Please see below for the background.
Below is the SUJA, finally published after 1 year of dormancy =) Quite unfortunately, it wasn't really selected for publication on the Science Faculty website so I will be publishing it myself here on my blog. As noted on the last post, my topic was on the extinction of the dinosaurs and how fast it happened. In other words were the dinosaurs in decline millions of years before the end of the Cretaceous period or the meteorite put an end to the age of dinosaurs to a sudden stop? UPDATE:(An updated version of this will be published in the Fall 2009 issue of the Triple Helix NUS. There will be substantial differences, so do have a look at the issue when it comes out as well)
INTRODUCTION
Dinosaurs are the most famous prehistoric animals ever known. Many books have been written about them for the laymen. People continue to be fascinated by dinosaurs which first evolved about 225 million years ago (MYA) in the Late Triassic and went extinct 65 MYA in the Late Cretaceous Period. During their 160 million years reign on Earth, they evolved into all shapes and sizes. The vast majority of scientists agree that modern-day birds are actually direct descendants from the dinosaurs. In other words, evolutionarily speaking, birds are dinosaurs. However, in this essay the word “dinosaur” is used to mean “non-avian dinosaurs”.
Since the discovery of dinosaur fossils in 1819, scientists have wondered what had caused their demise and have suggested over 80 theories just to do that.[1]Some of the more sensible ones are extraterrestrial body impact, volcanism, sea level changes and climatic changes. However, there are also the less credible theories like a mass death by a virus and aliens taking the dinosaurs away.[1] This divergence of theories is because there are many lines of evidence, both geological and biological and they can be interpreted in many ways. However, there is reason to believe that the most likely cause of their extinction is a catastrophic end by an extraterrestrial body as it accounts for the vast majority of the evidence and observations. The understanding of the extinction of the dinosaurs may give us an insight on what will happen if, however small the likelihood might be, if an asteroid were to hit the Earth. The study of mass extinctions also gives us insight on how biological systems may recover from an ecological crisis.
In the light of the current human-caused ecological crises with a high rate of species going extinct, this knowledge is invaluable.[1] In this article, the Cretaceous-Tertiary extinction is abbreviated as the K-T extinction. “Cretaceous” is abbreviated as “K” to avoid confusion with the Carboniferous Period (300 MYA). So far, the most accurate dating of the K-T extinction event is that it took place 65.95 MYA give or take 40,000 years. [2] But before we go into specific details of this extinction, we need to consider the nature of extinction and the potential pitfalls in determining it from the fossil record.
THE NATURE OF MASS EXTINCTIONS
A species goes extinct only when its last living member dies out. There are two types of mass extinction. One way is through background extinction. By nature, extinction is a normal outcome of evolution and usually species dies out in a constant way. Well known dinosaurs like Allosaurus, Iguanodon, Brachiosaurus and Stegosaurus probably went extinct through this process which is known as background extinction. However, when a large number of species goes extinct within a short period of time, it is said that a mass extinction had occurred. Researchers postulate that there have been 5 mass extinctions though geological history. The first occurred in the Late Ordovician 438 MYA and the fifth and most recent K/T extinction at Late Cretaceous 65 MYA. [3]Figure 1 below shows mass extinction events through the last 500 million years. The x-axis represents the time in million of years ago. The y-axis represents the percentage of marine animal genera going extinct. In general, extinction events have been shown to occur at regular intervals. The bigger the extinction event, the more infrequent it is. There is no major difference between a background extinction event and a mass extinction event, other than a mass extinction event is greater in magnitude.[4]
FOSSILS AND INTERPRETING OF THE FOSSIL RECORD
Researchers into extinctions obtain the evidence and develop their theories by studying fossils, the subject of the next section. What is a fossil? Fossils are basically the preserved remains of organisms from the prehistoric era. In general, organisms preserved this way are only called “fossils” only if they are older than 10,000 years ago. There are two types of fossils: body and trace fossils. Body fossils are the remains of an animal's body while trace fossils preserve an organism's activities like eggs and track prints. Fossils are preserved in general by the following process. Firstly, after death, the soft tissues of a dead body of an animal must decompose. Then the hard parts like bones, teeth and shells must be transported to their final resting place. The hard parts are rapidly buried and mineralized into rock. Finally, erosion by the elements eventually brings the fossils to the surface where they can be found. However, not all organisms have an equal chance of being fossilized [5]. A body can decay and break down very quickly. For example, in the tropical forest, a gorilla body can become a skeleton in ten days and disappear within three weeks --- without taking into account scavengers like hyena, which can chew its bones. The environment affects the changes of fossilization. Mountains are poor habitats for fossilization as no burial takes place, only erosion occurs. Forests are also not good for preservation as any bones can dissolve in the more acidic soils. Aquatic animals fossilize more readily than land animals as rapid burial takes place more often than on land. The type of organism also affects the likelihood of preservation. Animals without any hard parts like jellyfish leave little, if any, evidence in the fossil record.[6]
How are fossils studied? Well, scientists basically go to the fossils, dig them up and go home. However, fossil-hunting requires a lot of luck, patience and a trained eye. Fossils are studied in scientific institutions equipped with high technology equipment like computers, scanning electron microscope, geological analytical equipment and a good library [5].
