Life in the Cretaceous Seas
Many groups of marine organisms continued through from the Jurassic to the Cretaceous. Sharks of all kinds abounded, as well as many species of bony fishes. Mosasaurs, a new type of aquatic marine lizard, were widely distributed predators, with some species that reached over 14 meters in length. Equally dangerous and just as large were plesiosaurs (such as Kronosaurus) and crocodiles, but these were less common. Ichthyosaurs, which dominated Triassic and Jurassic oceans, had all but disappeared by the Early Cretaceous.

Reptiles were not the only marine giants of the Cretaceous. Strange-looking, often gigantic rudistid clams, reminiscent of Paleozoic horn corals, reached up to one meter in length and formed extensive reefs in shallow tropical oceans. Inoceramid clams over three meters long occurred in shallow, warm seas, including environments that were nearly devoid of oxygen. Ammonite cephalopods continued to diversify into amazing sizes and shapes, with some coiled forms over two meters across, and other forms that resembled an extended hook over two meters long.

The Cretaceous was also a high point for the evolution of plankton, at the other end of the size spectrum. Diatoms, a new group of photosynthesizing marine organisms, first appeared in the Early Cretaceous. The beautiful glassy skeletons of this group were far outnumbered by the limey-shelled coccoliths and foraminifera. Many other types of calcareous plankton reached their peaks at this time. These organisms dominated the plankton in most of the world’s oceans well into the succeeding Cenozoic Era, when the seas were much cooler and the patterns of ocean circulation much different.