Rule of thumb, it’s not perfect. Plus AFAIK archaeopteryx was more of a glider, rather than a dedicated flyer (the way pterodactyl was). You can also tell bird-like theropods from pterosaurs by the legs - birds have long-ish legs, while pterosaurs have short-ish legs (like bats).
As for the spinosaurs, while a swimmer (it’s hard to keep track, poor sod is getting retconned every other day) it was not an open sea kind of swimmer - just compare spinosaur’s limbs to that of a mosasaur
Rule of thumb, it’s not perfect. Plus AFAIK archaeopteryx was more of a glider, rather than a dedicated flyer (the way pterodactyl was). You can also tell bird-like theropods from pterosaurs by the legs - birds have long-ish legs, while pterosaurs have short-ish legs (like bats).
As for the spinosaurs, while a swimmer (it’s hard to keep track, poor sod is getting retconned every other day) it was not an open sea kind of swimmer - just compare spinosaur’s limbs to that of a mosasaur