In 2021, one team of scientists found that most turtles that have mental glands live in the water. They wanted to know if different turtles got mental glands separately or if one ancestor turtle got mental glands. They found that mental glands happened just once in turtles, in the ancestor of the family Testudinoidea. Then some of the species that came from this ancestor kept their glands and some lost them. They believe that all turtles that have mental glands develop them from the same tissue.