From the fossil record, we have learnt a lot about the history of life. For example, we have reconstructed the history of life on Earth. [1] Many evolution lineages have been reconstructed due to the fossil record, most notably that of horses, whales, humans and the horned dinosaurs.[1] We also use fossils to reconstruct phylogenies, which are studies to show how organisms are related to each other. And finally, fossils are used to deduce how extinct organisms might have lived and looked like. [5]
PITFALLS OF THE FOSSIL RECORD
Interpreting fossil record is an art and there are potential pitfalls in using the fossil record to interpret mass extinctions events. One issue is that rocks do not preserve all the time that has elapsed. The problem is that the time period is only preserved in the fossil during the deposition of sedimentary rocks. Without deposition, there can be no record. There are some times and some places that have no sedimentary deposition, so there will be no record preserved. In the case of the K-T extinction, the only good fossil sites that preserve the very latest part of the Late Cretaceous are in the Great Plains of North America[7]. It can be seen that trying to reach global conclusions with evidence largely coming from only North America is simply questionable.
Another important issue is that not all organisms that have been living are represented as a fossil. Only the occasional fossil site preserves almost all the original biodiversity, a conclusion that has been reached from the fact that their biodiversity is similar in extent and in number to modern ecosystems.[1] With both issues discussed above, we can conclude that it is not to easy to elucidate the events of a particular extinction. As a result, the less we can tell about the different events in an extinction, the more the apparent abruptness due to the lack of information.
The third important issue is what we call the Signor-Lipps effect. [8] The argument is that the fossils in question are distributed regularly in the fossil record. The dividing line separates the fossil-bearing rocks from the non- fossil bearing ones. After this line, the organism can been said to have gone extinct. Suppose that the dividing line is approached from the oldest rocks from the bottom, as it is approached closer, the likelihood of finding the fossil decreases as the amount of rock the fossil can possibly be found decreases. So instead of the fossils being found up to the dividing line, the last occurrence of the fossil will be found below the line. Even if the extinction event is abrupt in nature, to the uninitiated laymen, it will seem gradual. This can be seen from Figure 2 below. [1, 8]
How do researchers gauge the magnitude of an extinction? The intuitive way is to use taxa at species level to calculate the percentage of the biota that has gone extinct. However, the problem is that very often a bunch of old bones cannot be identified down to species level. For example, while a lion and tiger are different species, their skeletons shows a high degree of similarity. The usage of higher taxa is problematic as it is quite subjective due to the fact that there is no set definition on taxa like family and order etc. What can be one person's family can be another person's two families. [1]
IMPACT THEORY
The impact theory postulates that the cause of the K-T mass extinction is by an impact by a extraterrestrial body, hitting Earth in what is now the Yucatan peninsula in Mexico. Many short term and long term effects of the extraterrestrial body,most likely an asteroid ten kilometres wide or a comet,[9]have been suggested. For the dinosaurs, the impact and aftermath of the asteroid hitting the Earth would have made living conditions impossible, resulting in its extinction over a short period of time. Figure 3 illustrates what it may be like at the point of impact.
EVIDENCE OF THE IMPACT THEORY
With the above in consideration, the arguments in favour of the impact hypothesis can be presented. There is both geological and biological evidence to support this hypothesis.
Where K-T sediments are exposed, the following arrangement can be seen. Cretaceous sediments are at the bottom with Tertiary sediments at the very top. In between these two layers is a clay layer rich in iridium. Figure 4 shows the K-T boundary. This is unusual as iridium is one of the rarest elements on the surface of the Earth but is present in the Earth's interior and in extraterrestrial objects like meteorites and asteroids in great abundance [10]. It has been suggested that this is representative of an asteroid striking the earth and throwing up a dust cloud. This dust cloud is represented by the clay layer formed at impact [11].
Further geological evidence can been seen from the shocked quartz was found at the K-T boundary. This is a form of quartz, a type of mineral, that possess an altered structure due to the enormous pressure resulting from the impact[12] Microtektites, which are small, rounded and circular silicate glass particles, [13] were also found at the K-T boundary all over the world . They are believed to be material from the meteorite that had molten and solidified, being thrown up to the atmosphere during impact.
