3) Miocene Epoch

The Miocene (~22 to 5 million years ago) was a time of rich abundance and changes on land and in seas. The Himalayas were forming in Asia (as India collided with Asia), grasslands greatly replaced forests, and ocean diversity was the richest it's been in the last few 10's of millions of years, peaking around 15 million years ago. This time period started and ended with glaciation/ global cooling events, with several warm periods in-between.

One of the largest predators of all time, Megalodon sharks, existed and thrived on a rich whale selection to prey upon world-wide. But they were not alone near the top. Large predatory whales, new dolphins, other large sharks also patrolled these ancient seas. South Florida was underwater during much of the Miocene, and animals of that time, especially sharks (which constantly replace their teeth) left behind a great deal of fossils.

Megalodon shark feeding on cetothere whales

Why was marine life so abundant and sharks so huge during the Miocene? Widespread upwelling caused nutrient rich water was constantly coming to the surface in many areas around the world (the process is called upwelling) in unprecedented rates. With more nutrients in the ecosystem, more plankton thrive (& seagrass) as do higher organisms that feed off them and so on. Additionally the earth was much warmer than nowadays, meaning sea levels were higher and high energy consuming predators had more space and suitable conditions to thrive.

Diversity in both vertebrate and invertebrate life declined toward the end of the Miocene and into the Pliocene as the climate cooled. World-wide global cooling is most attributed to widespread burial of carbon in oceans and/or uplift of the Himalayas & Alps (taking out carbon dioxide from the atmosphere); although decline in salinity definitely occured in the late Miocene as the Mediterranean sea repeatedly became cut off from the Atlantic, favoring salt evaporite deposits to form, raising the freezing point of the oceans (Domning, 2000).