I've been fascinated with the Eugeneodontids (the buzz-saw chondrichthyans) and how they managed to practically become the apex predators of most oceanic environments from the Carboniferous to the Permian with famous members like Edestus and Helicoprion. Two genus of this extraordinary group even survived the Permian-Triassic Extinction 252 Million Years ago - Fadenia and Caseodus!
http://www.fossilworks.org/cgi-bin/bridge.pl?a=taxonInfo&taxon_no=34456
http://www.fossilworks.org/cgi-bin/bridge.pl?a=taxonInfo&taxon_no=34451
But by the Olenekian stage