Earth Detectives: Solving the Planet's Mysteries—Part 1
By Stewart Mittnacht, Staff Writer
Edited by Jess Peláez
Ancient lake sediments hold clues to the past. Sandwick Fish Bed, Orkney, Scotland.
The Earth has gone through enormous changes throughout its long history. Ecologists rely on Native American folklore, pollen samples from lake sediments, and genetic analysis to unravel how environments used to be. Restoring damaged ecosystems often requires a good deal of detective work alongside field work. A pressed flower left in the pages of an old journal can be just as illuminating as a walk through the woods. The sciences of paleontology and geology can even shed light on what might await us in our future.
But what if we could go back and visit ancient woods and explore habitats that were never seen by human eyes?
Let’s take a walk, three hundred million years ago. Welcome to the Middle Carboniferous!
Everywhere you look is a teeming rainforest. It’s not just here, however. This rainforest actually spans the entire planet. Don’t let the abundant foliage fool you, though. The name Carboniferous means “coal bearing” in Latin. All of that greenery will die and decay, but that will lead to an incredible transformation. Under heat and pressure, the leaf litter and other organic matter will change into fuel. That fuel will someday power the Industrial Revolution.
Back in the present, two types of rock dominate the rock layers of the Carboniferous: carbonates, such as limestone; and coal. Carbonate rocks form through deposits of marine plankton and shellfish. This evidence shows us that ancient Earth was a water world. Warm, shallow seas swallowed much of the land.
Back in the Carboniferous, continental uplift is steady, as larger tectonic plates begin to collide. In what would become the eastern United States, it’s an exciting time. A young mountain range is forming between the boundaries of the continents of Laurussia and Gondwana, and it is rife with volcanoes. Today, we call those mountains the Appalachians.
This growing landmass is nowhere near its present size. The shoreline extends about as far as today's eastern Texas. Here, the low-lying flatlands give way to a wide, shallow sea. This sea is home to both corals and more unusual reef-builders. Crinoids are strange relatives of starfish that resemble animal versions of lilies, and they are abundant.
Crinoid fossils from the early Carboniferous. Found in Iowa.
Primitive sharks are starting to diversify, too. They are stepping in for the now extinct armor-plated fish known as placoderms. Interestingly, sharks are not the only species moving in. The eurypterids are beginning to dominate the niche once occupied by the famous trilobite. T
he trilobite family is still around though, and should be able hold out for a few million years longer. Back in the present, we can see this fascinating survival story play out before our eyes. Fossils deep in the rocks formed during the Carboniferous tell the tale.
Heading back to the Carboniferous, you may be feeling the effects of the heat and humidity. Hopefully you brought along some bug spray, too! The Earth is rotating faster than it does today, so winds are stronger. That, combined with the hot oceans, whips up a constant stream of powerful tropical storms. Abundant rainfall blankets the Earth, and boggy marshland dominates the lowlands. This is the age of the insects.
These new herbivores dominate, armed with both plant-shearing mandibles and the power of flight. They have triggered an evolutionary arms race between plant and animal. The new plants in town have several advantages over the mosses that came before. Their deep root systems allow them to grow further inland, spreading across the continents.
The new reproductive defense of seeds protects young plants from hungry insect predators. Most important, we find lignin in these plants. Tough lignin fiber is the basis of wood, and for the plants of the Carboniferous, it is the ultimate defense. With no termites, fungi, or bacteria around to digest lignin, wood never rots. It accumulates where it falls, creating the largest carbon sink of all time. A rapid rise in oxygen accompanies this drop in carbon dioxide. The explosion of plant life on Earth has pumped up the concentration of this vital molecule.
An illustration of the Carboniferous landscape. "Carboniferous Pteridophyta (After Dana)" from the 1896 edition of Underwood's Native Ferns and their Allies.
Now let’s step back to the present. The Carboniferous coal forests were robust, biomass-rich habitats. These forests supported an enormous range of species for millions of years. Despite this, the collapse of this ecosystem was sudden and systemic. The exact cause is still debated, yet towards the end of the Carboniferous the Earth’s temperature began to drop. Glaciers formed in the southern continent of Gondwana. Cooler climate and lower sea levels shut down the coal swamp-supporting weather system. After that, the habitat began to fragment. What had once been an enormous and continuous rainforest retreated into small refugia. These refugia were clusters of forest clinging to the banks of rivers and lakes. This disaster is known as the Carboniferous Rainforest Collapse (CRC), and it was devastating. The once abundant amphibian population dropped and Earth’s biodiversity plummeted. But these losses would prove to be temporary, as a concept known as island biogeography took hold.
Island biogeography predicts when speciation is likely to occur. Speciation is the splitting of a genetic line. This can come to pass when a single species has two populations that separate. Speciation can happen when one organism's population establishes away from the mainland on a remote island. The rates of speciation will likely vary based on the remoteness of each island and its habitat suitability. The CRC reduced a once-global coal forest ecosystem to a series of small islands. This accelerated the rate of speciation for the survivors of the collapse. Reptiles were better adapted for the drier and cooler climate and diversified rapidly. They managed to supplant the amphibians. Our amphibious friends never recovered completely from the loss of their coal swamps.
Refugia formation in the Amazon due to human deforestation.
We cannot forget the lessons of the CRC. Man has, in effect, engineered a modern day CRC. Even a single highway road is enough of a barrier to separate a population for some species, such as snails and frogs. On a larger scale, the clearing of forests for farmland and the building of dams also serve as effective biological barriers. We can expect that as such activities continue the resulting losses of biodiversity will be devastating.
Steps can be taken to mitigate and even reverse these losses. We can build species corridors to prevent the formation of refugia. These species corridors come in a range of sizes and functions. Humble fish ladders are small, stepped canals alongside dams that fish can swim across. Massive planting projects can reconnect fragmented forests. With foresight and planning, we need not be the engineers of the next rainforest collapse.
In part two of our Earth Detectives series, we will fast forward 235 million years, to the heyday of the dinosaurs. Their demise will shed light on a bold new initiative to prevent our own eventual extinction.
Hat-wearing Stewart Mittnacht, shown here in Africa, is no stranger to fieldwork. He volunteers with Blueprint Earth as a science writer and field surveyor. He has a large collection of undergraduate degrees and grew up on a steady diet of Carl Sagan and Isaac Asimov.
References
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