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Decline and Fall of the Dinosaurs:
Catastrophism or Gradualism?
Sia Sin Wei
D
inosaurs are the most well-known
prehistoric animals. They have been
said to be the most successful animals
ever have to have evolved on the planet
earth. Since their discovery in the late 19th
century, one of the major ‘hot’ topics has
been their extinction 65 million years ago.
How they disappeared remains a controversy up to this day. While the fossil record
is the only and best source in uncovering
the reasons behind their extinction, the fact
that it is imperfect and incomplete hampers
our elucidation of theories to explain their
extinction. Despite this, there is a consensus
that an single extraterrestrial body impacted
Earth 65 million years ago and resulted in
an abrupt end to the age of dinosaurs
People continue to be fascinated by The x-axis represents the time in million of years ago. The y-axis represents the percentage of marine
these creatures which first evolved about animal. Reproduced from [32].
225 million years ago (MYA) in the Late Triassic. During their 160 million years reign
on Earth, they evolved into all kinds of shapes and sizes. In The Nature of Mass Extinctions
fact, the vast majority of scientists agree that modern-day A species goes extinct only when its last living member dies
birds are actually direct descendants from the dinosaurs out. There are two types of such an incidence. One way is
However, more captivating than their remaining legacy, is through background extinction, where a species slowly dies out
the story of how they could have perished 65 MYA in the at a constant rate as a result of natural selection. Well known
late Cretaceous Period. This essay examines various theories dinosaurs such as the Allosaurus, Iguanodon, Brachiosaurus
of dinosaur extinction and argues that the impact theory of and Stegosaurus probably went through background extinction.
In this article, however, I will focus on when a large number
catastrophic impact is stronger than the rest.
The study of mass extinctions has been scientifically of species goes extinct within a short period of time, which
interesting because it gives us insights on how biological is called mass extinction had occurred. The first and last
systems may recover from an ecological crisis and such of the five mass extinctions recorded in geological history
knowledge is invaluable in the light of the modern context occurred in the Late Ordovician 438 MYA and in the Late
of environmental damage and high number of endangered Cretaceous 65 MYA (K-T extinction) respectively [3]. I will
species [1]. Of all the mass extinctions that have occurred in focus specifically on the K-T extinction that wiped out the
the history of life on Earth, it is the extinction of the dinosaurs dinosaurs. In general, extinction events have been shown to
that have caught our imagination the most. Since the discovery occur at regular intervals of natural history, with big events
of dinosaur fossils in 1819, scientists have wondered what rather infrequent. There is no major difference between a
had caused their demise and have suggested over 80 theories background extinction event and a mass extinction event,
[1]. Some of the more sensible ones are extraterrestrial body other than the fact that the mass extinction event is greater
impact, volcanism, rising sea levels and climate changes. in magnitude [4].
There are also the less credible theories, such as mass deaths
by a virus and alien abductions. This abundance in theories Fossils and Interpreting of the Fossil Record
is due to the many lines of evidence, both geological and Researchers studying extinctions develop their theories by
biological, that can be widely interpreted. However, within studying fossils, the preserved remains of organisms older
the last 20 years, it has become clear that there was an impact than 10,000 years. Body fossils are the remains of an animal’s
by an extraterrestrial body on Earth at 65.95 MYA (give or body while trace fossils preserve an organism’s activities
take 40,000 years) [2]. Therefore, the most plausible theory like eggs and track prints. Fossils are preserved in general
is thus that the extinction of (non-avian) dinosaurs was by the following processes: First, after death, the soft tissues
caused by such an impact of the extraterrestrial body. The from the body of an animal decomposes. Then the hard
impact theory accounts for the vast majority of the evidence parts like bones, teeth and shells get transported by water
and observations and including the fossil record, which no to their final resting place. The hard parts are rapidly buried
and mineralized into rock. Finally, erosion by the elements
alternate theory can explain.
