July 23. Collapse of the Pennsylvanian Rainforest

About 305 million years ago, five or six million years before the end of the Pennsylvanian Period, the rainforests and coal swamps came to a relatively abrupt end. This was an extinction event that seems to have caused a step-wise decimation of the plant life that was so abundant earlier in the Period.

The cause of this rapid decline in the tropical rainforests is not clear. One good candidate is the increasing glaciation in the southern hemisphere, in Gondwana. A short but intense pulse of glacier formation would have lowered sea level, perhaps enough to affect the swamps as well. And it could have led to cooler overall climates that would be challenging for the warm-weather adapted forests. But it may have been the variability as much as anything – rapid warming following a short glacial episode. Forests acclimated to a pretty standard, unchanging environment might not have been able to cope with the variations. The overall climate appears to have become considerably drier at the same time it cooled, and that too would have impacted rainforests negatively. 

Some evidence seems to suggest that the collapse of rainforests may have taken only a few thousand years, and that variable climates followed. There were survivors, but they occupied ecological islands where diversity was restricted for the older rainforest flora. The amount of carbon dioxide in the atmosphere had been declining through the Pennsylvanian, and reached some of the lowest levels in the past 600 million years toward the end of the Pennsylvanian and into the Permian Period. Was that a cause of the decline of the rainforests - less CO2 for them to use? Or was the decline in CO2 a consequence of the abundance of plants, taking it out of the atmosphere? That’s not clear, but the CO2 decline was a long, gradual affair, while the crash of the rainforest ecosystem was quite rapid.

There is a well-known periodicity to extinctions in earth history, at about 26 to 27 million years. It’s suspected to relate to some cosmic-scale phenomenon such as the solar system’s position within the Milky Way Galaxy, but the ultimate cause of cyclic extinction events is not known with certainty. But the Carboniferous collapse of the rainforest ecosystem does fall within one of the maxima of the 26-million-year cycle.

What was a disaster for the ferns and scale trees of the Pennsylvanian swamps may have been a blessing for the tetrapods, the vertebrates that were establishing themselves on land during Mississippian and Pennsylvanian time. A 2010 study by scientists at the University of Bristol and University of London in the U.K. found that following an initial decimation that coincided with the rainforest collapse, land animals diversified rapidly into the new ecological niches created by the plant extinctions. Amphibians were hard hit by the changes, and while they survived, they were outpaced by the animals that were adapted to drier conditions – namely, the ancestors of the reptiles and mammals. Reptiles and mammals did not rely on water to reproduce, and they had skins that allowed for better management of both temperature and moisture. The Pennsylvanian rainforest collapse may have created the circumstances that led to reptilian and mammalian diversification, leading eventually to the dinosaurs, and to us humans too.

—Richard I. Gibson

Links:
Rainforest collapse triggers tetrapod diversification


Floodplain evolution 

The Carboniferous Crisis

May 19. Devonian plants: the first forests

The Devonian saw the most fundamental change in the appearance of the land in several hundred million years. Plants, which had gotten started on land maybe by very late Ordovician time and for sure by the Silurian, began to spread across the landscape. For the first time, soils with organic matter began to form in abundance. You can visualize the development of soil as a sort of symbiotic relationship with plants – chemicals from plants, plus the mechanical action of their root systems, broke up rocks and changed them to the stuff we’d call soil. Soil in turn served as a reservoir of nutrients for future plants, as well as a substrate that was softer than hard rock, a place for plants to grow. The cycle of plant growth, death, and soils had begun.

Devonian forest

Early Devonian plants were still pretty primitive, but by the end of the period many diverse plants with true leaves and root systems were covering large areas. Many were relatives of modern ferns and horsetail rushes, but early varieties of other plants, such as pro-gymnosperms, spore-bearing plants that eventually gave rise to conifers, were also around. And the first true seed-bearing plants had evolved by the end of the Devonian.

You remember Cooksonia, from April 25, back in the Silurian? In contrast to those spindly stalks a couple inches tall, the Devonian saw the development of the first woody plants – trees – and the first real forests. The oldest known tree is called Wattieza, a fern-like tree from New York dating to about 385 million years ago, the Middle to Late Devonian. Some of these early trees were more than 30 feet tall. The oldest known forest, at Gilboa, New York, has upright stumps, roots and trunks that are interpreted as part of an extensive ecosystem that can only reasonably be called a forest. The stumps and trunks had been known since 1870, but it wasn’t until 2007 that the crowns of the trees were found and connected to show the geometry of the entire tree.

Apart from bragging rights for being the oldest forest, this discovery has huge consequences for the history of the earth. If large plants were widespread on the earth’s surface, it would have had a significant impact on the atmosphere – carbon dioxide in, oxygen out, and dying plants would be returning their elements – largely carbon, into that new product on the land, soil. All of this is part of the carbon cycle, the shifting of carbon around in the atmosphere, hydrosphere, and soils and rocks of the solid earth. That, in turn, has a great impact on the nature of climate and the kinds of life that can inhabit various ecological zones.

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May 19, 1871, was the birthdate of Reginald Aldworth Daly, at Napanee, Ontario. Daly worked as a geologist surveying the Canada-U.S. boundary for many years, leading to a massive report entitled North America Cordillera: Forty-Ninth Parallel. His work also resulted in a definitive book called Igneous Rocks and their Origins. Daly served as the head of the geology department at Harvard for 30 years. He worked on impact theory, and there are craters on the moon and Mars named for him.

Carl Beck, my graduate school major professor of mineralogy, was also born on this day in 1916. He put me on the convoluted path that took me from kidney stone mineralogy to geophysics in oil exploration.

—Richard I. Gibson

Devonian landscape painted by Eduard Riou, 1872 (public domain)

Link: Gilboa fossil forest