This image is Mictlantecuhtli (the Lord of Death) as the Earth Monster. This is why you have the skull imagery. The Earth was conceived as a great monster that swallowed the sun as it descxended in the West preparatory to its 9 hour journey through the underworld before rising again in the East. Often the whole body is not shown and all you get is the mouth and eyes as seen here. The piece is oriented properly but in its original placement it would be face down in contact with the earth. Many Aztec scultures have this imagery on the bottom. See similar
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Bernard