The Crucifixion - earliest-known depiction?

Well, according to the page from which I have borrowed this image, it is.

This is one of the carved panels in the 5th century cypress-wood door of the Basilica of Santa Sabina, the oldest extant Roman basilica.

Since 1222, Santa Sabina has been the conventual church of the mother-house of the Dominicans, the Order of Preachers, on the Aventine Hill in Rome.