Xenacanthus, a freshwater shark that had an eel-like body.
Stethacanthus. The dorsal fin and top of the head were full of teeth, essentially modified dermal dentacles, theorized for use in either mating or hitching ride on larger animals. Freaky either way.
Helicoprion or whorl-toothed shark. This freaky animal had a spiral of teeth jutting from its jaw that was probably used for slashing through schools of fishing, or specially suited to getting through the yummy protections of the nautilus.
I did a small series of ancient sharks for the shark exhibit that I thought I would share here. I had a lot of fun imagining these crazy looking animals. These are all graphite on normal paper.