After spending millions of years attuning themselves to living on land, some reptiles returned to the sea, as well as inhabiting freshwater bodies. Some stayed close to the shoreline, feeding off the bounty in seashore sand.
The fact that marine reptiles and sharks were so large during the Jurassic and Cretaceous suggest that the marine food supply was ample. Over half of the oil now being exploited was laid down during the Cretaceous.
The seas were more densely populated by phytoplankton than at any other period in geological history. Diatoms emerged in the Late Jurassic, greatly radiating and becoming abundant during the Cretaceous. As today, tiny autotrophs were then the fodder that fed the oceanic food chain.