Koalas have a plate of cartilage across their lower backs and Matilda is sitting on that plate in this shot.
The closest relative to the koala is the wombat, and they are the only other marsupial that has one of these plates. The wombat however uses this plate for defence! If they are out eating and are threatened, they run to their burrow, go just inside and wedge themselves using this plate as a door to prevent predators trying to get into the burrow after them.
Koalas just sit on them 😂
