fern fossils because of the way its branches leave the trunk.

"If it turns out to be what we think it is, you know, then the reconstruction of the accepted structure or architecture of a tree fern, of a carboniferous tree fern, that is, will have to be rethought," said retired paleontology Prof. Erwin Zudrow of University College of Cape Breton.

When the fern was growing, Nova Scotia was covered by a steamy rain forest. During the carboniferous period, plants first developed the ability to grow seeds. Scientists hope the boulder-sized fossil will shed light on how the process evolved.

A tractor driver unearthed the 315-kilogram fossil while digging in an open pit mine.

"This is significant because it is the largest found in Nova Scotia and maybe Canada," said Debra Skilliter, head geologist at the National History Museum of Nova Scotia in Halifax.

At four to five metres, the fossil is bigger than anything in the museum's collection. Bigger fossils like the Cape Breton find allow scientists to see the whole plant.

It will take a lot of close study to interpret the stems, but the fossil has already attracted international interest. British scientists may join in the effort to piece the fossil's story together.