A surgical tool assembly, comprising: a surgical end effector comprising first and second jaws (1102, 1130) operably interfacing with each other to move between a fully open position and a fully closed position relative to each other upon application of closing and opening motions thereto; a closure assembly comprising: - a proximal closure member configured to move through a first closure stroke distance upon application of a closure input motion thereto; - a distal closure member operably interfacing with said surgical end effector; and a closure stroke reduction assembly comprising a closure reduction linkage operably interfacing with said proximal closure member and said distal closure member such that when said proximal closure member moves through said first closure stroke distance, said closure reduction linkage causes said distal closure member to axially move through a second closure stroke distance that is less than said first closure stroke distance to thereby move at least one of said first and second jaws from said fully open position to said fully closed position characterized in that said surgical end effector is coupled to a shaft assembly comprising: a shaft mounting portion configured for operable engagement with a source of said closure input motion; and a spine assembly operably coupled to said surgical end effector and said shaft mounting portion, said spine assembly movably supporting said proximal and distal closure members thereon, the closure assembly extending around the spine assembly.. a surgical end effector comprising first and second jaws (1102, 1130) operably interfacing with each other to move between a fully open position and a fully closed position relative to each other upon application of closing and opening motions thereto; a closure assembly comprising: - a proximal closure member configured to move through a first closure stroke distance upon application of a closure input motion thereto; - a distal closure member operably interfacing with said surgical end effector; and - a proximal closure member configured to move through a first closure stroke distance upon application of a closure input motion thereto; - a distal closure member operably interfacing with said surgical end effector; and a closure stroke reduction assembly comprising a closure reduction linkage operably interfacing with said proximal closure member and said distal closure member such that when said proximal closure member moves through said first closure stroke distance, said closure reduction linkage causes said distal closure member to axially move through a second closure stroke distance that is less than said first closure stroke distance to thereby move at least one of said first and second jaws from said fully open position to said fully closed position characterized in that said surgical end effector is coupled to a shaft assembly comprising: a shaft mounting portion configured for operable engagement with a source of said closure input motion; and a spine assembly operably coupled to said surgical end effector and said shaft mounting portion, said spine assembly movably supporting said proximal and distal closure members thereon, the closure assembly extending around the spine assembly.. a shaft mounting portion configured for operable engagement with a source of said closure input motion; and a spine assembly operably coupled to said surgical end effector and said shaft mounting portion, said spine assembly movably supporting said proximal and distal closure members thereon, the closure assembly extending around the spine assembly..