I'm trying to complete the fossil part of my museum and this is the only fossil that I'm lacking. I'm willing to pay in TBT, IGB or anything else that I have. If you have a spare dimetrodon tail and want to trade, name your price and post below!
Hi! Unfortunately I don't have this fossil, but to get my last fossil I used the soft-reset trick: once you've got the day's fossils, save your game, then have Blathers examine them. If it tells you that the collection already has those fossils, turn off without saving, turn on again and have the fossils examined again.
Do this until you find your last fossil!