Invasion of the Ordovician plants -

Life crept out of the oceans and onto dry land about 470 million years ago during the Ordovician period. But don’t imagine jungles teeming with tall, exotic trees, vines, and lush flowering plants. Instead, picture a Martian-like landscape with the rocks covered by moss and liverworts. The first Ordovician plants were simple nonvascular organisms, but they had a major impact on the world.

Ordovician Silurian Mass Extinction

About 35 million years after the Ordovician plants arrived, the earth fell into a glacial freeze, triggering a mass extinction. Thus, at the end of the Ordovician Period, 85 percent of all existing species disappeared.

About 435 million years ago earth’s temperature started to drop. Large continental land masses were concentrated in the southern hemisphere, with the proto South American and African continents being near the south pole. The earth entered into an ice age and glaciation on the southern continents locked up massive amounts of water causing sea levels to drop. Dropping temperatures and falling sea levels took their toll since most of life at that time was in the oceans.

In the Ordovician, many of the biosphere’s species existed in vast shallow seas within and at the edge of the continents, and many of these were endemic species (confined to a particular location and not spread worldwide). The cold climate and falling sea levels destroyed the habitats of these endemic species as the shallow seas dried up.

Despite the loss of so many species, this extinction event did not result in dramatic alterations of the marine ecological niches. Therefore, early Silurian oceanic ecosystems were similar to the preceding Ordovician ones. But life on land changed dramatically as vascular tree-like plants joined the moss and liverworts.

Moss and climate change

Mosses, hornworts, and liverworts are all examples of non-vascular plants. They are small in size since their growth is limited by poor internal transport of water, gases, and nutrients. Also, they reproduce via spores, not seeds. Thus, at first glance, it seems unlikely that such humble plants could drive a mass extinction.

The Ordovician plants thrived on both solid rock and rocky soil. Even though their roots were quite shallow, they still secreted a variety of organic acids. These acidic secretions enhanced the natural weathering processes by accelerating the chemical breakdown of rock surfaces. A 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 weathering on the mossy granite.

These observations still don’t get us to mass extinction in the oceans. But faster weathering of the earth’s rock surfaces paved the way for another chemical reaction. Because 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 lowered the CO2 content of the atmosphere.

Non-condensable greenhouse gases, like CO2, play a critical role in maintaining the 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 the Ordovician plants removed enough CO2, the earth slipped into another glacial age.

Lessons learned

Small, humble Ordovician plants made a big splash when they entered onto the evolutionary stage of life by triggering climate change and then a mass extinction. Climate change via global cooling or global warming rearranges the biosphere, destroying some ecosystems and opening a pathway for new ecosystem to evolve.