The Centaurs (partially lost footage from unfinished Winsor McCay animated short; 1918-1921)
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Winsor McCay was animation's first true innovator. His shorts, including How a Mosquito Operates and Gertie The Dinosaur, helped innovate animation technique. His detailed style and passion inspired future animators as Tex Avery, Otto Messmer, and Walt Disney.
In 1947, a collection of unfinished/unreleased works by McCay were found to be in the possession of long-time collaborator Irving Mendelsohn, who received them years earlier by McCay's son Robert. Unfortunately, the films had not been stored well and deteriorated by the time. When they were rediscovered, several portions of the reels had decomposed completely. Among the unreleased and partially destroyed material was 90 seconds of footage from McCay's (now well-known) production, The Centaurs, a fragment showing animated centaurs wandering through a live-action forest.
The fragment's animation was gorgeous but seemed to have no plot. For decades, historians were baffled as to what production the footage had been created for. Finally, in the mid-2000s, a small set of production notes from an unfinished Winsor McCay film titled The Centaurs surfaced. Given the animation style and title, the fragment has been attributed to that production.
It is unclear as to why exactly the film was never completed. It was probably due to expenses or scenes including nudity. The production notes say the finished footage amounted to over three minutes, of which only the aforementioned 90 seconds survive. Unfortunately, due to deterioration and its lack of completion, it's likely that the rest of the film will never be seen.