The Ordovician Period, spanning approximately 485 to 443 million years ago, saw the emergence of Earth’s earliest known land plants. These pioneering organisms began the slow process of terrestrialization, transforming barren landmasses into more hospitable environments. Their presence on land initiated profound changes for the planet, setting the stage for diverse terrestrial ecosystems.
Primitive Nature and Form
Ordovician plants were largely non-vascular, lacking specialized internal tissues like xylem and phloem for efficient water and nutrient transport. This meant they remained small and low-lying, unable to grow tall like modern trees. They likely formed dense, green mats resembling present-day liverworts, hornworts, and mosses, which are also non-vascular plants.
These early land colonizers possessed simple structures without true roots, stems, or leaves. They anchored themselves with hair-like structures called rhizoids and absorbed water and nutrients directly from their immediate surroundings. Their dependence on surface moisture for reproduction further limited their habitat to damp or moist environments.
Unveiling the Past Through Fossils
Our understanding of Ordovician plants is primarily derived from a fragmented fossil record. The most common evidence consists of microscopic spores, which are indicative of land plant reproduction. These tiny spores are resilient due to a tough outer coating made of sporopollenin, a durable biopolymer.
Scientists also find occasional fragments of plant tissue, including waxy cuticles that prevented desiccation. Reconstructing the complete appearance of these ancient plants from such limited remains poses a significant challenge. However, comparisons of spore wall structures and other features to modern bryophytes provide clues about their likely forms.
Shaping Earth’s Ancient Environment
The colonization of land by Ordovician plants had a substantial impact on Earth’s early environment. These simple organisms began to alter the terrestrial surface, contributing to the formation of early soils. Their photosynthetic activity directly influenced atmospheric composition by drawing down carbon dioxide (CO2).
A significant hypothesis suggests this CO2 drawdown contributed to a global cooling event, possibly leading to the Late Ordovician glaciation. As these plants spread, they enhanced the chemical weathering of silicate rocks, a process that removes CO2 from the atmosphere. This increased weathering, combined with CO2 absorption through photosynthesis, caused atmospheric CO2 levels to decrease, triggering a temperature drop and the formation of extensive ice sheets.