Ah, thanks!

So the meat of this is actually the mouse models, looks like. Strains of mice with genetically missing teeth got them back.

They basically just juiced normal ferrets to see what would happen, and they grew an extra incisor as you can see in the images posted. I'm guessing developing genetically toothless ferrets wasn't in the scope of their funding. You can get all kinds of weird lab mice "off the shelf", though.