Last I heard it was was completely made up, there's basically no way to tell from fossilized remains. Edit: It's explained a bit more in the book, where they say "they have great visual acuity, but a simple amphibian visual system, that's highly attuned to motion". The dinosaurs in JP were partially mixed with amphibian DNA, so it was probably intended to be a consequence of that in universe. But in real life, there's no way to know.