Final answer:
The Ordovician Period is renowned for the longest sustained rise in marine biodiversity, with a significant expansion of marine invertebrates, the emergence of fish, and plants colonizing land.
Step-by-step explanation:
The Ordovician Period saw the longest sustained increase in biodiversity of marine genera and species richness in geological time yet documented. As the sea levels rose and the continents were mostly submerged, vast marine habitats emerged, offering a plethora of niches for organisms to exploit. This diversification included not only a significant increase in different types of invertebrates but also marked the first appearance of fish and the colonization of land by plants.
One of the key features during the Ordovician was the dramatic rise in numbers of genera (which correlates with species richness), stretching from the Cambrian through to the mid-Devonian, spanning approximately from 525 to 400 million years ago. The rise in richness was notable until a large extinction event occurred in the mid-Devonian period. This period of diversification set the stage for complex ecosystems and underlies the present-day biodiversity we observe in the Phanerozoic eon.