This is a large extinction event and 75% of species became extinct around 65million years ago. This marks the end of the Mesozoic era. Once again the result was gradual, showing a decline in species over several million years, leading finally to an abrupt extinction event. Marine casualties include ichtyosaurs, brachiopods, forminifera, belmnites and some bivalves. Loses on land include dinosaurs, pterosaurs and plants. The extinction of dinosaurs did leave a large ecological niche, which mammals largely filled.
Hypothesis to explain the Cretaceous-Tertiary mass extinction
Impact of an asteroid or meteorite- this remains a popular theory and there is a lot of evidence to suggest that a large object did hit the earth 65ma.
- A layer of iridium can be found concentrated in clay's at the boundary. Most iridium comes from space
- shocked grains of quartz are found at the boundary as a thin layer within sediments
- the presence of tektites
- large scale sedimentary evidence in Texas that there was a huge tsunami at this time
- a large meteorite crater can be found on the Yucatan Peninsula in Mexico, at Chicxulub. Although there are other contenders this is the most likely impact site for the meteorite.
Major volcanic activity( Deccan traps)- there was another enormous series of eruptions in India, covering an area of roughly 500,000km^2. The eruptions took place over about 30,000 years. The effects of this volcanism was probably the same as with the Siberian Traps.