This leads us to the million dollar question: Where is the crater if there was an asteroid hit the Earth? A most likely candidate is the Chicxulub crater near the Yucatan peninsula. It has been dated at 65 MYA which correlates well with the K-T boundary. [1] Glass from Haiti had been proven to come from the crater and shocked quartz had also been found in the crater itself.[1] Evidence for a tsunami after the impact have been found in certain sites in North America. Also, the K-T boundary in the crater was thicker compared to the other parts of the world. This represents material that did not got thrown up into the atmosphere. [9] The crater was close enough to have caused all of the observations after the impact. The formation of the shocked quartz can be attributed to the asteroid impact. Large amounts of water could have been displaced during an impact at sea, causing tsunamis to form.[3] There are also several lines of biological evidence to prove catastrophism by a asteroid. There is some proof from the fossil record of marine micro-organisms called foraminferia. In today's ocean, they are a constituent of the plankton in the oceans. Since the 1970s, most scientists have agreed that according to the fossil record for marine micro-organisms,the extinctions at the K-T boundary was abrupt.[1]. This is supported by a study of calcareous nanofossils, another group of organisms that corroborates the case for an abrupt extinction. [14] Both groups of micro-organisms show an abrupt pattern of extinction. Another study, using foraminferia, observed a very serious decrease in the primary productivity i.e. a decrease of organic matter synthesized by organisms using inorganic chemicals and sunlight over 3 millions years at and after the K-T Boundary [15]. This may be caused by the blockage of sunlight by the dust clouds and atmospheric aerosols thrown up by the asteroid which deprived them of sunlight which is an essential ingredient for photosynthesis. In general, the consensus is that according to the evidence from the marine microbes, the extinction is abrupt. But how abrupt would the extinction appear to be with regards to other groups of organisms?
The plant record also supports a catastrophic impact event. Studies have shown an increase in the prevalence of fern spores in many parts of the world during the K-T boundary. This increase in the amount of fern spores is taken as an indicator of fern populations at the time. Therefore it can be deduced that there is an increase in the population of ferns post impact. The pattern of the changes seen in plant biota was very similar to what may be expected by conditions seen after an “impact winter” or global wildfire [16] possibly caused by the heat build up during impact. Similar patterns of succession with ferns as a pioneer species have also been seen after major disasters like volcanic eruptions [17]. Also the plant record in North America suggests there is a major, abrupt plant extinction at the K-T boundary [18] These results suggest the rise of ferns at the expense of flowering plants due to a global disaster like wildfires at the K-T boundary caused by the impact.
Does the vertebrate record support the impact theory? In general it has been shown that animals that depended on detritus i.e. dead organic matter like marine scavengers, small mammals, and river animals were better off. The presence of a dust cloud or perhaps atmospheric aerosol thrown up by the impact of an extraterrestrial body may cause photosynthesis to cease. As a result, animals that depended on plants died off whereas animals depending on detritus would still have a food source due to the large amounts of dead organisms[19] Also animals that are small in size or were living in aquatic conditions had a high rate of survivability due to the simple fact they can avoid the adverse conditions at the surface just by hiding in the soil or just staying deep underwater.[20,21] Taken together, the evidence suggests that life on earth pre impact was undergoing a “business as usual” condition and most taxa underwent a drastic decline in diversity after the impact.
Despite being the “stars” of the K-T extinction, dinosaurs have not been well studied. While initially, they seemed to show a gradual pattern of extinction [22], the current consensus is that dinosaurs did not show a gradual decline but it was “business per usual” up to the point of impact [1, 23]. This is shown through statistical analysis of dinosaur diversity at both genus and family level. The number of taxa did not change significantly during the last few million years of the Cretaceous. In fact they have seemed to have maintained high diversity during this period. [23,24] However , the main obstacle to unlock the true pattern of extinction is that the only good dinosaur fossils site dating from the latest Cretaceous are only restricted to North America. There is a general lack of material from the other parts of the world. More study on fossils from across the world will be needed for a more accurate determination of dinosaur diversity and the pattern of dinosaur extinction.
