Protostega is a close relative to the modern day Sea Turttles.  Like there moderin day relatives they ate jelly fish.  They are well protected thanks to its shell.  Like modern day sea turtles they lay there eggs on land and leave them alone to return to the ocean as well as the same breeding habbits to return to the same breeding beach every year.  These large Turtles lved in the western interior Sea dur the creteacous period.