But I want to introduce you all to the Cambrian. In my humble opinion this period may win the prize for most interesting geological time (wow, I think I really just out nerded myself) for a couple of reasons. First, this period is where nearly every phylum that exists on earth evolved. We often talk about certain groups of animals being more or less ancient than others but in reality they mostly started here. There are a few exceptions such as jellies and sponges but nearly everyone else originated in the Cambrian. In the early 1900s a fossil bed in Canada now known as the Burgess Shale was discovered that began the study of what we call the "Cambrian Explosion," a huge increase in the amount of large shelled animals.
The other thing about the Cambrian is that the top predators of the time, besides the Terror Shrimp Anomalocaris, were the Ammonites, a group of (you guessed it) cephalopods. These ancient ancestors of squids and octopuses looked a little more like we imagine a mollusk with a spiral shell protecting their soft bodies. Here's an image from wikimedia: http://en.wikipedia.org/wiki/File:Asteroceras_BW.jpg.
Because there were no jawed animals yet, the cephalopods had pretty much free reign over the Cambrian sea food. Yum!
There is still a family of cephalopods today that retains the shell, the Chambered Nautilus, but most other cephalopods have lost their external shell. However both squids and cuttlefish in addition to several other orders of cephalopod such as the spirula have an internal shell.
And of course, the whole cambrian sea was littered with trilobites and opabinia. What a time it must have been to be extant.