Gradual evolution of costal aspiration breathing in tetrapods.

The evolution of costal aspiration breathing has lacked a complete, plausible scenario for the intermediate steps and selective pressures that led to the transformation from buccal pump breathing in anamniotes to the use of ribs and intercostal muscles for aspiration breathing in amniotes. Problems have included the selective pressure that would have produced such a profound change from a head pump to a body pump, the adoption of costal aspiration despite some kind of diaphragmatic solution arguably being more plausible, and the possibility that the transition may have been abrupt, without a prolonged period of buccal pumping and costal aspiration being used together. Here I propose a plausible, but as yet untested, set of intermediate steps and selective pressures that could have produced this great transformation. The key points are that increasingly terrestrial feeding may have selected against effective inspiration by buccal pumping, costal aspiration may have been the path of least resistance for evolving an accessory mechanism to assist the diminishing role of buccal pumping, and buccal pumping may have been retained as an accessory lung inflation mechanism in stem amniote lineages, providing time for costal aspiration to become an effective lung inflation mechanism.