and emblems, which indicated their rank and function in the court, but not their individual identities. Although there have been attempts to link some of the depictions with historical figures, these identifications have been speculative and unverified. In certain cases, the lack of information even extends to the functional roles of some figures, which cannot conclusively be determined. The bronze heads were reserved for ancestral altars. They were also used as a base for engraved elephant tusks, which were placed in openings in the heads. The commemorative heads of the king or the queen mother were not individual portraits, although they show a stylized naturalism. Instead, they are archetypical depictions; the style of their design changed over the centuries, which also occurred with the insignia of the depicted royalty. The elephant tusks with decorative carvings, which may have begun being used as a decorative element in the eighteenth century, show distinct scenes from the reign of a deceased king. As a prerequisite for royal succession, each new Oba had to install an altar in honor of his predecessor. According to popular belief, a person's head was the receptacle of the supernatural guide for rational behavior. The head of an Oba was especially sacred, since the survival, security, and prosperity of all Edo citizens and their families, depended on his wisdom. In the annual festivals to reinforce the mystical power of the Oba, the king made ritual offerings in these sanctuaries, which were considered essential for the continuation of his reign. The stylistic variation of these bronze heads is such an important characteristic of Beninese art that it constitutes the primary scientific basis for establishing a chronology. The leopard is a motif that occurs throughout many of the Benin Bronzes, because it is the animal which symbolizes the Oba. Another recurring motif is the royal triad: the Oba in the centre, flanked by two assistants, highlighting the support of those who the king trusted in order to govern. According to some sources, the Benin artists may have been inspired by items brought during the arrival of the Portuguese, including European illuminated books, small ivory caskets with carved lids from India, and Indian miniature paintings. The quatrefoil "river leaves" might have originated from European or Islamic art, but by contrast, Babatunde Lawal cites examples of relief carving in southern Nigerian art to support his theory that the plaques are indigenous to Benin.