In summary, post-impact, Earth was shrouded by aerosols of dust. This blocked out the sunlight. In the ocean, the deprivation of sunlight caused a decrease in the biomass of phytoplankton of the ocean. The decline of these primary producers would have caused a collapse of marine food chains. [24] Only animals which are hardy generalists and could feed on dead organic matter which could have piled up in huge amounts could survive the extinction. On land, tsunamis could have killed any land organisms living on the coast that have survived the initial blast. Any organisms that survived all of that must contend with wildfires set off due to the great amounts of heat produced by the impact. Plant diversity suffered due to the lack of sunlight needed for photosynthesis and the wildfires. For the herbivorous dinosaurs, they would have died out due to the lack of food. When the last carcasses of dead prey ran out, the carnivorous dinosaurs also went extinct due to starvation. The animals which could survive the extinction were mainly small such that they did not need a large amount of food to survive or could burrow underground to escape the adverse conditions above. As the earth recovered, large amounts of land were vacant and the first plants to resettle on the land were hardy species like ferns. Flowering plants settled after the ferns. The main survivors of the extinction like the mammals quickly diversified to fill the ecological niches left vacant by the now extinct dinosaurs.[25]
COUNTER ARGUMENTS OF THE IMPACT THEORY
What are some arguments against the impact theory? There were some suggestions that the shocked quartz and microtektites can be attributed to tectonic activity. Indeed, the Late Cretaceous was marked by an increase in tectonic activity in general. In India there is a series of lava flows called the Deccan Traps formed during the period between 65-60 MYA . The plateau that makes up a major portion of Southern India was caused by the lava flowing out of holes in the Earth's crust. Any volatile gases produced could have affected temperature globally. However, there are two problems to this. The Deccan traps have been dated to be slightly younger than the K-T boundary. Secondly, the shock metamorphic features that volcanic activity produces were different from the ones at the K-T boundary. [1]
Another theory is that marine regression was responsible for the death of the dinosaurs. There is a general consensus that there was a drop and rise in sea levels at the K-T boundary. There are some scientists who believe that this caused changes in the distribution of the seas and continents. Seas shrank while new land was opened up for terrestrial organisms. These changes resulted in ecological havoc to weaken the dinosaurs and made them vulnerable to environmental changes [26]. However, there is no geological evidence in North America that supports their theory that the drop of sea levels caused the decline of the dinosaurs.
It has also been suggested that climatic change may have contributed to dinosaur extinction. A cooling trend at the end of the Cretaceous has been generally recognized. A low ratio of 18O/16O in the shells of molluscs and brachiopods suggests a decrease of temperature in the ocean. The plant record in North America confirms the same thing on land.[3] However, just as we noted already, the results were taken mainly from the North American fossil record and it is not wise to assume a global climate change based on such localized data.
It has also been alleged that the dinosaurs gradually declined due to the rise of the angiosperms (flowering plants). Dinosaurs were thought to be not used to feeding on angiosperms and were believed to have fed predominantly on gymnosperms (non flowering seed bearing plants) as shown by teeth design of plant-eating dinosaurs and the presence of gymnosperm remains in coprolites (fossil faeces) [27]. However angiosperms were very likely to have evolved about 40 million years earlier and dinosaurs should have time to adapt to the appearance of angiosperms . There was also an increase in dinosaur diversity during the late Cretaceous especially in the hadrosaur (duck-billed dinosaur) and ceratopsian (horned dinosaur)groups. This would debunk the theory the angiosperm diversification resulted in dinosaur decline.
CONCLUSION
In conclusion, the other theories offered for the extinction of the dinosaurs are difficult to substantiate in comparison to the impact theory of catastrophic impact by an asteroid 65 MYA. Whether the dinosaurs were already in decline before this or not, the impact ended the reign of dinosaurs and changed the course of the history of life forever. For without the impact, mammals would not have evolved to dominate the Earth and therefore humans would not have evolved to prove that a group of reptiles once ruled his planet. No matter how they died out, palaeontologists will be continuing to work to uncover the mystery of dinosaur extinction.
