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
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.