Complex sentence: They evolved during the Late Jurassic but were not very common then, but during the Cretaceous they rapidly diversified and become one of the most distinctive components of Cretaceous marine faunas.
Split into simpler sentences: They evolved during the Late Jurassic but were not very common. However, during the Cretaceous, they rapidly diversified and become one of the most distinctive components of Cretaceous marine faunas.