Complex sentence: Ancient depictions of the god Mercury and of Roma (personification of the city) depict them wearing winged helmets, however in the 19th century the winged helmet became widely used to depict ancient Celtic, Germanic and Viking warriors.
Split into simpler sentences: Ancient depictions of the god Mercury and of Roma (personification of the city) depict them wearing winged helmets. There is some limited evidence of such decorative motifs being used on actual helmets, but it was only in the nineteenth century that the winged helmet became widely used to depict ancient Celtic, Germanic and Viking warriors.