Carboniferous rainforest collapse

The Carboniferous rainforest collapse could be the result of colder, dryer climate

The Carboniferous Rainforest Collapse (CRC) was an extinction event that occurred around 305 million years ago in the Carboniferous period. Vast coal forests (so called because the compacted remains of the dense vegetation formed coal seams) covered the equatorial region of Euramerica (Europe and America). Climate change devastated tropical rainforests, fragmenting the forests into isolated 'islands' and causing the extinction of many plant and animal species. The change was abrupt, happening during the Moscovian and Kasimovian stages of the Pennsylvanian.

Extinction Patterns

Ferns and treeferns from Mount Field National Park, giving an impression of how a Carboniferous rainforest may have looked.

In the Carboniferous, the great tropical rainforests of Euramerica supported towering lycopsids, a heterogeneous mix of vegetation, as well as a great diversity of animal life: giant dragonflies, millipedes, cockroaches, amphibians, and the first reptiles.

Plants

The rise of rainforests in the Carboniferous greatly altered the landscapes by creating low-energy, organic-rich anastomosing river systems with multiple channels and stable alluvial islands. The continuing evolution of tree-like plants increased floodplain stability by the density of floodplain forests, the production of woody debris, and an increase in complexity and diversity of root assemblages.

Collapse occurred through a series of step changes. First there was a gradual rise in the frequency of opportunistic ferns in late Moscovian times. This was followed in the earliest Kasimovian by a major, abrupt extinction of the dominant lycopsids and a change to treefern dominated ecosystems. This is confirmed by a recent study showing that the presence of braided, meandering, and branching rivers, occurrences of large woody debris, and records of log jams decrease significantly at the Moscovian-Kasimovian boundary. Rainforests were fragmented forming shrinking islands further and further apart and in latest Kasimovian time, rainforests vanished from the fossil record.

Invertebrates

Before the collapse, terrestrial invertebrates were diverse and included annelids, molluscs, and arthropods,including giant arthropleurids. Most were detritivorous, eating 'litter' off of the forest floor, however, some had evolved herbivorous and predatory forms.

Vertebrates

Diet

Before the extinction event, terrestrial vertebrates were predominantly amphibians and a few basal amniotes (‘reptiles’). Amphibians were tied to waterside habitats and were primarily piscivores, though a few had evolved insectivory.

Diversity

Before the collapse, animal species distribution was very cosmopolitan: the same species existed everywhere across tropical Pangaea, but after the collapse each surviving rainforest island developed its own unique mix of species. Many amphibian species became extinct while reptiles diversified into more species after the initial crisis. These patterns are explained by the theory of island biogeography, a concept that explains how evolution progresses when populations are restricted into isolated pockets. This theory was originally developed for oceanic islands but can be applied equally to any other ecosystem that is fragmented, only existing in small patches, surrounded by another habitat. According to this theory, the initial impact of habitat fragmentation is devastating, with most life dying out quickly from lack of resources. Then, as surviving plants and animals reestablish themselves, they adapt to their restricted environment to take advantage of the new allotment of resources and diversify. After the Carboniferous Rainforest Collapse, each pocket of life evolved in its own way, resulting in a unique species mix which ecologists term endemism.

Biotic Recovery & Evolutionary Consequences

Animals

Terrestrially adapted early mammal-like reptiles like Archaeothyris were among the groups who quickly recovered after the collapse.

This sudden collapse affected several large groups, labyrinthodont amphibians were particularly devastated, while the first reptiles fared better, being ecologically adapted to the drier conditions that followed. Amphibians must return to water to lay eggs; in contrast, reptiles - whose amniote eggs have a membrane ensuring gas exchange out of water and can therefore be laid on land - were better adapted to the new conditions. Reptiles acquired new niches at a faster rate than before the collapse and at a much faster rate than amphibians. They acquired new feeding strategies including herbivory and carnivory, previously only having been insectivores and piscivores.

The depletion of the plant life also contributed to the deteriorating levels of Oxygen in the atmosphere, which facilitated the enormous arthropods of the time. Due to the decreasing Oxygen, these sizes could no longer be accommodated, and thus between this and the loss of habitat, the giant arthropods were wiped out in this event, most notably the giant dragonflies and millipedes (Meganeura and Arthropleura).

This extinction event had longterm implications for the evolution of amphibians. Under prolonged cold conditions, amphibians can survive by decreasing metabolic rates and resorting to overwintering strategies (i.e. spending most of the year inactive in burrows or under logs). However, this is only a short term strategy and not an effective way of dealing with longterm unfavorable conditions, especially desiccation. Since amphibians had a limited capacity to adapt to the drier conditions that dominated Permian environments, many amphibian families failed to occupy new ecological niches and went extinct.

Possible Causes

Atmosphere

Atmospheric levels of carbon dioxide crashed to one of its all time global lows in the Pennsylvanian and early Permian.

Climate

There are several hypotheses about the nature and cause of the Carboniferous Rainforest Collapse, some of which include climate change. Specifically, at this time climate became cooler and drier, this is reflected in the rock record as the Earth entered into a short, intense ice age. Sea levels dropped by a hundred metres and glacial ice covered most of the southern continent of Gondwana.

