Contemporary vertebrates included armored jawless fish called ostracoderms. This allowed placoderms to lift their heads, unlike ostracoderms. Later, about 420 million years ago, the jawed fish evolved from one of the ostracoderms. In those days, placoderms were thought to be shelled jawless fish akin to ostracoderms. Similar ostracoderms were preserved near Canon City, Colorado. Other ostracoderms, such as the Galeaspida are now known to have a paired olfactory organ. Ostracoderms have heads covered with a bony shield. The first fossil fishes that were discovered were ostracoderms. Ostracoderms existed in two major groups, the more primitive heterostracans and the cephalaspids. But, in the middle or late Ordovician, ostracoderms swam over South Dakota.