Excess damage is calculated in a way similar to the way trample is handled. Start with the amount of damage being dealt to the creature and determine what is lethal. Usually, this is the creature's toughness minus the damage already marked on it, ignoring any replacement or prevention effects that will modify this damage. Also ignore whether the creature won't actually be destroyed by this damage, perhaps because it has indestructible.

Ravenous Tyrannosaurus can't devour creatures entering the battlefield at the same time as it.

Once you've determined how much damage is excess, Ravenous Tyrannosaurus simultaneously deals damage to the target creature and its controller. This damage may be modified by replacement or prevention effects.