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THE TRIPLE HELIX Fall 2010 13
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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 in particular conditions. For example, in the
tropical forest, a gorilla body can become a skeleton in ten
days and disappear by decaying within three weeks before
fossilization can take place and that is without even taking
into account scavengers like the hyena. The environment
thus affects the changes of fossilization. Mountains are poor
locations for fossilization as no burial takes place, only erosion
occurs. Forests are also bad for preservation since bones can
dissolve in the acidic soils. Aquatic animals, meanwhile,
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 the jellyfish leave little, if any, evidence
in the fossil record [5, 6].
From the fossil record, we have been able to reconstruct
a history of life on Earth [1]. Many evolution lineages have
been developed, most notably that of horses, whales, humans and the horned dinosaurs showing how organisms
are related to each other. And finally, fossils are used to
deduce how extinct organisms might have lived and what
they might have looked like [5].
Pitfalls of the Fossil
Interpreting fossil record is an art. 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
places that have no sedimentary deposition and 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].
Therefore, trying to reach global conclusions with evidence
largely coming from only North America is 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 and
this is a conclusion that has been reached that can only be
based on comparisons in extent and in number to modern
ecosystems [1]. With both issues discussed above, we can
conclude that it is not too easy to elucidate the events of a
particular extinction.
The third important issue is 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 non- fossil bearing ones. After
this line, the organism can be said to have gone extinct. Suppose that the dividing line is approached from the oldest
rocks from the bottom. As it approaches, 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].
So how do researchers gauge the magnitude of extinction? The intuitive way is to use the number of species to
calculate the percentage of the total number species present
that have 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 , order, etc. What can be one creature’s family can be
another person’s two families [1].The Signor-Lipps effect,
the issue with deposition, and the incomplete picture of the
entire biodiversity are serious issues that must be noted.
Even with these shortcomings, however, the fossil record
is an impressive monument to the history of life on Earth.
Figure 1: The Signor-Lipps Effect. Reproduced from [31].
14 THE TRIPLE HELIX Fall 2010
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Impact Theory
The impact theory postulates that the cause of the K-T mass
extinction is an extra-terrestrial body hitting Earth, in what
is now the Yucatan peninsula in Mexico. Many short term
and long term effects of the extra-terrestrial body, most
likely an asteroid ten kilometres wide or a comet, have been
suggested [9]. 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.
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. Evidence from a tsunami after
the impact has been found in certain sites in North America.
Also, the K-T boundary in the crater is thicker compared to
the other parts of the world. This represents material that
did not get thrown up into the atmosphere [9]. The crater
was close enough to have caused all of the observations
after the impact [3].
There are also several lines of biological evidence to
prove catastrophism by an asteroid. There is some proof
Reproduced from [33]
Evidence of the Impact Theory
We can now interpret the vast amount of observations and
evidence from the fossil record. There is vast support from
the geological and biological evidence for the impact theory.
Where K-T sediments are exposed, the following arrangement can be seen. Cretaceous sediments are at the
bottom and Tertiary sediments at the very top. In between
these two layers is a clay layer rich in iridium. 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]. An asteroid striking the earth may have
thrown up a dust cloud that settled into the clay layer seen
in the sediments. [11].
Further geological evidence can be seen from the
shocked quartz that was found at the K-T boundary. This
is a form of quartz that possesses an altered structure due
to the enormous pressure resulting from the impact [12].
Microtektites, which are small, rounded and circular silicate
glass particles, were also found at the K-T boundary all over
the world [13]. They are believed to be material from the
meteorite that had molten and solidified, being thrown up
to the atmosphere during impact. These unique geological
materials lend support to the notion of an asteroid impact.
This then 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
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from the fossil record of marine micro-organisms. In today’s
oceans, 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 Nano fossils, another group of organisms that corroborates the case for an abrupt extinction [14].
Thus we can conclude that marine micro-organisms show
an abrupt pattern of extinction. 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 million years at and after the K-T Boundary
was shown using fossils of marine unicellular organisms called
foraminferia to further substantiate the impact theory [15].
This may be have been caused by the blockage of sunlight
by the dust clouds and atmospheric aerosols thrown up by
the asteroid which deprived the microorganisms of sunlight.
