My guess is that the rainbow faces who were supposed to be in that scene had taken the day off (taking a joyride on Halley's Comet, I suppose

). They hadn't bothered to inform the cast and crew (that would be telling, wouldn't it?

), and with the entire populace of the Great Valley assembled for shooting, the filmmakers had to find replacements on short notice. It just so happened, though, that the two fast biters from the previous day's shooting were still hanging around, and in their infinite wisdom, the producers figured, "Oh, they're both medium-sized two-footed dinosaurs; we'll just have the folks in make-up give 'em a quick touch-up and nobody'll notice."

Unfortunately, after shooting that scene, most of the dinosaurs, tired of waiting, just ate their tree sweets and left, forcing them to use those atrocious CGI stand-ins we see in the next scene of the finished film.

I really need to get out more...