Cephalopods in the Ordovician seasThe Cambrian Period was followed, 485 million years ago, by the Ordovician Period. At that time many species died out, to be replaced by new ones in another evolutionary “explosion”. During the Ordovician, the seas surrounding North America and Europe were rich in life, especially trilobites, brachiopods and reef-forming corals. Molluscs, especially bivalves, gastropods and cephalopods, became common. The first jawless fish, which had already appeared in the Late Cambrian, were followed in the Late Ordovician by jawed fish. Life appeared on land for the first time, in the form of simple plants. The Ordovician ended, after a series of major extinctions, about 443 million years ago. The Silurian (443–419 million years ago) and Devonian (419–359 million years) followed.
Trilobites first appeared in the Cambrian and soon became, for the next 250 million years, some of the most numerous of animals. Trilobites were arthropods—creatures with a hard external skeleton and jointed limbs. A trilobite used its legs to scuttle along the sea bed or paddle through the water. Having no jaws, its legs brought food into its mouth. A trilobite’s body was covered by a hard, jointed carapace (shield) divided into three lengthwise strips—hence its name, which means “three lobes”.
Tiktaalik was the earliest fish to have a neck. It was the first known land-dwelling vertebrate.
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