Hello from Portugal! A 100% layman here. Not even sure if this is in fact a fossil, but am very intrigued by its shape.
Seems partially fossilized - the sides are stoney and symmetrically flanking what appears to be a fragment of some form of segmented exoskeleton (?) which seems quite hard and strong despite its flakey appearance.
I found it in the Algarve, at Galé Beach / Praia de Galé in Albufeira, Portugal.
Have you seen similar finds? So interested in hearing your thoughts!
PS I tried to provide useful photos, definitely let me know if I can do better