|PREHISTORIC LIFE HOME||PREHISTORIC LIFE TABLE OF CONTENTS||OBL HOME||OBL REFERENCES|
THE CRETACEOUS-TERTIARY (K/T) EXTINCTION
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). About 15% of marine invertebrate families become extinct; the ammonoids, rudists (reef building clams), and inoceramid clams were especially effected. Marine reptiles (mosasaurs, plesiosaurs), foraminiferan groups, dinosaurs, pterosaurs, several types of marsupial animals, and some plants all become extinct. Dinosaurs had adapted to such a broad range of habitats, including polar environments, which a sudden extinction event is more likely than a gradual changing of climate. A number of species may have survived the K/T boundary only to become extinct shortly afterwards (Kaiho, 1999; Jablonski, 1997; Jablonski, 1995; Drake, 1983; Marshall, 1996).
What caused the second worst extinction of prehistoric life? There are a number of possible explanations:
The end of the Cretaceous witnessed increased volcanic activity due to the movement of continents. Volcanoes were active in the rising Rocky Mountains and in the southwest of India, volcanic eruptions produced a bout 480,000 cubic miles of basaltic lava covering an area about the size of California (with accumulated lava flows more than 1.5 miles thick in some areas. These Deccan traps are thought to have released about 21 trillion tons of hydrogen sulfate and 300 billion tons of hydrochloric acid leading to acid rain and the acidification of the oceans. In addition, 33 trillion tons of carbon dioxide were released which, after an initial cooling of 3-5 0C because of the volcanic debris blocking sunlight, may have raised global temperature by 5 0C. These eruptions, which are the largest known set of volcanic eruptions in earth’s history, occurred at the end of the Cretaceous over a period of 500,000 years between 66 and 65 million years ago (Dingus, 1998; Feduccia, 2003).
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
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.
Some have suggested that there was no mass extinction of dinosaurs, just a gradual decline. In other words, some wonder whether the dinosaurs went out with a bang or a whimper. Some who have analyzed the fossil record suggest this, while others disagree.
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).
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. Ejecta and evidence of tsunamis around the Caribbean, Central America, and the southern United States support the conclusion that the meteor struck the Yucatan Penninsula (Chatterjee, 1997; Swisher, 1992; Melosh, 1997).
The 10 km asteroid that impacted the earth struck at a speed of 90,000 km/h and a force of 100 million megatons. What would have been the effects of such an impact? Effects of the impact probably included temperatures at the impact site of several thousand degrees, global wildfires, an immediate spike in global temperature, followed by a freezing period of several months to a year because of the debris cloud which would have blocked sunlight, a longer term global warming period, and intense acid rain which would have acidified the ocean. It is estimated that sunlight would have been blocked for 3 months because of the amount of debris ejected. Tsunamis, global fires, a short-term temperature spike, and debris clouds which blocked the sun and caused a longer-term temperature drop. 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”. Tsunamis are thought to be responsible for an unusual layer of sandstone which underlies the iridium rich layer in sites in the Southeast U.S. The tsunami is thought to have reached a height of up to 300 feet and flooded regions which were 62 miles inland. (Dingus, 1998; Wolfe, 1986; Tschudy, 1984; Vajda, 2004; Feduccia, 2003).
Short term global warming would have resulted from the impact. There might have been global wildfires and some K/T sites are soot rich.
At one site near Antarctica, a layer of fish bone indicating a mass death assemblage occurs immediately after the iridium layer marking the impact. Because some of the other common marine organisms of the Late Cretaceous (such as the ammonites) last appear just before this event, it is suggested that the meteorite impact was “the straw that broke the camel’s back” and added one stress too many to an already stressed world. For example, although there was a massive volcanic eruption (the Millbrig/Big Bentonite event) in the Middle Ordovician in North America which covered several million square kilometers, global biodiversity did not decrease as greatly as at the K/T extinction event. Perhaps the K/T extinction event would have been less dramatic had the earth’s biosphere been stable prior to the impact (Zinmeister, 1998).
Frogs, salamanders, turtles, and crocodiles seemed to have survived the K/T extinction with the least number of extinctions (Dingus, 1998).
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.