Hello.

I think this bone is a hyoplastron.

Hello.

I think it is part of a sea turtle shell. I can´t go further on that.

Good luck with the ID determination!

Best regards,

Fernando.

Or a hypoplastron (a component of the ventral shield of a marine turtle, probably Caretta caretta)

Thank you all :)

Agreed...this looks like the hypoplastron of a sea turtle.  Try Googling "hypoplastron"...you will see plenty of similar bones.