Board Thread:General Discussion/@comment-26200221-20151211162449/@comment-34326521-20160221121952

Wikipedia has an article on species with natural projectiles.

There are many species that can evolve retractable fingers which fire off explosive projectiles. In fact, it's all of them. All they'd need is luck. I can't imagine what led sharks to evolve detachable jaws either, since they're already some of the largest predators in the ocean, but the fact is, they did. And even if Vaxasaurians were at the very top of the food chain, that doesn't mean anything. Any other species might get lucky and evolve something that will be their (Vaxasaurians') downfall.

The odds of fully completed rocket launchers "popping up" in any living creature, without intentional DNA manipulation, is probably zero, or very close to it. However, evolution doesn't work that way. Bits and pieces, that are useful on their own, always come first.

The thing about the energy cost is: something is only "too energy costly" if there's either not enough energy available in the ecosystem, or if the creature can't ingest and process enough calories for it to work. The human brain consumes ridiculous amounts of energy, relative to its mass, but it's still cost-effective, because we can ingest enough calories for it and the rest of our bodies. The functions it performs are also extremely important. As for Ultimate Humungousaur, rocket launchers are probably a good way to keep predators at bay.