No great extinction or burst of diversity separated the Cretaceous from the Jurassic Period that had preceded it. In some ways, things went on as they had. Dinosaurs both great and small moved through forests of ferns, cycads, and conifers. Ammonites, belemnites, other molluscs, and fish were hunted by great "marine reptiles," and pterosaurs and birds flapped and soared in the air above. Yet the Cretaceous saw the first appearance of many lifeforms that would go on to play key roles in the coming Cenozoic world.
Perhaps the most important of these events, at least for terrestrial life, was the first appearance of the flowering plants, also called the angiosperms or Anthophyta. First appearing in the Lower Cretaceous around 125 million years ago, the flowering plants first radiated in the middle Cretaceous, about 100 million years ago. By the close of the Cretaceous, a number of forms had evolved that any modern botanist would recognize.
At about the same time, many modern groups of insects were beginning to diversify, and we find the oldest known ants and butterflies. Aphids, grasshoppers, and gall wasps appear in the Cretaceous, as well as termites and ants in the later part of this period. Another important insect to evolve was the eusocial bee, which was integral to the ecology and evolution of flowering plants.
The Cretaceous also saw the first radiation of the diatoms in the oceans (freshwater diatoms did not appear until the Miocene).
The Cretaceous-Tertiary Extinction
The most famous, if not the largest, of all mass extinctions marks the end of the Cretaceous Period, 65 million years ago. As everyone knows, this was the great extinction in which the dinosaurs died out. (Except for the birds, of course.) The other lineages of "marine reptiles", such as the ichthyosaurs, plesiosaurs, and mosasaurs, also were extinct by the end of the Cretaceous, as did the flying pterosaurs -- although some, like the ichthyosaurs, were probably extinct a little before the end of the Cretaceous. Many species of foraminiferans went extinct at the end of the Cretaceous, as did the ammonites. But many groups of organisms, such as flowering plants, gastropods and pelecypods (snails and clams), amphibians, lizards and snakes, crocodilians, and mammals "sailed through" the Cretaceous-Tertiary boundary, with few or no apparent extinctions at all.
What on earth -- or not on Earth -- caused this extinction? And how can we know? Read on!
Read about the K-T Mass Extinction at the Hooper Virtual Paleontology Museum.
Find out more about the Cretaceous paleontology and geology of North America at the Paleontology Portal.