Chances are, if you’ve turned on a lightbulb or a computer,
you’ve harnessed the power of some very ancient, and very strange … trees.
The vast majority of the electricity produced in human history has come from coal.
And like all fossil fuels, coal is made from the remains of extinct organisms that have
been exposed to millions of years of heat and pressure.
But in the case of coal, these organisms consisted largely of some downright bizarre plants that
once covered the Earth, from Colorado to China.
Meet the Lepidodendrales, also known as scale trees.
Named for their scaly looking bark, scale trees were tall plants topped with crowns
of branches that sported long, narrow leaves.
They thrived during the Carboniferous Period, from 299 million to 359 million years ago.
And they’re partly why we call the Carboniferous ... the Carboniferous!
This period is named after the carbon-rich layers of sediment that formed from countless
millions of scale trees, ferns, and other early plants that died, piled up, and fossilized.
You know these layers as coal.
During the Carboniferous, much of the Earth was covered in lush forests and swamps, and
this is where the scale trees flourished, sometimes growing as tall as a 15-story building.
Their remains have been found throughout the Northern Hemisphere, but they’re especially
common in North America, where fossilized roots, trunks, and leaves have been found
from almost every stage of the plant’s life cycle.
And because we know so much about the anatomy of these trees, we know that they were very
different from the trees that you and I know today.
For one thing, they had no need for seeds!
This is because scale trees were a type of plant called lycopods, which originated way
back in the Silurian Period -- before seeds were even a thing!
Some lycopods are still around today, like the low-lying, water-loving club mosses and
quillworts.
But instead of having seeds, these plants reproduce by releasing tiny, single-celled
spores.
Scale trees produced cone-like pods that dropped their spores into the water below, to be whisked
away to the nearest patch of soil.
But lycopods were also the earliest vascular plants, which means they were the first plants
to contain xylem, a special tissue that moves water and nutrients from the roots to
the leaves.
This tissue is what allows modern trees to grow tall and straight, because it contains
lignin, a rigid organic polymer that gives the plant structural support.
But scale trees only produced a small amount of lignin.
And they also had much shallower anchoring roots than today’s trees.
So how did they manage to grow so large without simply falling over?
Their secret was in their bark.
The outer layer of the scale tree, the periderm, was extremely thick and actually made up most
of the trunk.
This allowed scale trees to reach great heights -- some growing as large as 50 meters high
and more than 2 metres across!
But they were still a lot more delicate than the hardwoods you find in today’s forests.
A single storm could probably knock down a whole swath of them.
And they didn’t always look quite so majestic.
As juveniles, scale trees looked more like giant spikes sprouting out of the ground.
At this stage, they had no branches to speak of, so their leaves just grew right out of
the bark, in a spiral pattern.
As the trees matured and shed their leaves, diamond-shaped marks were left in the bark,
called leaf scars, giving the scale trees their name.
So, scale trees were weird by the standards of your modern oak or pine.
But when you’re talking about evolutionary history, you can’t really argue with results.
Scale trees were built to grow fast and die young, allowing them to spread across North
America, Europe and Asia, leaving huge swaths of organic matter in their wake.
At their peak, they made up nearly half the biomass in Europe and North America, and provided
food and shelter for the giant insects and early reptiles that called these forests home.
But the scale trees’ success may also have led to their downfall.
Their vast forests pumped out huge amounts of oxygen, which was great for us animals.
But at the same time, the scale trees were also sucking up tons of CO2 from the atmosphere.
And back in the Carboniferous, many of the decomposing microbes that could break down
the hardy lignin -- and release CO2 as a result -- had yet to evolve.
So, over time, scads of scale trees collapsed and piled up, without decomposing, keeping
much of the CO2 that they had taken up, locked away in their tissues.
In addition to really putting the carbon in the Carboniferous, this storage of CO2 also
created a sort of reverse-greenhouse effect.
Falling CO2 levels, combined with a sudden burst in volcanic activity, plunged the Earth
into an early ice age.
The resulting extinction became known as the Carboniferous Rainforest Collapse, and the
scale trees were among its casualties.
A few species did manage to hang on, in places like Asia and South America, into the Permian
Period, but by 272 million years ago, the last of the scale trees were gone.
Nonetheless, these bizarre plants left behind a big legacy.
After living and dying in prolific numbers for millions of years, scale trees and other
early plants formed a thick layer of carbon-rich peat across much of our swampy world.
The peat was eventually buried, compacted, and exposed to extreme heat, turning into
coal.
And eventually, some hairless ape dug it up and burned it, releasing the energy that the
scale trees absorbed from the Carboniferous sun over 300 million years ago.
Think about that the next time you turn on a light, or charge your phone, or … watch
a video about natural history.
As plants go, scale trees were powerful.
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