During the introductory scene, in the closeup shot of Halftooth's animatronic head bellowing at the terror birds, there is a hairy piece of dirt on one of his teeth. In the next shot, which is computer-animated, his teeth are clean.
Phorusrhacos was already extinct several million years before the timeframe of this episode. (The bird featured in the episode is actually its relative, Titanis. But since it is constantly referred to as Phorosrhacos, even in the book, this is a legitimate error.)