Other mistake: When Buck tells about his fight with Rudy, he swings while saying "Back, forth" in front of the others. They are looking the wrong way. He is also swinging on nothing at all.
Other mistake: As Sid gets Scrat stuck on his foot in the beginning of the movie, the sky and mountains shouldn't be visible, Scrat being at the bottom of the ravine he just fell in. (00:03:15)
Answer: 1. We don't know it was trying to eat the group. It may have just been trying to scare them away from its territory. 2. This isn't a true ankylosaur. It's something with 65 million additional years of evolution. There is no reason for it to be exactly the same as the Cretacean version.
LorgSkyegon