The cooler, drier climate conditions were not favourable to the growth of rainforests and much of the biodiversity within them. Rainforests shrank into isolated patches, these islands of rainforest were mostly confined to wet valleys further and further apart. Little of the original lycopsid rainforest biome survived this initial climate crisis, only to survive in isolated refugia.

Then a succeeding period of global warming reversed the climatic trend; the remaining rainforests, unable to survive the rapidly changing conditions, were finally wiped out. As the climate aridified through the later Paleozoic, the rainforests were eventually replaced by seasonally-dry biomes. Though the exact speed and nature of the collapse is not clear, it thought to have occurred relatively quickly in geologic terms, only a few thousand years at most.

Impact

Major meteoroid events near the time of the CRC included the formation of the Weaubleau-Osceola structure, a serial impact which has been dated to 330-335 million years ago and would have affected the Euramerican continent.

Volcanism

After restoring the center of the Skagerrak-Centered Large Igneous Province (SCLIP)using a new reference frame, it has been shown that the Skagerrak plume rose from the core–mantle boundary (CMB) to its ~300 Ma position. The major eruption interval took place in very narrow time interval, of 297 Ma ± 4 Ma. This rift formation coincides with the Moskovian/Kasimovian boundary and the Carboniferous Rainforest Collapse.

Multiple Causes

In recent years, scientists have put forth the view the idea that many of Earth's largest extinction events were due to multiple causes that coincided in time. Proponents of this view suggest multiples causes because they either don't see a single cause as sufficient in strength to cause the mass extinctions or believe that a single cause is likely to produce the taxonomic pattern of the extinction. Two of Earth's largest extinction events have been hypothesized to be multi-causal in nature:

The cause of the Permo-Triassic extinction is unclear and some authors have indicated that it may be best explained by a "Murder on the Orient Express Scenario" where multiple causes contributed to a devastating impact on life. Possible causes supported by strong evidence include the large scale volcanism at the Siberian Traps, the releases of noxious gases, global warming, and anoxia.

David Archibald and David E. Fastovsky discussed a scenario combining three major causes to the K-T extinction: volcanism, marine regression, and extraterrestrial impact, together wiping out the non-avian dinosaurs 65 million years ago.

The specific cause of the CRC is not known, but certainly a multiple cause scenario is a possibility.

Fossil Sites

Fossil lycopsid, probably Sigillaria, from Joggins, with attached stigmarian roots

Many fossil sites around the world reflect the changing conditions of the Carboniferous Rainforest Collapse.

• Hamilton, USA
• Jarrow, UK
• Linton, USA
• Newsham, USA
• Nyrany, Czechoslovakia
• Joggins, Canada

The Joggins Fossil Cliffs on Nova Scotia's Bay of Fundy, a UNESCO World Heritage Site is a particularly well-preserved fossil site. Fossil skeletons embedded in the crumbling sea cliffs were discovered by Sir Charles Lyell in 1852. In 1859, his colleague William Dawson discovered the oldest known reptile, Hylonomus lyelli, and since then hundreds more skeletons have been found.

Current Relevance

Fossil Fuels

The plant material that was lost during this extinction event was transported by water to low lying areas in bogs, marshes, lakes and inland seas. It decayed and as more material covered it, it was compressed, heated and eventually became coal, a fossil fuels in the form of a combustible black or brownish-black rock. Coal is the largest source of energy for the generation of electricity worldwide, as well as one of the largest worldwide anthropogenic sources of carbon dioxide releases. Gross carbon dioxide emissions from coal usage are slightly more than those from petroleum and about double the amount from natural gas.

Hypothesized Future Rainforest Collapse

Modern man-made deforestation for agriculture in southern Mexico.

Tropical and temperate rainforests have been subjected to heavy logging and agricultural clearance throughout the 20th century and the area covered by rainforests around the world is shrinking. A classic pattern of fragmentation is occurring in many rainforests including those of the Amazon, specifically a 'fishbone' pattern formed by the development of roads into the forest.[2] and littoral rainforests growing along coastal areas of eastern Australia is now rare due to urban development. A number of scientific and environmental groups have hypothesized that a combination of anthropogenic climate change and deforestation may lead to future rain forest collapse.

Modern rainforest collapse may result in massive loss of biodiversity; this is of concern not only for the loss of a biome with many untapped resources but also because animal species extinction is known to correlate with habitat fragmentation. Biologists have estimated that large numbers of species are being driven to extinction (possibly more than 50,000 a year; at that rate, says E. O. Wilson of Harvard University, a quarter or more of all species on Earth could be exterminated within 50 years) due to the removal of habitat with destruction of the rainforests.

The application of palaeodata to the present conditions of this planet is still a science in its infancy, but presumably a better understanding of the process of habitat fragmentation and rainforest collapse in the past may help determine the best course of action to preserve similar ecosystems today. Specifically, examining the conditions that led to the Carboniferous Rainforest Collapse and the recovery of ecosystems after the extinction, may help contribute suitable solutions to resolving the current crisis.

Further reading

• "The Carboniferous Crises"
• "Rainforest collapse kickstarted reptile evolution"
• "Ancient Rainforest Collapse Increased Reptile Diversity"
• "Brave new reptilian world"
• "Carboniferous climates and amniote origins"
• "Early forests tamed wild rivers"