Ammonite
Ammonites began life as tiny planktonic creatures less than 1mm in diameter. In their infancy they would have been vulnerable to attack from other predators, including mosasaurs and fish. However, their shell gave their soft parts some protection. The existence of sexual dimorphism, with larger females and smaller males, has been much discussed.p244 The matter is still open, but at least in some species deposits are found with two sizes and no intermediates.