Shieldon and Bastiodon are horrifically grotesque to look at (especially Bastiodon with all those spikes protruding from its 'shield') and I've never had access to anybody with Pearl (when I was a hardcore Diamond player, I didn't understand how to get GTS functioning, so I was starved of the opportunity to trade, and nobody I knew had Pearl), so I'd always go with the Skull Fossil. Cranidos and Rampardos are far more useful in-game as it is, anyway, as they're far more powerful that Shieldon and Bastiodon ever will be (power is the essence in-game; it gets things done).