Minor nitpick, ammonites are extinct, its nautilus that aren't. They look similar but they're not even all that closely related! Nautalis are kind of their own weird group, meanwhile ammonites are more closely related to other extant cephalopods (squids/octopus/cuttlefish)

Ammonite shells are one of the go-to ways to depict fossils, and fossils are a pretty common motif in earth so its still fitting imo. I mean, the earth exalt pillar even has ammonite impressions.