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.
These Ordovician [fossils] can be found in the Manitou Formation in the Colorado Springs area.
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.
Corals and coral-like organisms occur in the Ordovician Fremont Limestone.