The sea deepened and widened as the Ordovician Period began, and a series of limestones and dolomites was deposited, either on top of the Sawatch Sandstone or, where the Sawatch had not been deposited, directly on the Precambrian. These rocks are now called the Manitou Formation. The fossils in these rocks are much more varied than those in the Sawatch Sandstone: snails, echinoderms, sponges, cephalopods, brachiopods, and trilobites are common. The Ordovician sea must have teemed with life, as many rocks deposited at this time are more than half composed of animal remains. In addition to hard-shelled animals which formed fossils, there were probably abundant soft-bodied animals such as jellyfish and worms, which left no record of their presence. After deposition of the Manitou Formation, the seas receded slightly. A new series of sands was deposited above the Manitou in central Colorado. These now form the Harding Sandstone, a formation of unusual interest because it contains remains of the earth’s earliest known vertebrates, primitive jawless fish called Agnathids. In places in the Harding Sandstone there are dense accumulations of the tiny polygonal armor plates from these fish. Although no whole fish have been found, we can reconstruct their appearance by comparing individual plates or groups of plates with later, better known relatives. Also present in great quantities in the Harding Sandstone are conodonts, peculiar tiny brown tooth-like fossils. Relationships of the conodonts are unknown; they may be parts of the Agnathids, or perhaps they represent some entirely different group of animals, with no living relatives. After deposition of the sands of the Harding Sandstone, the sea deepened locally and the Fremont Limestone, a massive gray crystalline limestone containing many marine fossils, was deposited. Mollusks (some quite large), brachiopods, and corals contributed their shells to the Fremont Limestone. The chain coral _Catenipora_ and the horn coral _Streptelasma_ may often be used to identify the formation. The Fremont Limestone was deposited very late in the Ordovician Period. Probably the seas were much more extensive then than present deposits indicate; subsequent erosion has at several times erased the evidence in uplifted areas. [Illustration: These Ordovician fossils can be found in the Manitou Formation in the Colorado Springs area.] [Illustration: The earliest known fish remains come from the Ordovician Harding Sandstone of central Colorado. These fragments of the protective plates have been magnified about five times.] [Illustration: Corals and coral-like organisms occur in the Ordovician Fremont Limestone.] Silurian Period (400-440 million years ago)