65 million years ago


The mass extinction that occurred at the end of the Cretaceous ended the Age of Reptiles and marked the beginning of the Age of Mammals. The mammals which survived the extinction event--probably the impact of a large meteorite or comet--found themselves in a changed world which lacked large herbivores, large carnivores, or competition from specialized animals adapted to specific habitats. Mammals would diversify in a way unlike their first 160 million year history.


The Cretaceous-Tertiary (K/T) extinction killed off a number of groups of organisms. Given the great diversity of organisms which died in this extinction, it is unlikely that the extinction of the dinosaurs was something that would only have effected them (such as a virus or increased predation on dinosaur eggs by mammals).

What caused the second worst extinction of prehistoric life? There are a number of possible explanations:

At the end of the Cretaceous, the earth’s continents were approaching their modern forms. The Rockies, Alps, and Andes were all rising. The oceans were regressing and the continents were more exposed than they had been in the previous 60 million years. There are some volcanoes in India that erupted in this time (60-65 mya) and could have released the iridium discussed below. All of this geologic activity could have affected climate and the volcanic activity could have released toxic gases into the atmosphere.

By the end of the Cretaceous, flowering plants had replaced the once dominant Mesozoic gymnosperms and ferns. Although such a drastic change in the vegetation seems a likely candidate for the extinction of some organisms, it certainly wouldn’t have affected marine life and flying pterosaurs. If anything, the diversity of herbivorous dinosaurs (such as hadrosaurs, ceratopsians, and pachycephalosaurs) was increasing in the Late Cretaceous.

It is possible that mammals increased their predation on dinosaur eggs. However, since dinosaurs had coexisted with mammals for more than 100 million years, it is unlikely that they were the primary cause of the extinction of dinosaurs.

It seems that the end Cretaceous was about 5 degrees cooler than the Mid Cretaceous and there may have been polar ice caps. This could have affected dinosaur reproduction. The gender of about half the kinds of reptiles is controlled by nest temperature rather than sex chromosomes. If such a mechanism for gender determination existed in dinosaurs, changes in temperature might have affected gender ratios in dinosaur populations. Although this is a reasonable hypothesis, it should be remembered that dinosaurs dominated a diversity of habitats ranging from the equator to the arctic and Antarctic circles. Dinosaurs demonstrated that they were capable of thriving in a broad range of temperatures.

The Impact of a Comet or Meteorite
All over the world (more than 100 sites), an extremely high iridium content is found in the soil layer at K/T boundary. Most iridium comes from meteorites or other matter from outer space. Shocked quartz crystals are also found in K/T strata. This deformation in quartz grains is observed at nuclear test sites, meteorite impacts, and high speed shock in labs. The K/T strata also contain microtektites--small blobs of silica rich glass believed to be material thrown up from a meteorite impact. The presence of chromium in the K/T strata is consistent with the impact of a carbonaceous chondrite meteor (Shukolyukov, 1998; Pillmore, 1984; Alvarez, 1980; Alvarez 1984; Alvarez, 1995; Izett, 1991).

Did a meteorite/comet impact cause the end-Cretaceous extinction? There are a number of craters that could be the right age and there may have been multiple impacts. The Chicxulub crater off the Yucatan peninsula (Mexico) has the appearance of an impact crater and is 65 million years old. This crater is about 100 km in diameter, but is surprisingly deep. Its rocks are full of iridium and shocked mineral grains. Sixty-five million year old signs of tidal waves in the Gulf of Mexico and the western Atlantic support the idea that there was an impact in this area. Ejecta found in regions nearby that suggest material was thrown from an extraterrestrial impact (Swisher, 1992; Melosh, 1997).

What would have been the effects of such an impact? It is estimated that sunlight would have been blocked for 3 months because of the amount of debris ejected. In the Western U.S., the layers of Cretaceous pollen lie under an iridium layer, which lies under a layer with plant and fungal types which are consistent with a cooling of an “impact winter” (Wolfe, 1986; Tschudy, 1984; Vajda, 2004).

Short term global warming would have resulted from the impact. There might have been global wildfires and some K/T sites are soot rich.

Are there any known fossils of Paleocene dinosaurs? Although there have a few claims of fossil dinosaurs in Cenozoic strata none conclusively substantiated. Many argue that the possibility that a few dinosaurs survived the impact for a short period in the Cenozoic would be of minor consequence. The fact would still remain that a sudden event at the end of the Cretaceous doomed many dominant groups of the Mesozoic Era, and ushered in the Age of Mammals in the Cenozoic.