Yes, this is what Shubin explains - conodonts were the most common fossil from that period - http://en.wikipedia.org/wiki/Conodont
describes them and http://en.wikipedia.org/wiki/Chordata
shows where they fit on the tree of life. They lived in the Paleozoic - http://en.wikipedia.org/wiki/Paleozoic
Shubin's story about how scientists thought that conodonts were organisms until they found a fossil with the soft imprint of the body showed that conodonts were the teeth, which of course survived much better as fossils than the rotting flesh.
shows where Ostracoderms fit.
Check out http://www.geocities.com/CapeCanaveral/ ... hatAre.htm
for a good intro to conodonts and how to find them.
Shubin explains how teeth evolved - starting in conodonts as a new and efficient method for eating other creatures, and then gradually spreading. The amazing story is how ostracoderms used this innovation to build a skull as protection from other toothy eaters - the first skull was made of teeth! It is like the hand example - like the bone of the upper arm evolving to produce the two bones of the forearm, the group of bones at the wrist and then the fingers, the earliest teeth gradually evolved into the whole skeleton of vertebrates.
Ostracoderms are a great example of maladaption in practice. When fish with jaws evolved, the ostracoderm went into decline, because there was a new and better organism able to exploit its niches.