 He subsequently proposed the names of Sauropsida and Ichthyopsida for the latter two groups.In 1866, Haeckel demonstrated that vertebrates could be divided based on their reproductive strategies, and that reptiles, birds, and mammals were united by the amniotic egg.The terms "Sauropsida" ('lizard faces') and "Theropsida" ('beast faces') were used again in 1916 by E.S.