The ends of geologic eras, such as the Paleozoic and Mesozoic, are marked by mass extinction events. These events are characterized by a significant and rapid decline in biodiversity, leading to the extinction of a large proportion of plant and animal species on Earth.

Here's a breakdown:

* Paleozoic Era: Ended with the Permian-Triassic Extinction, the largest known extinction event in Earth's history. About 96% of marine species and 70% of terrestrial vertebrate species went extinct.

* Mesozoic Era: Ended with the Cretaceous-Paleogene Extinction, also known as the K-Pg Extinction, which famously wiped out the dinosaurs. This event is attributed to a massive asteroid impact.

While mass extinctions are the most dramatic markers, other factors can also contribute to the boundaries between eras:

* Significant changes in climate and sea level: These can alter habitats and lead to the rise and fall of different species.

* Major tectonic activity: Shifts in continents and ocean basins can alter ecosystems and create new environments.

* Evolutionary innovations: The development of new adaptations can lead to the diversification of life and the emergence of new dominant groups.

Ultimately, the boundaries between geological eras are defined by significant shifts in the Earth's history and the resulting changes in its life forms.