Cretaceous Extinction Event

Introduction

The Cretaceous extinction event occurred ~65 million years ago and was a large-scale mass extinction of animal and plant species with 46% of genera and 70% of species lost (Barnosky et al. 2011). The event marks the end of the Mesozoic. Widely known as the K-T extinction, it is associated with a geological signature usually present as a thin band of sedimentation found in various parts of the world, known as the K-T boundary (Fortey 1999).

Scientists theorise that the extinctions of the end Cretaceous period were caused by one or more catastrophic events, such as massive asteroid impacts or increased volcanic activity. A number of impact craters, and evidence of massive volcanic activity (such as the Deccan traps) have been dated to the approximate time of the Cretaceous extinction event. These geological events would have likely reduced sunlight, hindering photosynthesis. resulting in a massive disruption in the Earth's ecology. Other more gradual cases have also been put forward including changes in sea level or climate (Macleod et al. 1997).

After the Cretaceous extinction event, biodiversity required substantial time to recover, despite the existence of abundant vacant ecological niches.


What was lost?

There was significant variability in the rate of extinction between and within different clades. Those species that were dependent on photosynthesis were more severely affected as atmospheric particles blocked sunlight reducing the amount of solar energy that reached the Earth's surface. This significant plant extinction led to a major rearrangement of the dominant plant groups, severely affected herbivorous animals. Consequently top predators such as the Tyrannosaurus rex also perished.

(Image showing how a Tyrannosaurus rex may have appeared. From www.planet-inspire.blogspot.com)

• Microbiota - The Cretaceous extinction event was one of the most dramatic turnovers in the fossil record for various calcareous nanoplankton that formed the calcium deposits which gave the Cretaceous its name. Statistical analysis of marine losses during this extinction event suggest that the decrease in diversity was caused more by a rapid increase in extinctions as opposed to a decrease in speciation. ~ 55% of diatom species were lost during the extinction along with numerous species of planktonic and benthic foraminifera, suggesting a significant turnover but not a catastrophic extinction of these organisms (Macleod et al. 1997). 
• Marine invertebrates - ~98% of colonial coral species (those that inhibit warm, shallow tropical waters) became extinct during the Cretaceous extinction event. However, solitary corals which generally do not form reefs and tend to inhabit cooler and deeper areas of the ocean were not as significantly impacted. ~35% of echinoderm genera became extinct during this period as well as all but two species of the molluscan class Cephalopoda (those that did survive diverged into modern octopodes, squids, and cuttlefish) (Ward et al. 1991). Other invertebrates also became extinct during this period including rudists (reef-building clams) and inoceramids (relatives of modern scallops). 
• Fish - ~80% of shark, ray, and skate families survived the extinction, along with more than 90% of bony fish families. The marine and freshwater environments that these fish inhabited are believed to have mitigated the environmental effects of this extinction event. 
• Terrestrial plants - There are large amounts of evidence that the Cretaceous extinction severely disrupted plant communities. However, there were important regional differences, with 57% of North American plant species going extinction, yet New Zealand and Antarctica experienced no significant turnover of species. 
• Amphibians - Strong evidence that the majority of amphibians survived the extinction event relatively unscathed (Macleod et al. 1997). Archibald and Bryant (1990) conducted an indepth study of salamander genera in fossil beds in Montana, and found that six of seven genera were unchanged after the Cretaceous extinction event. 
• Non-archosaur reptiles - More than 80% of Cretaceous turtle species survived the extinction event, and all of those species which survived are represented in current species (Novacek 1999). The order Squamata (which is today represented by lizards and snakes were successful throughout the Cretaceous and survived the extinction event and are currently the most successful and diverse group of living reptiles. Their small size, adaptable metabolism, and ability to move to more favourable habitats were key factors in their survival. The mosasaurs and plesiosaurs (giant aquatic reptiles) were not so successful and became extinct by the end of the Cretaceous. 
• Crocodyliforms - Five of the ten families of crocodilians died out during the late-Cretaceous period, with the only apparent trend being that no large crocodile survived (Macleod et al. 1997). 
• Avian dinosaurs - All non-neornithean birds became extinct during the Cretaceous period including a number of groups which had previously flourished. Neornthine birds survived the extinction event due to their abilities to dive, swim,and seek shelter. 
• Non-avian dinosaurs - The majority of scientists agree that all non-avian dinosaurs became extinct during the Cretaceous extinction event. There is no evidence that these creatures could burrow, swim, or dive, and were therefore unable to shelter themselves from the environmental stress that occurred during this period. It is possible that small dinosaurs did survive, however, they would have been deprived of food as both herbivorous and carnivorous dinosaurs would have quickly found that their food was in short supply. Whether the extinction occurred gradually or very suddenly is a contested topic, as both views have support in the fossil record. 
• Mammals - All major cretaceous mammalian lineages survived the Cretaceous extinction event, although they suffered significant losses. In particular the marsupials, which largely disappeared from North America. The mammalian species of this period were generally small, comparable in size to rats. This small size would have helped them to find shelter in protected environments. 