3,501 words
References:
[1] D. Fastovsky & D. Weishempel (2005) The Evolution and Extinction of Dinosaurs 2nd Edition
[2] http://www.sciencedaily.com/releases/2008/04/080424140400.htm “When Did Dinosaurs Go Extinct? Cretaceous-Tertiary Boundary Dating Refined” Accessed on 17 Feb 2009
[3] A.Hallam (1997) Mass Extinctions and their Aftermath
[4] D. Raup & J. Sepkoski (1984) “Periodicity of Extinctions in the Geological past” Proceedings of the National Academy of Sciences, USA 81,801-805
[5] M. Benton & D. Harper (1997) Basic Palaeontology
[6] O.Judson (2008) http://judson.blogs.nytimes.com/2008/12/30/reflections-on-an-oyster “Reflections on an Oyster” Accessed 21st Mar 2009
[7] D. Fastovsky & P. Sheehan (2005) “The extinction of the Dinosaurs in North America” GSA Today March 2005
[8] P. Siggs & J. Lipps (1982) “Sampling bias, Gradual Extinction Patterns and Catastrophes in the Fossil record” In L. Silver & P. Schaltz (eds) “Geological Implications of Impacts of Large Asteroids &Comets on the Earth” Geological Society of America Special Paper No.190 pgs 291-296
[9] D. Carlisle (1995) Dinosaurs, Diamonds and Things from Outer Space
[10] N. Greenwood & S. Earnshaw (1997) Chemistry of the Elements
[11] L. Alvarez (1983) “Experimental Evidence that an Asteroid Impact led to the Extinction of Many Species 65 million years ago” Proceedings of the National Academy of Sciences USA 627-642
[12] www.scn.org/~bh162/shocked_quartz.html “WHAT IS "SHOCKED QUARTZ"?” Accessed on 1st Mar 2009
[13] http://www.agu.org/revgeophys/claeys00/node4.html “Tektites, Microtektites and Impact Glasses” Accessed on 6 March 2009
[14] J. Pospichal “Calcenous Nanofossils and Clastic Sediments at K-T Boundary, NE Mexico” Geology Vol 24 no. 3 pg 235-258
[15] S. D'Hondt, P. Donaghay, J. Zachos, D. Luttenberg, & M. Lindinge (1998) “Organic Carbon Fluxes and Ecological Recovery from the Cretaceous-Tertiary Mass Extinction” Science Vol. 282. no. 5387, pp. 276 - 279
[16] V. Vajda, I. Raine, & C. Hollis (2001) “Indication of Global Deforestation at the Cretaceous-Tertiary Boundary by New Zealand Fern Spike” Vol. 294. no. 5547, pp. 1700 – 1702
[17] G. Smather , D. Mueller-Dombois(1974). Invasion and Recovery of Vegetation after a Volcanic Eruption in Hawaii, Scientific Monograph Number 5. United States National Park Service. Retrieved on 6 March 2009
[18] P. Wilf and K. Johnson (2004) “Land plant extinction at the end of the Cretaceous: a quantitative analysis of the North Dakota megafloral record” Paleobiology 30:347-368.
[19] P. Sheehan and T. Hansen (1986) “Detritus feeding as a buffer to extinction at the end of the Cretaceous” Vol. 14,No. 10, p. 868-870
[20] D. Robertson, M. McKenna,O. Toon,S. Hope and J. Lillegraven (2004) “Survival in the first hours of the Cenozoic” GSA Bulletin,Vol. 116, No. 5-6, p. 760-768
[21] P. Sheehan and D. Fastovsky (1992) “Major extinctions of land-dwelling vertebrates at the Cretaceous-Tertiary boundary, Eastern Montana” Geology,Vol. 20, No. 6, p. 556-560
[22] R. Sloan, J. Rigby Jr, L.Van Valen, and D. Gabriel “Gradual Dinosaur Extinction and Simultaneous Ungulate Radiation in the Hell Creek Formation” (1986) Science Vol. 232, no. 4750, pp. 629 – 633
[23] P. Sheehan, D. Fastovsky, R. Hoffmann, C. Berghaus, and D. Gabriel (1991) “Sudden extinction of the dinosaurs: latest Cretaceous, upper Great Plains, USA” Science Vol. 254. No. 5033, pp. 835 – 839
[24] K. Pope, S. Hundt and C. Marshall “Meteorite Impact & Mass Extinction of Species of The Creteceous-Tertiary Boundary” Proceedings of the National Academy of Sciences USA Vol 15 No.19
[25] M. Novacek (1999) “100 Million Years of Land Vertebrate Evolution: The Cretaceous-Early Tertiary Transition”Annals of the Missouri Botanical Garden, Vol. 86, No. 2 (Spring, 1999), pp. 230-258
[26] W. Clemens, J. Archibald and L. Hickey (1981) “Out with a Whimper not a Bang” Paleobiology, Vol. 7, No. 3 (Summer, 1981), pp. 293-298
[27] G. Lloyd, K. Davis, D. Pisani, J. Tarver, M.Ruta, M. Sakamoto, D. Hone, R. Jennings, M. Benton. (2008) ”Dinosaurs and the Cretaceous Terrestrial Revolution”. Proc R Soc B 275:2482 7
Edit(22-8-2010): By the time of writing, this topic was quite controversial. However in March this year, a team of 41 scientists has published in Science to reaffirm the single asteroid theory of dinosaur extinction. It is now clear to almost all scientists in this field that a single asteroid caused dinosaurs went extinct because of the overwhelming evidence. As such, the topic is no longer contentious as before
