Many people use the term "dinosaur" to refer to something that is outdated or past its time: "This dinosaur of a computer never works properly!" But the dinosaurs and other groups of animals that went extinct at this time were actually doing very well. Only the Ichthyosaurs were showing signs of fading out.
The Cretaceous-Triassic Extinction (often called the K-T Extinction or K-T Boundary) is most famous for the extinction of the dinosaurs. Other groups of vertebrate animals which went extinct at this time included the reptile groups of the Pterosaurs, Plesiosaurs, and Mosasaurs, as well as many species of Crocodilians.
Many invertebrate species also went extinct at this time as well, including many species of Ammonites, which were on the rebound from their decimation at the end of the Permian Period. (Be sure to read my article "Extinction I: The Permian Extinction Event".)
It has been estimated that the Cretaceous-Tertiary Extinction witnessed the extinction of 80-90% of marine species, about 50% of the marine genera and about 15% of the marine families. For terrestrial (land) animals, about 85% of the species, about 56% of the genera and about 25% of the families died out. Animals that were over about 55 pounds (25 kg) all became extinct at this time.
Many reasons are often cited as the "cause" of the K-T Extinction, but it was most likely due to many events which coincided at this time. The K-T Extinction occurred at a time when the supercontinent Pangaea was breaking up due to Continental Drift. This would have resulted in continents moving closer to the poles, leading to colder winters in these areas. Also, mammals were spreading and would have competed with the many types of reptiles that went extinct at this time.
In my next article, I will be discussing some of the hypotheses of what caused the Cretaceous-Tertiary Extinction.
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