The Pokedex is unreliable and self-contradictory. In Pokemon White version the Pokedex says "They upgraded the cannon on its back." implying it already had a cannon. The Black/White 2 entries contradict it, but I think the cannon already being present is more interesting so I went with the White version entry.
Yeah, I wish they could have somehow fitted near each other. But one is a trilobite and one is an insect. Just not close enough to squeeze them together. We have done Scyther's ancestor though, and it looks pretty sweet. It's related to Leavanny.