to get shieldon you need to under ground and find the armor fossil then put it in your bag next go up to oreburgh city and talk too the guy in the museum, but you need five Pokemon in your party then tell him about the armor fossil and go out of the museum then go back in then talk to him again he will give you shieldon lv.20,you can do this with any other fossils you find.
sources:
http://www.serebii.net/pokedex-dp/410.shtml
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its shieldon not sheldon. you get it by going underground and searching for fossils.
how to get a togepi in Pokemon platinum
There is no grunt that you can give a Pokemon to in Pokemon platinum.
That pokemon is Azumarill in pokemon platinum in the national pokedex
No, you cannot get Jirachi in Pokemon platinum.