The Ordovician Period, a chapter in Earth’s deep past, spanned approximately 485 to 443 million years ago. During this ancient interval, the planet’s geography looked quite different from today, with most landmasses clustered into the supercontinent Gondwana, which gradually shifted towards the South Pole. Vast, shallow, and warm seas covered much of the continents, creating expansive marine environments. This period represents a significant time of evolutionary change for animal life, particularly within these widespread aquatic realms.
The Great Ordovician Biodiversification Event
The Ordovician Period witnessed a significant increase in marine animal diversity, an event known as the Great Ordovician Biodiversification Event (GOBE). This period saw a rise in the number of marine animal families and genera. The diversification was not instantaneous or uniform globally, occurring at different times across various regions.
Several scientific theories attempt to explain this diversification. One contributing factor was likely a long-term cooling trend that began in the Early Ordovician, potentially increasing oxygen levels in the oceans. The breakup and dispersal of continents also created numerous new shallow-sea habitats, fostering isolation and the formation of new species. Intense volcanic activity may have supplied abundant nutrients to the oceans, supporting this expansion.
The diversification of phytoplankton, the base of the marine food web, played an important role, providing a richer food source for zooplankton and suspension feeders. This led to the development of more complex food webs and ecosystems with specialized and tiered species. The collective influence of these geological and biological changes created an environment conducive to the expansion of marine biodiversity.
Dominant Marine Invertebrates
The shallow Ordovician seas were home to diverse invertebrate life, the most abundant animal groups of the time. Trilobites were widespread and varied in shape and size. They occupied various ecological roles, with many species scuttling on the seafloor as scavengers or deposit feeders, while some may have been predators.
Brachiopods were another successful group, often resembling clams but belonging to a distinct phylum. These filter-feeding marine invertebrates possessed two bilaterally symmetrical valves, or shells, and were abundant on the seafloor. Many brachiopods attached themselves to the substrate with a fleshy stalk, while others rested unattached, extracting plankton and oxygen from the water using a specialized feeding organ called a lophophore.
Early cephalopods, particularly the straight-shelled nautiloids known as orthocones, emerged as important predators in these ancient oceans. Some of these large creatures, like Endoceras, could reach great lengths, with estimations suggesting some individuals grew up to 6 meters (20 feet) or even potentially 9 meters (30 feet). These large orthocones likely preyed upon trilobites and other marine invertebrates, becoming apex predators.
Crinoids, often called “sea lilies” due to their flower-like appearance, were echinoderms related to modern starfish and sea urchins. These stalked, filter-feeding animals formed dense “gardens” on the seafloor, capturing food particles with their feathery arms. Their skeletal elements, composed of calcified plates, frequently preserved as fossils, indicating their widespread presence in Ordovician marine environments.
Other important invertebrate groups included bryozoans, tiny colonial animals that built intricate, calcareous structures resembling moss or lace. These filter feeders contributed to carbonate sedimentation and formed various colony shapes, although their colonies did not reach the large sizes seen in corals. The first reef-building corals also appeared, with tabulate and rugose corals becoming prominent. Tabulate corals formed colonial structures with honeycomb-like cells, while rugose corals could be solitary “horn corals” or colonial, contributing to reef ecosystems alongside algae and sponges.
The Rise of Early Vertebrates
Amidst the diverse invertebrate communities, the Ordovician Period also marked the appearance of the earliest known vertebrates. These were primarily jawless fish, collectively referred to as agnathans or ostracoderms. Ostracoderms were small, fish-like animals, ranging from 7 to 25 centimeters (3 to 10 inches) in length, and were characterized by flat, thick bodies.
A primary feature of these primitive vertebrates was their external bony armor or head shields, which provided protection. Their internal skeletons were likely cartilaginous. Ostracoderms are thought to have been bottom-dwellers, using their jawless mouths to suck up detritus or small, slow-moving prey from the seafloor. Their gills were used exclusively for respiration, a departure from earlier chordates that used gill precursors for both feeding and breathing.
Conodonts, another group now understood to be primitive, eel-like vertebrates, also diversified during the early Ordovician. These organisms are primarily known from their distinctive, tooth-like microfossils, composed of calcium phosphate, found in marine sedimentary rocks. While their soft tissues rarely preserved, these elements were present in their oral cavity and used for processing food.
The End-Ordovician Mass Extinction
The diverse life of the Ordovician Period was curtailed by one of Earth’s five largest mass extinction events, occurring around 445 million years ago. This End-Ordovician mass extinction, also known as the Ordovician-Silurian extinction, unfolded in two distinct pulses over a period of one to two million years. It primarily affected marine life, as terrestrial ecosystems were still in their nascent stages.
The leading scientific explanation for this extinction links it to a major ice age centered on the supercontinent Gondwana, which had drifted over the South Pole. The first extinction pulse coincided with global cooling and a notable fall in sea levels. As vast ice sheets formed, sea levels dropped, destroying shallow marine habitats that many diverse invertebrate communities relied upon.
The second pulse of extinction occurred as the climate warmed again and sea levels rose. This post-glacial warming led to changes in ocean chemistry, including widespread oxygen depletion (anoxia) and the production of toxic sulfides. While animal groups like trilobites, brachiopods, corals, and graptolites survived the event, each lost a significant number of their families and species, with an estimated 85 percent of marine species disappearing overall.