Old Earth Ministries Online Dinosaur Curriculum

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Lesson 5 - Dinosaur Extinction

   The Cretaceous傍ertiary extinction event, which occurred approximately 65.5 million years ago (Ma), was a large-scale mass extinction of animal and plant species in a geologically short period of time. Widely known as the K傍 extinction event, it is associated with a geological signature known as the K傍 boundary, usually a thin band of sedimentation found in various parts of the world, that is rich in iridium, a signature element found in meteorites. K is the traditional abbreviation for the Cretaceous Period derived from the German name Kreidezeit, and T is the abbreviation for the Tertiary Period (a historical term for the period of time now covered by the Paleogene and Neogene periods). The event marks the end of the Mesozoic Era and the beginning of the Cenozoic Era. With "Tertiary" being discouraged as a formal time or rock unit by the International Commission on Stratigraphy, the K傍 event is now called the Cretaceous鳳aleogene (or K鳳g) extinction event by many researchers.

VIDEO
The End of the Dinosaurs

 

Above: A Wyoming (USA) rock with an intermediate claystone layer that contains 1000 times more iridium than the upper and lower layers. Picture taken at the San Diego Natural History Museum (click to enlarge) (Picture Source)

     Because there were no known fossil non-avian dinosaur fossils in rocks younger than the K傍 boundary, scientists originally concluded that non-avian dinosaurs became extinct immediately before, or during the event.  Recent discoveries of some non-avian dinosaur fossils above the K-T boundary have been found, indicating that a few species may have survived for a few hundred thousand years after the K-T (see Dinosaur Tombstone).

     Scientists theorize that the K傍 extinctions were caused by one or more catastrophic events, such as massive asteroid impacts (like the Chicxulub impact), or increased volcanic activity. Several impact craters and massive volcanic activity, such as that in the Deccan traps, have been dated to the approximate time of the extinction event. These geological events may have reduced sunlight and hindered photosynthesis, leading to a massive disruption in Earth's ecology. Other researchers believe the extinction was more gradual, resulting from slower changes in sea level or climate.

     This lesson will not examine the entire scope of the event.  For a more complete description, see the K-T Extinction Event page on Wikipedia. 

 

Duration

 

     The length of time taken for the extinction to occur is a controversial issue, because some theories about the extinction's causes require a rapid extinction over a relatively short period (from a few years to a few thousand years) while others require longer periods. The issue is difficult to resolve because of the Signor-Lipps effect; that is, the fossil record is so incomplete that most extinct species probably died out long after the most recent fossil that has been found. Scientists have also found very few continuous beds of fossil-bearing rock which cover a time range from several million years before the K傍 extinction to a few million years after it.

 

Causes

 

     There have been several theories on the cause of the K傍 boundary which led to the massive extinction. These theories have centered on either impact events or increased volcanism; some include elements of both. There is even a scenario combining three major postulated causes: volcanism, marine regression, and extraterrestrial impact. 

      By far, the most widely accepted theory is the impact theory.  In 1980 a team of researchers consisting of Nobel prize-winning physicist Luis Alvarez, his son geologist Walter Alvarez, and chemists Frank Asaro and Helen Michel discovered that sedimentary layers found all over the world at the Cretaceous傍ertiary boundary contain a concentration of iridium many times greater than normal (30 times and 130 times background in the two sections originally studied). Iridium is extremely rare in the earth's crust, but it is abundant in most asteroids and comets.  The Alvarez team suggested that an asteroid struck the earth at the time of the K傍 boundary. There were other earlier speculations on the possibility of an impact event, but this was the first evidence uncovered.  Such an impact would have inhibited photosynthesis by generating a dust cloud, which would block sunlight for a year or less, and by injecting sulfuric acid aerosols into the stratosphere, which would reduce sunlight reaching the Earth's surface by 1020%. It would take at least ten years for those aerosols to dissipate, which would account for the extinction of plants and phytoplankton, and of organisms dependent on them (including predatory animals as well as herbivores). Small creatures whose food chains were based on detritus would have a reasonable chance of survival. The consequences of reentry of ejecta into Earth's atmosphere would include a brief (hours long) but intense pulse of infrared radiation, killing exposed organisms. Global firestorms may have resulted from the heat pulse and the fall back to Earth of incendiary fragments from the blast. High O2 levels during the late Cretaceous would have supported intense combustion. The level of atmospheric O2 plummeted in the early Tertiary Period. If widespread fires occurred, they would have increased the CO2 content of the atmosphere and caused a temporary greenhouse effect once the dust cloud settled, and this would have exterminated the most vulnerable organisms that survived the period immediately after the impact.

