Not just plains tuna, but desert tuna.

The tuna evolved eventually into buffalo but it took a long time.

They first established a beachhead and began hunting large cats by constructing a series of breathing apparatuses out of kelp.

It didn't always go that way - I understand that tuna developed a breathing apparatus with kelp, they were able to trap certain amounts of oxygen. Not going on land for days at a time, an hour, hour 45. Enough time to establish a beachhead and aggressively hunt cats and their family.