Though no one knows who truly created ska, guitarist Ernest Ranglin has every right to lay claim. He was already a first-call ax-slinger for West Indian jazz bands during the ’50s, and when the Jamaican recording industry exploded at the end of the decade, Ranglin became its No. 1 guitarist and the frequent arranger for ska hits. Joining Ranglin on some late-’50s and early-’60s sessions for the legendary Studio One label was a precocious likkle pianist named Monty Alexander, who was barely in his teens then. Their friendship and musical partnership has endured for more than 40 years now, and the guys still make crucial, reggae-steeped jazz, as their new Rocksteady demonstrates. Prepare to get up and drop some legs when the Monty Alexander Group plays at 8 and 10 p.m. (see City List for other dates) at Blues Alley, 1073 Wisconsin Ave. NW. $22.25. (202) 337-4141. (Christopher Porter)