Added the following 2 entries to the 19th (2nd half) and 20th century timelines, based on information from Kurt Dietrich’s excellent Jazz ‘Bones: The World of Jazz Trombone:

c. 1895—New Orleans, Louisiana: Trombonist William Cornish joins trumpeter Buddy Bolden’s band as possibly the earliest known jazz trombonist (Bolden himself often being considered the first jazz musician). Jazz pioneer Kid Ory later describes seeing and hearing Cornish perform, usually on valve trombone, at around the turn of the century (Dietrich 15) (see facing photograph of band; public domain; source: wikimedia commons).

1917—New Orleans, Louisiana: Trombonist Eddy Edwards, performing as a member of the Original Dixieland Jazz Band, participates in the first commercial recordings of what is advertised as jazz. Edwards is considered by some to be the first jazz trombonist to gain national fame (Dietrich 17).