Ordovician Freeze
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The Big Ordovician Freeze

Little Plants Take the Stage

Published in The EarthSphere Blog. Feature Image: Moss by CF Lovelace & WM House (© ArcheanArt)


We dwelt upon the Ordovician in our last Forgotten Origins article. Part of the Ordovician story tells a tale of how life dragged itself onto dry land after 4 billion years, and the consequences of this invasion.

Non-Vascular Plants

Life crept out of the oceans and onto dry land about 470 million years ago during the Ordovician Period. If you’re imagining dappled sunlight dancing across the floor of a lush, primordial jungle teeming with tall, exotic trees and shaded by a thick canopy of leaves, you are wrong. Instead, picture a Martian-like landscape covered by rocks, boulders, and fine dust. But when you inspect more closely, small insignificant-looking plants appear, growing from the crevasses and on the boulders. The ancient ancestors of moss and liverworts covered the Ordovician landscape. The first Ordovician plants were simple non-vascular organisms, but they had a major impact on the world.

Mosses, hornworts, and liverworts are all examples of non-vascular plants. They grow close to the ground with shallow root systems that could never support a larger plant. Since they are non-vascular, their size is also limited by their inefficient internal transport system for water, gases, and nutrients. Vascular plants can pump water and nutrients from their roots deep underground to leafy canopies hundreds of feet in the air. Non-vascular plants are limited to living along the ground, close to their shallow roots. Still, the first plants on land were pioneers, and they had an entirely new world to exploit.

These early continental settlers reproduced by spores, not seeds. We know seeds hold certain advantages for plant reproduction, with their protective shell enclosing nutrients and an embryo. It’s a self-sufficient unit ready to produce new life and continue the species. Seeds are multicellular, but a spore is a single cell surrounded by a tough membrane. Spores are created asexually through a process called sporogenesis. There are not as durable and hardy as seeds, but each spore can grow into a new plant under the right conditions.

Life on a Barren Rock

Despite the sophistication of seeds, reproduction by spores had its advantages for these first Ordovician plants. Each spore is small, and many spores are easily transported by the wind. Seeds often rely on animals for transport to new locations, but there were no land animals in the Ordovician. Reproduction by spores provided a way for these new species to propagate themselves far and wide, filling Earth’s ecological niches for as far as the wind blew. Spores allowed these new organisms to conquer dry land.

One of the lessons learned by humans in the Anthropocene is the interconnected nature of our biosphere. Through a process popularly named the ‘butterfly effect,’ seemingly small changes can set off a series of events that culminate in major environmental alterations. The idea applies to deterministic non-linear systems, and it links to chaos theory.

Some complex dynamical systems exhibit unpredictable behaviors such that small variances in the initial conditions could have profound and widely divergent effects on the system’s outcomes. Because of the sensitivity of these systems, outcomes are unpredictable. This idea became the basis for a branch of mathematics known as chaos theory, which has been applied in countless scenarios since its introduction.(Understanding the Butterfly Effect)

Ordovician plants thrived on both solid rock and rocky soil. Even though their roots were quite shallow, these first simple plants made a big difference. In every location they occupied, they secreted a variety of organic acids into the soil and rocks below. This process was new. The norm on Earth’s continents for four billion years was a land barren of life. But during the Ordovician, this norm came under attack.

One Thing Leads to Another — Chemical Weathering

Foreign invaders took over the surface of Earth’s continents. These plants figured out how to live outside of the oceans, bringing new life processes with them. By-products from their way of life changed the surface of the planet. Acidic secretions from these simple plants enhanced the natural weathering process by accelerating the chemical breakdown of rock surfaces.

research team at the University of Exeter, UK, grew moss on granite for 130 days and then measured the weathering effects. Observations from the mossy granite, when compared to a control granite surface with no plants, showed significantly more chemical weathering on the mossy granite. Non-vascular plants enhance weathering.

The roots of these ancient plants were shallow, and at first glance, small amounts of secreted organic acids may seem insignificant. But the world held unlimited potential for these plants, and with no predators to stop them, they spread across the planet, building our first dry-land ecosystems. The effects from individual plants were small, but eventually, their collective effect could not be ignored.

Faster weathering of the Earth’s rock surfaces paved the way for another chemical reaction. As weathered rock is chemically altered, it reacts with carbon dioxide (CO2) and removes it from the atmosphere. The moss and liverworts accelerated the weathering process and gradually helped lowered the CO2 content of the atmosphere.

Non-condensable greenhouse gases, like CO2, play a critical role in maintaining Earth’s surface temperature in a range that supports life as we know it. Without these non-condensable gases, the Earth’s average temperature would be about -18 degrees Celsius. So, when weathering removed enough CO2, the first plants helped Earth slip into another glacial age.

(The Forgotten Origins series is also available on ArcheanWeb)



What Are Spores? — Definition & Types (Source: Study.com) 

First land plants plunged Earth into ice age (By Michael Marshall; New Scientist) 

Non-Vascular Plants (Source: Basic Biology) 

Understanding the Butterfly Effect ( by Jamie L. Vernon; American Scientist) 

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.