Why would adding two small air-storage sacks have 'negative utility'? It appears it would add a small positive benefit and be selected for. This is true for both the posterior and anterior sacs. So yes, initially they'd be small, but selective pressure would cause for them to become enlarged as in modern birds. alligators appear to be a 'dead-end' that happened across a strong environmental niche, surviving into the present time little changed from anciently, when they ate dinosaurs and primitive mammals that stumbled their way. far more sluggish, and far less environmental pressure to cause them to change over time.