Shallow seas covered large area of the world, lichen-like plants had begun to adapt themselves to life on land.
Post-Cambrian trilobites had diversified into many forms including the Cryptolithus shown in bottom of Figure 02a.
Echinoderms were characteristic inhabitants in this period. To the left is a primitive species that carried its bulging body on a short stalk. Another species (right) was also rounded or pear-shaped, but anchored itself by the tapering end of its body. The sea lilies (the red ones on extreme left) belong to the same group.
Brachiopods (white shells, right) furnish the most abundant invertebrate remains in Ordovician sediments.
Cephalopods (nautilus) were the early version of the present-day cuttlefish. They are active hunters with arms