During the Ordovician period, part of the Paleozoic era, a rich variety of marine life flourished in the vast seas and the first primitive plants began to appear on land—before the second largest mass extinction of all time ended the period.
Most of the world's landmasses came together to create the supercontinent of Gondwana, which included the continents of Africa, South America, Antarctica, and Australia. Gondwana drifted south throughout the period, finally settling on the South Pole. The landmass that would become North America was combined into the supercontinent of Laurentia, which was separated from Gondwana by the narrow Iapitus Ocean. Proto-North America straddled the Equator, though to begin with it lay largely underwater.
For the most part the Earth's climate was warm and wet, with sea levels rising as much as 1,970 feet (600 meters) above those of today. But once Gondwana took up its polar position in the late Ordovician, massive glaciers formed over Africa at the supercontinent's center. This heralded a 20-million-year ice age during which shallow, life-rich seas shrank away.
Life at the start of the Ordovician remained confined to the seas with new animals evolving in place of those that didn't survive the Cambrian. Chief among them were the squidlike nautiloids, a type of tentacled mollusk. The nautiloids lifted off from life on the seabed as gas-filled chambers in their conical shells made them buoyant. They were accomplished swimmers, propelling themselves by jetting water through their body cavity. Equipped with grasping tentacles, the nautiloids were effective predators.
Another group of marine hunters were the mysterious conodonts, known mainly from the tiny fossil teeth they left behind. The few complete fossils that have been found suggest they were finned, eel-like creatures with large eyes for locating prey. The conodonts are now thought to have been true vertebrates; however, this line of backboned animals later went extinct.
Fish started becoming more widespread in the fossil record. They were small and had downward-pointing, jawless mouths, indicating they lived by sucking and filtering food from the seabed. Bony shields covered the front of their bodies—the beginnings of a fashion for armor plating among fish. Lampreys and hagfish are these fishes' living descendants.
The archaic sponge reef-dwellers of the Cambrian gave way to bryozoans—tiny, group-living animals that built coral-like structures. Ordovician reefs were also home to large sea lilies, relatives of sea stars. Anchored to the bottom inside calcareous tubes, they collected food particles with feathery arms that waved in the ocean currents.
From Sea to Land
The hard-bodied arthropods started eyeing opportunities on land. Edging into freshwater and shallow lagoons, they likely included horseshoe crabs, which, despite their name, are more closely related to spiders and scorpions. A few species of these "living fossils" still survive today, such as along the eastern seaboard of the United States, where each spring horseshoe crabs crawl ashore to spawn.
There is also evidence that the first primitive plants began to appear on the previously barren land.
These first steps toward life on land were cut short by the freezing conditions that gripped the planet toward the end of the Ordovician. This resulted in the second largest mass extinction of all time, wiping out at least half of all marine animal species about 443 million years ago
The Devonian Period:
The Age of Fish
The Devonian Period of the Paleozoic Era lasted from 417 million years ago to 354 million years ago. It is named for Devon, England where the old red sandstone of the Devonian was first studied.
The Continents of The Devonian
During the Devonian there were important changes in the land masses on the globe. North America and Europe had collided forming a large continent called Euramerica. This caused the formation of the Appalachian Mountain Range. The other large land mass was Gondwana. It was made up of South America, Africa, Antarctica, India and Australia. These two large land masses lay close to one another near the equator.
The two continents were moving toward each other throughout the Devonian Period. The waterway between the two continents covered a subduction zone. This is an area where one plate is moving underneath the other. Eventually this would mean that the two continents would collide to form the supercontinent Pangea in the Permian Period. That event is more than 64 million years after the Devonian Period.
The Climate of The Devonian Period: Plants Cover The Land
Laying so close to the equator meant that the climate of the Devonian was warm. The warm temperatures made life on land particularly good for the plants. They developed vascular tissues to carry water and food through roots and leaves. The most important development was the seed. Now plants were not dependent on the presence of water for reproduction and they could move further inland. Ferns and the first trees began to cover the land.
Insects and Other Animals Find Homes On Land
The plant-covered lands made a good home for the first wingless insects and spiders. Even a primitive vertebrate, the tetrapod or four-footed vertebrate, developed the ability to live outside the water and move on land.
The Age of Fishes
The Devonian Period is known as the Age of Fishes. It is famous for the thousands of species of fish that developed in Devonian seas. When fish first started to develop, they had no jaws and the support structure was made of cartilage. This material doesn’t fossilize well, so the earliest fossils were of fish whose outside skin was protected by scales and plates made of boney tissue. These fish were called Ostracoderms. Their name means “shell-skins.” These animals appear in rock from the late Silurian and early Devonian periods.
Fish with Jaws
The next development was the fish with jaws, gills and paired fins. The Placoderms were the first fish to have all three of these characteristics. They still had the “shell skin” of the Ostracoderms, but it mainly covered the head and neck area. The largest of the Placoderms was the Dunkleosteus. It was a huge predator in the Devonian seas. It could be as long as 10 meters. Instead of teeth, it had large boney plates that stuck down in the front of its mouth opening. The powerful jaws were deadly to other fish, sharks and even other Dunkleosteus.
Sharks, or Chondricthyes, developed during the Devonian also. Sharks are thought to be descendents of the large Placoderms, but they lost the ability to form the boney armor on the outside of the body and were unable to form bones on the inside also. Their body is supported by cartilage. Because of the skeletons of cartilage, very little fossil evidence is available. They did leave behind their teeth. Much of the information we have about ancient sharks comes from the many different types of fossil teeth that have been found. Sharks first appear in the middle Devonian period.
The Bony Fish; Osteichthyes
The bony fish appear during the middle Devonian Period. The first of these are the lobe-fins. These fish have pairs of fins with fleshy lobes at the base and more typical fin membranes at the ends. The lobes contain jointed bones. These lobe-fins are thought to have evolved into “legs” and eventually into amphibians that spend their lives both in and out of the water.
The coelacanth is a lobe-fin fish that developed during the Devonian Period. For years it was thought to have gone extinct at the end of the Mesozoic Era along with the dinosaurs, but in 1938 a living coelacanth was caught. Since then coelacanths have been seen from time to time in the Indian Ocean.
The Lung Fish
The Dipterus was a lungfish that developed during the Devonian Period. In many ways it looked like the lobe-fins with bony flesh at the base of its fins. But the Dipterus had lung sacks branching off of its throat that got air from the gills. During the Devonian Period, there were huge swings of floods and drought. During drought times, when lakes turned into ponds, the plants used all the oxygen in the little water that remained. A Dipterus that was stranded in such a pool could stick its head out of the water and get the air it needed to stay alive.
The Reef Builders
The work of the sponges and corals went on through the Devonian Period. They built some of the largest reefs in the world. Invertebrates grew well in Devonian seas too, so many new species developed. The ammonite is one of these.
Mass Extinction Ends The Devonian Period
Species had begun to branch out and include both land and water habitats. The Devonian Period ended with a mass extinction. The Devonian extinction hurt the water habits much more than those on land. The sponges and corals were the most affected. No major reef building happened again for thousands of years.