what is the name of an important Mesozoic index fossil?

Answer 1

Ammonites would be the classic example. But in the early Mesozoic (Early - Middle Triassic) conodonts are the most important. And in the later part of the Mesozoic, planktonic foraminifera, calcareous nannofossils, and Inoceramus bivalves are also important.

If you want the name of one fossil species, I dunno... Rotalipora globotruncanoides, a planktonic foraminifera, first appears at the boundary between the Lower Cretaceous and Upper Cretaceous.

Answer 2

trilobites, ammonites & brachiopods