Okay, I kind of understand some environmental pressures on Paleozoic amphibians such as Tiktaalik (i.e. pushed by predators and lack of competition on land, correct me if I'm wrong), but I am not sure about the environmental pressures that pushed the cetaceans to the waters. Were the pressures similar if not virtually identical to that of Tiktaalik, or was it something different? To add to that, what possible pressures today or in the foreseeable future could push the cetaceans back on land?