Dry scrublands were a part of the Carboniferous landscape
9 October 2009, by Sara Coelho
Many natural history museums show the Earth during the Carboniferous period, 300 million years ago, as covered by tropical forests. But new research suggests that the exhibits need a makeover because dry scrublands may have been equally important.
Carboniferous coal forests.
'The coal forests that covered Earth 300 million years ago were the first tropical rainforests to evolve on the planet,' says Dr Howard Falcon-Lang, a palaeontologist from Royal Holloway, University of London.
The forests developed into the massive coal deposits we burn for power today, but millions of years ago they harboured a rich ecosystem with many species of plants and animals, including giant dragonflies and early reptiles. Nowadays, coal forests are the poster image of the Late Carboniferous period, known as the Pennsylvanian in North America, and feature in museum exhibits all over the world.
'What we found is evidence for a dynamic system, with coal forests and dry shrublands contracting and expanding with climate events in 100,000 year cycles.'
But Falcon-Lang and his United States colleagues found evidence for a more complex scenario. 'We challenge the long held view that coal forests were continuous over millions of years,' he says.
The findings come from a rock outcrop in the US state of Illinois. 'We were looking for evidences of climate cycles in Pennsylvanian rocks,' he says. At the time the Earth was going through a succession of glacial and interglacial periods, each with different climate features. During glacial peaks the tropics were dry and the sea level was low; in between glaciations, the tropics had a wet climate and sea level rose as ice caps melted.
Falcon-Lang and his team were interested in the deep valleys cut in the landscape during glacial stages. They found that at the bottom of these valleys lived a plant community very different from coal forests.
Unlike the ferns, clubmosses and horsetails of the coal forests, this 'plant community was dominated by seed plants, especially conifer trees which were a rare and novel group at the time,' says Falcon-Lang. These plants were well-adapted to dry conditions and there is evidence that fires swept regularly across the landscape.
The scenery was 'mostly sparse forests made of small trees and shrubs,' describes Falcon-Lang. 'The soils were arid and the climate was dry. This was not a continuous coal-like forest,' he adds.
The scrubland landscape took over the coal forests during the peaks of glaciation, when the climate was dry. And when glacials melted and the climate turned warmer, tropical coal forests regained the upper hand, suggests the report published in the October edition of the journal Geology.
'What we found is evidence for a dynamic system, with coal forests and dry shrublands contracting and expanding with climate events in 100,000 year cycles,' says Falcon-Lang. 'The coal forest was not dominant for millions of years as thought before,' adding that the dry scrublands were probably as important.
Falcon-Lang thinks that coal forests became icons of the Pennsylvanian because they have been studied for the past 200 years and yield many famous well-preserved fossils. But in fact, they are over-represented in the fossil record because it is easier for a plant fossil to be preserved in a boggy swamp than in an arid valley.
H.J. Falcon-Lang, W.J. Nelson, S. Elrick, C.V. Looy, P.R. Ames and W.A. DiMichele. Incised channel fills containing conifers indicate that seasonally dry vegetation dominated Pennsylvanian tropical lowlands. Geology 2009, vol 37 pp 923-926. doi: 10.1130/G30117A.1
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