I can't say that I am familiar with your specific set-up, but would guess that either:
1) the toilet is bolted/screwed to the floor with a standard wax seal between it and the holding tank, or
2) the toilet is glued to the holding tank.
If the toilet is screwed or bolted down the lower section is likely a shroud that conceals the mounting points. Unless someone can offer more definite info, I'd closely examine the toilet to see if the lower section can be removed.
If the toilet and tank are glued together --- good luck! I'd probably opt for salvaging the holding tank and sacrificing the toilet.
Have you tried Googling the toilet model for a parts diagram or schematic?