In the Silurian and Devonian, organisms continued to evolve rapidly in the shallow sea that covered much of the continent, and like the late Ordovician, brachiopods and bryozoans were the most common organisms in the shallow seas. However, echinoderms became increasingly more important and abundant in the Silurian. By Silurian times the nautiloids and cephalopods had nearly disappeared, and the grapto-
lites were virtually extinct. One line of descent of the nautiloids survived and evolved into groups of coiled ammonoids in the Devonian. These ammonoids were swimming and floating organisms, and they evolved rapidly, so they formed useful index fossils for this period. Another unusual but widespread group of animals, the small, toothlike conodonts, were abundant in this period, and these seem to represent some disaggregated parts of a larger type of floating or swimming organism.
In Silurian-Devonian times coral reefs became widespread and were populated mainly by rugose and tabulate corals. The Devonian was also important for the evolution of life in that fish experienced
I Oceanic rocks
Simplified cross section of the evolution of the Iapetus Ocean, including an Ordovician arc-continent collision and the Devonian collision with Avalonia, in the Acadian Orogeny widespread development, and the land was extensively invaded by plants. The Devonian has many examples of armored fish fossils, which show the development of internal skeletons, jaws, and evolved bodies better suited for swimming than their older Paleozoic counterparts. Many plants began to invade low-lying areas near the seas and lakes, and by the end of the Devonian, huge forests of thick trees covered much of the land, and the land became inhabited by insects, scorpions, and spiders that evolved from sea creatures. As an example, scorpions evolved from giant, three-foot (1-m) long eurypterids known as sea scorpions. By the late Devonian amphibians were crawling on the land, and these may have evolved from fish that crawled out of the sea.
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