No great extinction or burst of diversity separated the Cretaceous
from the Jurassic Period that had preceded it.
If there are no strong elements, some key feature are:
diatoms in the oceans
Perhaps the most important of these events, at least for terrestrial
life, was the first appearance of the flowering plants, also called
Short answer, there isn't one.
The J-K boundary does not have a well defined fingerprint or marker, because of this it is in flux with different studies constantly offering to tweek or redefine its date based on their own criteria.