The Ordovician period saw a mass extinction event, followed by diversification and adaptive radiation of marine organisms. Jointed brachiopods replaced trilobites, while cephalopods and crinoids emerged. The period also saw the emergence of the first animals with sophisticated nervous systems and land plants.
Ordovician organisms lived during the Ordovician period, which lasted between about 488.3 and 443.7 million years ago. It started with a mass extinction called the Cambrian-Ordovician extinction event, which wiped out 50% of all multicellular organisms in the fossil record. At the time, all known multicellular organisms were exclusively marine. The number of known fossils from the Ordovician is about 500, substantially more than the 200 or so available from the Cambrian before it.
The Ordovician period is characterized by episodes of adaptive radiation and diversification surpassed only by the so-called Cambrian explosion, which occurred about 60 million years earlier. The mass extinctions of the late Cambrian severely impacted brachiopods (a stationary shelled organism that superficially resembles bivalves), trilobites (which were never the same again), and the first jawless fish called conodonts. During the Ordovician, other animals, such as jointed brachiopods, cephalopods (sophisticated molluscs), and crinoids (sea lilies) largely replaced the Cambrian animals that preceded them. Notably, jointed brachiopods almost completely displaced trilobites in the shallow seas where both lived. These organisms would continue to dominate marine fauna throughout the remainder of the Paleozoic Era.
While trilobites weren’t as successful as they were during the Cambrian, they were still numerous and diverse, sharing the bottom of the food chain with jointed brachiopods. Numerous gastropods (snail-like animals) evolved during this period. There were numerous fish without jaws, with jawed forms appearing towards the end of the period. The Ordovician saw the evolution of the first corals and the first coral reefs, the first coral reefs to be built over tens of millions of years, from the flourishing of the ancient archaeocyathids during the Early Cambrian. The Ordovician strata are filled with graptolites, the remains of colonial marine animals called arrowworms.
The Ordovician was marked by the emergence of the first animals with sophisticated nervous systems and a substantial amount of brain tissue, as evidenced by the fossils of nautiloid cephalopods, the dominant predators of the period. These relatively intelligent, tentacled, large-eyed, hard-shelled molluscs may have outperformed earlier apex predators such as anomalocarids. Large cephalopods had appeared during the Late Cambrian but diversified greatly throughout the Ordovician.
The Ordovician saw the emergence of the first land plants, non-vascular plants that would have resembled modern lichens, liverworts and mosses. These simple plants tended to cluster near the sea or streams. No fossils of fused bodies of Ordovician land animals have been found, although some of the earliest terrestrial trace fossils appear during this period, probably made by early myriapods such as millipedes. The oldest currently known fossil of a land animal is a 1-cm fossil millipede dated to 428 million years ago, just 16 million years after the end of the period. Ordovician fossils of land animals may yet be discovered.
The discovery of fungal hyphae very similar to a special type of fungi found in the roots of most all plants suggests that this ancient symbiotic relationship began during the Ordovician.
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