I am having tons of trouble looking for a argento head fossil. I have even..

It is rare, but I have found two-one for Argento, a second to superevolve Into Shenlui. Did you know you have to collect doubles of all the superevolvers (bank the evolved form in order to make space for another original vivosaur in your holder) in order to get the completist mask? I can't remember if I used the yellow or the head mask to find him. I also know that I had completed the main story line, I may have also completed the three Battle Royales and the Champion's Tournament. Additionally , I just tend to do the side missions as they show up, so I don't know if you have to complete a special one of them before Argento shows up.
Sorry this is so full of "not sure's", but I think those are the events/missions you have to evaluate to see when Argento becomes available to you at Dusty Dunes. Good luck!

So far I have had no luck finding argento but. . . I can give as much detail of it as I possibly can. Like BJML said it's (rare) aka it's a secret vivosaur. Though I have discovered one of argento's fossil's. Its legs is what I found. And believe me when they said it's a rare vivosaur, man they meant it. Trust me on this one argento will be a pain to get. It took me one hour to find it's legs fossil. I suggest that this might be your best and only chance of getting it easily. Dig and clean all of the (earth) type vivosaurs there are at dusty dunes. After that. . . Well I think you get the idea. If not here is what I mean: Cleaning up all earth fossils=new earth fossil rock= most likly argento. This is well all I know. And if you think I'm bluffing I'm not. You will know sooner or later that this vivosaur will be a great challenge of patience. I wish you good luck on the hunt.You will need it.

Let me know if any of this helped =).