Huge mosses soaring more than 130ft into the air dwarfed everything else in a rainforest that was swallowed by the sea 300 million years ago.

Palaeontologists have found the fossilised remains of gigantic club mosses alongside thousands of other plants in an extraordinary frozen forest.

It is a snapshot of life 300 million years ago during the Carboniferous era and is by far the biggest fossil forest yet discovered. Covering 10,000 hectares, it is the size of Bristol.

The rainforest was bursting with life until an earthquake caused an enormous chunk of land to drop downwards 15-30ft, taking primitive trees and shrubs with it.

Most of the forest was covered immediately by the sea and the rest sank within two months to be preserved in thick mud. It has now been found more than 200ft below ground in coalmines in Illinois.

The peaty soil turned to coal, which has been dug out, leaving passages along which fossilised trees and shrubs can be seen, complete with their trunks, branches and leaves.

Researchers say that the extensive finds have “transformed our understanding of the ecology of the Earth’s first rainforests”. They have identified 50 species of plant. Although modern rainforests have 600 species per hectare, this was a huge number for the Carboniferous era.

Club mosses survive today but grow only to about 2in tall, whereas in the ancient forest from the Carboniferous era they were giants. They rose high above the rest of the rainforest plants, which included ancient tree ferns and horsetails that grew up to 66ft tall.

Lower to the ground were a variety of shrubs, including pteridosperms, a seed-bearing plant that is the extinct link between ferns and conifers.

It was a time when the first rainforests were still evolving and the first reptiles had just appeared. It would be another 60 million years before the advent of dinosaurs.

Animal life would have been dominated by giant insects and arthropods, including a dragon-fly with a 2ft wingspan, a wood-louse more than 6ft long, and the ancestors of spiders.

Few fossils of insects have yet been found in the drowned forest but remains of eurytterids, a lobster-like creature that occasionally hauled itself out of the water to clamber around the undergrowth, have been located.

Dr Howard Falcon-Lang, of the University of Bristol, said: “You walk underground and see the plants from the bottom up. It was an amazing experience. The fossil forest was rooted on top of the coal seam, so where the coal had been mined away the fossilised forest was visible in the ceiling.”

He added: “This discovery allows us to track how the species make-up of the forest changed across the landscape, and how that species make-up is affected by subtle differences in the local environment.”

He and colleagues from the US mapped the forest via hundreds of miles of passages. Their report is published online today in the journal Geology.

At the time of the earthquake, Europe and North America were joined into a supercontinent over the equator. Similar fossils, though on a much smaller scale, were noted in mines in Britain in the 19th century but were reburied.

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