Unfortunately, if the fossil you're finding in the Underground is the Skull Fossil, you will not be able to obtain an Armor Fossil in your Platinum save file, and therefore unable to obtain Shieldon and Bastiodon outside of trading.
In Platinum, similar to in the original Diamond and Pearl games, you are only able to obtain one of either the Shield or Armor Fossil, though it is obviously not based on which version you're playing. Which one it will be is instead based on the last digit of your Trainer ID number; if it's an odd number, you'll be able to find the Skull Fossil, and likewise an even number yields Armor Fossils. As your ID number cannot be changed outside of restarting the save file, if it ends in an odd number, unfortunately you're out of luck if you'd like to get Shieldon or Bastiodon.
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