Postulated Causes

There have been several theories on the cause of the K-T boundary which led to the mass extinction, these have typically centred on either impact events, increased volcanism, or a combination of both.

• Impact event - In 1980 Luis Alvarez, Frank Asaro, and Helen Michel discovered that sedimentary layers found all over the world at the Cretaceous-Teritary boundary contained a concentration of iridium 30-130 times background levels (Alvarez et al. 1980). Iridium is very rare in the Earth's crust as it is a siderophile element, and the majority of it travelled with the iron and sank into the Earth's core during planetary differentiation. Iridium remains abundant in most asteroids and comets, and the Alvarez team suggested that an asteroid hit the Earth at the time of the K-T boundary (Alvarez et al. 1980). Subsequent research identified the Chicxulub Crater buried under Chicxulub on the coast of Yucatan, Mexico as the impact crater which matched the Alvarez hypothesis size (180 km in diameter) and dating (Pope et al. 1996). The asteroid landed in the ocean and would have caused megatsunamis, evidence of which has been found in several locations in the Caribbean and Eastern USA. The asteroid landed in a bed of gypsum which would have produced vast amounts of Sulfur Dioxide, further reducing the sunlight reaching the planet, and then precipitated as acid rain. There is an ongoing dispute as to whether the impact was the sole cause of extinctions. However, in March 2010 an international panel of scientists endorsed the asteroid hypothesis as being the cause of the extinction, ruling out other theories such as volcanism. They determined that a 10-15 km space rock hurtled to Earth and hit at Chicxulub in the Yucatan peninsula.
• Deccan Traps - Recent evidence shows that the Deccan traps erupted over a period of 800,000 years, spanning the K-T boundary, and may potentially be responsible for the extinction and delayed biotic recovery experienced during the late-Cretaceous (Macleod et al. 1997). It is likely that large volumes of dust and sulfuric aerosols would have been released into the air, which may have blocked sunlight and reduced photosynthesis in plants. Carbon Dioxide may have also been emitted, increasing the greenhouse effect when the dust and aerosols had cleared from the atmosphere. 
• Multiple impact event - A number of other craters appear to have been formed around the time of the K-T boundary, suggesting the possibility of multiple near simultaneous impacts (perhaps from a fragmented asteroidal object). These include the 24 km wide Boltysh crater in Ukraine (65.17 mya +/- 0.64 ma) and the controversial much large 600 km wide Shiva crater beneath the Indian Ocean (~65 mya) (Macleod et al. 1997). 
• Maastrichtian sea-level regression - There is numerous evidence that sea levels fell in the late-Cretaceous by more than any other time in the Mesozoic. This would have greatly reduced the continental shelf area, which is the most species-rich part of the sea, which could have caused a mass extinction. Climate change would have also resulted partly by disrupting winds and ocean currents, and also by reducing the albedo of the Earth resulting in increased temperatures.

As well as these individual causes of the mass extinction a number of scientists feel that it was a combination of multiple causes that led to this, arguably the most famous mass extinction, resulting in the demise of the dinosaurs. 

References

Alvarez, L. W., W. Alvarez and H. V. Michel (1980). Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208: 1095-1108.
Archibald, J. D. and L. J. Bryant. (1990). Differential Cretaceous-Tertiary extinction of nonmarine vertebrates; evidence from Northeastern Montana. In Sharpton, V. L. and P. D. Ward (Eds.): Global catastrophes in Earth history: an interdisciplinary conference on impacts, volcanism, and mass mortality. Geological Society of America. Special Paper. 247: 549-562.
Barnosky, A. D. et al. (2011). Has the Earth's sixth mass extinction already arrived? Nature 471: 51-57.
Fortey, R. (1999). Life: A natural history of the first four billion years of life on Earth. Vintage.
Macleod, N. et al. (1997). The cretaceous-Tertiary biotic transition. Journal of the Geological Society. 154: 265-292.
Novacek, M. J. (1999). 100 million years of land vertebrate evolution: the Creaceous-Early Tertiary transition. Annals of the Missouri botanical garden. 86: 230-258.
Pope, K. O. et al. (1996). Surface expression of the Chicxulub crater. The Geological Society of America. 24: 527-530.
Ward, P. D. et al. (1991). Ammonite and inoceramid bivalve extinction patterns in Cretaceous/Tertiary boundary sections of the Biscay region. Geology. 19: 1181-1184.
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