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
THE TRIPLE HELIX Fall 2010 15
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Reproduced from [30]
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
possibly caused by the heat build up during impact [16].
Similar patterns of succession with ferns as a pioneer species have also been seen after major disasters like volcanic
eruptions [17]. These results suggest that plant life began
anew with pioneer species like the ferns due to wildfires at
the K-T boundary caused by the force of the impact upon
the earth [18].
Does the vertebrate record support the impact theory?
In general it has been shown that animals that depended
on dead organic matter (detritus) seem to have been better
off after the K-T extinction. The presence of a dust cloud
or perhaps atmospheric aerosol thrown up by the impact
of an extra-terrestrial 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 during the pre-impact phase was undergoing
a “business as usual” condition and most taxa underwent
a drastic decline in diversity only 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, 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, 22, 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 in unlocking 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.
16 THE TRIPLE HELIX Fall 2010
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.
From the evidence from the
fossil record supporting the impact
theory, a post impact scenario can
be constructed. Post-impact, Earth
was shrouded by aerosols of dust.
This blocked out the sunlight. In the
ocean, the darkness caused a decrease
in the biomass of phytoplankton. 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 may 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].
Counterarguments of the Impact Theory
One of the major weaknesses of the Impact Theory is that
in all other mass extinction events, no other evidence for
impacts has been detected. In contrary, in other known impact
events, there was also no extinction events associated with
them. In fact, all of the five extinctions occurred at times
of sea level changes and at least three occurred at times
of increased volcanic activity [26]. As a result, many other
theories have been purposed to be the main cause of the K-T
extinction. Some of the more prevalent theories are volcanism,
climate change, sea level changes and the evolution of the
angiosperms. However, compared to the impact theory, they
have serious weaknesses and are unable to account for the
evidence from the fossil record as well as the impact theory.
Volcanism has been proposed to be the major event
that resulted in dinosaur extinction. There were some suggestions that the shocked quartz and microtektites which
supports the impact theory 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
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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. There are problems
reconciling the protracted eruption history with a single
mass extinction horizon at the K-T boundary. In addition,
the rocky features altered by high heat and pressure 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, weakening
the dinosaurs and made them vulnerable to environmental
changes [27]. However, while sea level changes have been
associated with marine mass extinctions, as sea level changes
occur frequently, this association would seem to be more
coincidental than causal. Sea level changes have been shown
to fail to adequately explain the terrestrial vegetation disruption and faunal change across the K-T boundary [28].
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, as aforementioned, 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.
Finally, 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)
[29]. 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.
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-anoyster “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. Archibald JD, Clemens WA , Padian K, Rowe T, Macleod N et al. Cretaceous
extinctions : multiple causes. Science. 2010; 328 : 973
27. 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
28. Schulte P, Alegret L, Arenillas I, Arz JA, Baltion PJ, Bown PR et al. Responses
to letters Science. 2010; 328 : 976
29. 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
30. Extinction intensity. [accessed 6 Mar 2009, cited 6 Mar 2009] Available from :
http://en.wikipedia.org/wiki/File:Extinction_intensity.svg
31. Signor-Lipps Effect . Available from : Fastovsky DE & Weishempel DB The
Evolution and Extinction of the Dinosaurs pp 419
32. K-T boundary. [accessed 6 Mar 2009, cited 6 Mar 2009] Available from :
http://commons.wikimedia.org/wiki/File:K-T_boundary.jpg
33. Artist impression of point of impact. [accessed 6 Mar 2009, cited 6 Mar 2009]
Available
from : http://en.wikipedia.org/wiki/File:Impact_event.jpg
© 2010, The Triple Helix, Inc. All rights reserved.
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 humans would not have evolved
to prove that a group of reptiles once ruled this planet.
Palaeontologists will continue working towards uncovering
the mystery of dinosaur extinction until we can confirm
the impact theory, or any other promising ideas, for sure.
Sia Sin Wei is an undergraduate at the National University of
Singapore.
THE TRIPLE HELIX Fall 2010 17