Late to this party, but here are my thoughts:
1. Most of the marks are rodent chews, but the placement of the holes is odd. Maybe there is a natural thin spot in the shell there, or some really big gator (or something with a wide bite pattern) bit it there puncturing two holes there, but the placement is still odd.
2. Most turtle bone gorgets that I have heard of come from big sites, they aren't common isolated finds. If it came from a site, there should be other bone tools, pottery, and points that identify it to the woodland or Mississippian periods.
3. It could also be something historic, lots of people have used turtle shells for rattles, decoration, etc.
4. Turtle shells often break apart naturally into 'geometric' plates when they are fresh. If they are dried carefully, they hold up well. (Someone else mentioned it, but their joints between shell plates are soft when fresh and often fall apart if they decompose naturally. If they are cleaned and get a chance to dry, the joints harden and the shell stays together.)
If you find other relics there that fit the pattern, maybe it's a gorget, otherwise I'd say it's probably something else.
