The clamshell shaped cavity is a mold fossil owing to the fact that the clamshell died, its body deteriorated but while it is deteriorating it is being covered by sediment. As a result when the clamshell completely disappears it will leave an impression on the sediment, in other words, it is a mold of the prehistoric clamshell.
The leaf-tailed gecko would have an advantage when hiding from predators, since it would blend in with the foliage, unlike the other geckos which would stand out.