     Subsequent research identified the Chicxulub Crater buried under Chicxulub on the coast
Chicxulub crater
Radar topography reveals the 180 kilometer (112 mi) diameter ring of the crater; clustered around the crater's trough are numerous sinkholes, suggesting a prehistoric oceanic basin in the depression left by the impact.
 of Yucat疣, Mexico as the impact crater which matched the Alvarez hypothesis dating. Identified in 1990, this crater is oval, with an average diameter of about 180 kilometers (112 mi), about the size calculated by the Alvarez team. The shape and location of the crater indicate further causes of devastation in addition to the dust cloud. The asteroid landed in the ocean and would have caused megatsunamis, for which evidence has been found in several locations in the Caribbean and eastern United States洋arine sand in locations which were then inland, and vegetation debris and terrestrial rocks in marine sediments dated to the time of the impact. The asteroid landed in a bed of gypsum (calcium sulfate), which would have produced a vast sulfur dioxide aerosol. This would have further reduced the sunlight reaching the Earth's surface and then precipitated as acid rain, killing vegetation, plankton and organisms which build shells from calcium carbonate  For a more in-depth study of the possible causes, see Causes of Extinction.

 

Avian Dinosaurs (Birds)

 

      Most paleontologists regard birds as the only surviving dinosaurs. However, all non-neornithean birds became extinct, including flourishing groups like enantiornithines and hesperornithiforms. Several analyses of bird fossils show divergence of species prior to the K傍 boundary, and that duck, chicken and ratite bird relatives coexisted with non-avian dinosaurs. Neornithine birds survived the K傍 boundary as a result of their abilities to dive, swim, or seek shelter in water and marshlands. Many species of birds can build burrows, or nest in tree holes or termite nests, all of which provided shelter from the environmental effects at the K傍 boundary. Long-term survival past the boundary was assured as a result of filling ecological niches left empty by extinction of non-avian dinosaurs.

 

Non-avian Dinosaurs

 

     More has been published about the extinction of dinosaurs at the K傍 boundary than any other group of organisms. Excluding a few controversial claims, it is agreed that all non-avian dinosaurs became extinct at the K傍 boundary. The dinosaur fossil record has been interpreted to show both a decline in diversity and no decline in diversity during the last few million years of the Cretaceous, and it may be that the quality of the dinosaur fossil record is simply not good enough to permit researchers to distinguish between the choices. Since there is no evidence that late Maastrichtian nonavian dinosaurs could burrow, swim or dive, they were unable to shelter themselves from the worst parts of any environmental stress that occurred at the K傍 boundary. It is possible that small dinosaurs (other than birds) did survive, but they would have been deprived of food as both herbivorous dinosaurs would have found plant material scarce, and carnivores would have quickly found prey to be in short supply. The growing consensus about the endothermy of dinosaurs (see dinosaur physiology) helps to understand their full extinction in contrast with their close relatives, the crocodilians. Ectothermic ("cold-blooded") crocodiles have very limited needs for food (they can survive several months without eating) while endothermic ("warm-blooded") animals of similar size need much more food in order to sustain their faster metabolism. Thus, under the circumstances of food chain disruption previously mentioned, non-avian dinosaurs died while some crocodiles survived. In this context, the survival of other endothermic animals, such as some birds and mammals, could be due, among other reasons, to their smaller needs for food, related to their small size at the extinction epoch.

     Several researchers have stated that the extinction of dinosaurs was gradual, so that there were Paleocene dinosaurs. These arguments are based on discoveries in two rock formations.  First, the discovery of dinosaur remains in the Hell Creek Formation up to 1.3 metres (4 ft 3 in) above the K-T boundary indicates some dinosaurs survived at least 40,000 years after the event. More recent discoveries in the Ojo Alamo Sandstone indicate that at least one species of hadrosaur lived past the K-T boundary, approximately 65.118 Ma (about 400,000 years after the K傍 event).

 

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