What are the key elements defining the separation between Jurassic and Cretaceous?

Question

1. We have one mass extinction that seperates Trias from Jurassic, roughly huge volcanims over a long period of time, and Pangea starting to divide.
2. Another mass extinction that marks the end of Cretaceous period, linked to the fall of an asteroid.

However there is no mass extinction between the two: What are the crucial elements for defining the limit between Jurassic and Cretaceous?

Answer 1

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.

https://www.sciencedirect.com/science/article/pii/019566719190001S

https://ui.adsabs.harvard.edu/abs/2017EGUGA..1912252L/abstract

https://www.researchgate.net/publication/236003223_Fixing_a_basal_Berriasian_and_Jurassic-Cretaceous_J-K_boundary-_perhaps_there_is_some_light_at_the_end_of_the_tunnel

Answer 2

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:

• flower plants
• 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 the angiosperms or Anthophyta. First appearing in the Lower Cretaceous around 125 million years ago, the flowering plants first radiated in the middle Cretaceous, about 100 million years ago.

The source article also states:

Early angiosperms did not develop shrub- or tree-like morphologies, but by the close of the Cretaceous, a number of forms had evolved that any modern botanist would recognize. The angiosperms thrived in a variety of environments such as areas with damper climates, habitats favored by cycads and cycadeoids, and riparian zones. High southern latitudes were not invaded by angiosperms until the end of the Cretaceous. Ferns dominated open, dry and/or low-nutrient lands. Typical Jurassic vegetation, including conifers, cycads, and other gymnosperms, continued on into the Lower Cretaceous without significant changes. At the beginning of this period, conifer diversity was fairly low in the higher latitudes of the Northern Hemisphere, but by the middle of the period, species diversification was increasing exponentially. Swamps were dominated by conifers and angiosperm dicots.