Le squelette joyeux (The Merry Skeleton) is a simple minute-long short from film pioneers the Lumière brothers. The brothers filmed mostly documentary scenes to promote the possibilities of motion pictures, so you can say that any film they did was "experimental."
This dancing skeleton is sometimes captioned as an early experiment in stop-motion animation, but it is not. It is clearly a puppet, specifically a marionette. Since the puppet is far from anatomically correct, Silent-ology looked into the availability of such puppets in 1898.
It got me wondering if this type of skeleton puppet was common at all in the 19th century, something the Lumières were familiar with and thought would be fun to capture–and with a black cloth in the background too, so it’d really pop. Searches for “skeleton marionette” in vintage newspapers revealed that, why yes, dissembling puppets were a thing. (“Marionette” is the word of choice, don’t bother with “puppet.” 19th century and all.)
There are surviving examples of the skeleton marionette, which you can see at Slient-ology. They also found a couple of skeleton movies that were made earlier than The Merry Skeleton. This one is a live-action short from G.A. Smith called The X Rays from 1897.