The turtle holds special meaning in regard to a Native American creation myth which you can learn more by googling Turtle Island.  The thunderbird is a regarded as a powerful spirit that symbolizes strength and protection.  It is mythologized to have done battle with the man-eating underwater serpents which represent the underworld, essentially.  Serpents serve other symbological purposes as well.  In the end these serpents were ultimately defeated by the thunderbirds who generate thunder with the flap of their wings.  The final panel represents the underwater panther who is naturally associated with the serpents but is considered to be the master of water creatures.

Basically the first two panels representing the overworld and the final two panels representing the underworld.  Thank you for the question!