The Ordovician Period, spanning approximately 485.4 to 443.8 million years ago, represents the second segment of the Paleozoic Era. This geological interval was a time of significant expansion in marine biodiversity. Life during the Ordovician was predominantly confined to the planet’s vast oceans, where complex marine communities flourished. These evolutionary developments laid foundations for future biological diversity.
The Ordovician World
During the Ordovician Period, Earth’s continental arrangement differed significantly from today, with landmasses like Gondwana, Laurentia, Baltica, and Siberia positioned differently. Gondwana, a large supercontinent, was situated in the Southern Hemisphere and gradually shifted towards the South Pole, influencing global climate patterns. High global sea levels led to widespread shallow, warm epicontinental seas covering much of the continents, providing abundant habitats for marine life. While the climate was generally warm and moist during the Early and Middle Ordovician, later in the period, as Gondwana moved poleward, global cooling and glaciation occurred. This led to fluctuating sea levels and, in some deeper ocean basins, periods of anoxic (low oxygen) conditions due to sluggish ocean circulation.
Proliferation of Life Forms
The Ordovician Period witnessed a significant increase in marine biodiversity, known as the “Great Ordovician Biodiversification Event” (GOBE). This period saw a rise in the number of marine animal genera, quadrupling in diversity and establishing many modern animal phyla and classes. The GOBE was not a single, instantaneous event but rather a series of diversifications occurring at different times and locations, spanning the entire Ordovician Period. This evolutionary turning point involved the development of new ecological roles and more complex marine ecosystems, including the expansion of filter-feeding organisms. Factors contributing to this diversification included increased nutrient availability, the creation of new ecological niches, and various evolutionary innovations.
The expansion of life during the GOBE led to more organized community structures, with the development of diverse feeding strategies. Organisms began to occupy different “tiers” both above and within the seafloor, creating more complex food webs. For instance, the diversification of phytoplankton and planktotrophic larvae provided a broader food base for suspension-feeding animals. Reef communities also transformed, shifting from microbial-dominated structures to those dominated by metazoans in the Late Ordovician, further promoting biodiversity.
Key Inhabitants of Ordovician Seas
Ordovician seas were teeming with diverse invertebrates. Trilobites, common during the Cambrian, continued to be diverse, occupying various ecological roles and scavenging the seafloor. Brachiopods were abundant, covering the seafloor and acting as articulate filter feeders, straining food particles from the water.
Large nautiloid cephalopods emerged as apex predators in the Ordovician oceans. These mollusks, related to modern squid and octopuses, played a significant role in the marine food web. Graptolites, colonial hemichordates, were widespread and diversified, becoming important index fossils for dating rock strata due to their global distribution and rapid evolution.
Echinoderms, a group that includes modern starfish and sea urchins, also diversified during this time. Crinoids, or “sea lilies,” became dominant on the ocean floor, filtering food. Early vertebrates made their appearance in the Ordovician, represented by jawless, armored fish called ostracoderms. These early fish had bony shields on their heads and a slit-like mouth for feeding.
The Ordovician Extinction Event
The Ordovician Period concluded with one of Earth’s largest mass extinction events, primarily affecting marine life. This extinction, which eliminated an estimated 85 percent of all Ordovician species, occurred in two distinct phases. The initial pulse of extinction is linked to global cooling and extensive glaciation that expanded over Gondwana, shifting Earth from a greenhouse to an icehouse climate. This cooling and the subsequent drop in sea level led to significant habitat loss, particularly for organisms in shallow continental seas.
The second pulse of extinction occurred as the glaciation receded and warmer conditions returned. This phase was associated with widespread oceanic anoxia (oxygen depletion), which persisted into the early Silurian Period. The two phases of environmental change—first cooling and sea-level fall, then rapid warming and oxygen depletion—had a significant impact on marine biodiversity, setting the stage for the subsequent Silurian Period.