Here's one interpretation:
"In reality, the relief shows the World Tree, which the Maya believed had its roots in the underworld, trunk on the earthly plane, and branches high in paradise, and Pacal's relationship to it in death. The king is depicted either at the moment of his death falling from the earthly plane down into Xibalba or at the moment of his resurrection from the underworld, climbing up the World Tree toward paradise. The adornments along the edges represent the sky and other glyphs the sun and moon and, still others, past rulers of Palenque and Pacal's place among them. The bird at the top of the tree is the Bird of Heaven (also known as The Celestial Bird or Principal Bird Deity) who represents the realm of the gods in this piece, and the `urn' beneath Pacal is the entrance to Xibalba." link
That sounds similar to their explanation of the Dendera Reliefs.
"The supposed Crooke’s tube is mentioned in the texts describing the scene as a sun barge, the boat in which the sun god Ra‘ travelled across the sky. The form of the barge is in no way unusual. In many representations, the solar bark consists of a string like object with a bow and stern, while gods and objects connected with the sun or the sunrise stand on the horizontal platform. One of these objects, usually seen at the stern, is the lotus flower, which is what Bielek describes as the lamp socket."
And then we have this guy:
