Proposal on Dinosaur Extinction
Chirag Shah
Viren Amin
Harit Desai
9/10/97
Abstract
The extinction of the dinosaurs is an import issue that can shed light on the repeated
occurrences of mass extinctions. It is very important that we don’t blindly accept theories
because they are lavish and extravagant. Our aim is to show that dinosaurs didn’t die out due
one catastrophic celestial event. On the contrary we intend to prove that their extinction was a
much more complex process that took place gradually over millions of years.We propose to
show that this gradual demise was caused by colder climates and accompanying vegetative
changes brought on by the onset of the great tectonic activity of the Cretaceous period.Fossil
evidence documents a change in habitat and vegetation occurring worldwide at this time. We
also show evidence to propose that dinosaurs were cold-blooded and thus further affected by the
alterations in the climate. In the fossil record there also lies proof that dinosaur diversity
decreases rapidly over the end of the Cretaceous ending with a gap 2- 3 m below the iridium
layer of the asteriodal impact 65 million years ago. We will go on to discredit the asteroid theory
and at the same time add credence to the gradual theory throughout the paper.
Dinosaurs died out gradually because of climatic cooling and accompanying changes in
vegetation due to plate tectonics. 65 million years ago marked the end of thecretaceous period
as well as the end of the reign of the dinosaurs. Many theories have been proposed to explain the
end of these majestic beasts. To date the most well known of these theories has been the asteroid
theory, which proposes that some gigantic celestial body collided with the earth to put an end to
it’s ruling inhabitants. It’s a dramatic answer to the extinction question and the image of one
huge blow obliterating those “terrible lizards” is almost irresistible. But could it have been that
simple? Was that really all it took to wipe out 65 percent of the Earth’s species?
Asteroid theorists like Luis Alvarez have hypothesized that an asteroid collided with the
earth ejecting large quantities of dust and iridium into the atmosphere, which completely blocked
out sunlight for months resulting in profound short-term climatic effects and widespread
extinction (Williams, 1994: 183). The settling of this dust which was high in iridium resulted in a
cretaceous-tertiary boundary (KT- boundary) that these theorists regard as the ending point in the
timeline of dinosaurs. We believe this “lights-out” scenario is insufficiently selective. Many
organisms survived into the Tertiary period and did not show the adverse effects expected from
such an apocalyptic, unselective catastrophe. Careful analysis of stratigraphic data from
Montana, where most of the direct evidence for dinosaur extinction has been found, documented
both a gradual change in climate for at least the second half of the Maastrichtian stage (see fig
1 .4), and a parallel decline in the numbers and diversity of dinosaurs over a period of about a
million years (Dodson, 1989:65). Thus, it is believed by gradualists that dinosaur extinction
resulted from changing environments: regressing seas, decreasing habitat diversity, increasing
seasonality, and climatic cooling (Dodson, 1989:62).
The Mesozoic period was a time of rapid movement of the great plates of the Earth’s
crust. The sheets of the Earth’s crust are like gigantic, slow moving (a few centimeters a year)
conveyor belts that carry the continents about on them. This drift produced a major geological
realignment during the reign of the dinosaurs. A general cooling trend was initiated during the
first half of the Cretaceous period, and there is evidence of increasing seasonality by the end of
2
the Cretaceous. Cooler climates at the end of the Maastrichtian were associated with a change in
oceanic circulation from an equatorially dominated to a polar dominated circulation pattern and
with the drainage of the thermally buffering epicontinental seas from the western interior of
North America (Paladino, 1989:64).
Orogeny, the process of continental uplift and mountain building, became a climatic
influence in Cretaceous times (Colbert, 1961:35). Newly uplifting mountain chains affected the
dynamics of atmospheric flow, latitudinal heat, moisture transport, and hence the climate. As
you increase in altitude the atmospheric temperature begins to decrease, and so the increase in
the mean elevation of the continents caused a relative cooling of the continent.Secondary
effects such as alterations in vegetation from a prehistoric evergreen landscape to a more modern
deciduous fauna most likely arose from the elevation of the continental plates (Srivastava, 1994:
13 8). The above affects of continental drift and orogeny would have resulted in alterations in the
circulation of the oceans and thus a worldwide continental cooling effect in climate.We believe
that through plate tectonics and its net cooling effect the dinosaurs’ natural environment was
severely altered, and thus the dinosaurs that once reigned theJurassic, met their tragic demise in
the Cretaceous era.
Over a period of many millions of years the super-continent of Pangea began gradually
separating and tremendously stressing the Earth’s delicate ecosystems. With the Rocky
Mountains beginning to be formed during the Cretaceous period the great Inland Sea of North
America was being steadily drained (Colbert, 1961:35). The drainage of this Inland Sea caused
the surrounding landscape to lose its climatic buffer and temperature began dropping as a result.
Well-preserved assemblages of various fossil biotas, including dinosaurs, from the Edmonton
Group of Alberta, Canada illustrate this point (Srivastava, 1994: 137).Within the Edmonton
group solidified microfossils including spores, pollen, algal cysts, fungal elements, and plant
cells suggested the beginning of a cooling trend in the earlier prevailing subtropical climate
(Srivastava, 1994: 141). The palynofloras of this formation also document a gradual deterioration
from a humid subtropical climate to the appearance of warm temperate elements in about a
3
330,000
years span represented by the Whitemud - Battle unit (see figure 1.1) (Srivastava,
1994: 143). Elements of temperate flora appeared earlier in this formation before the KTboundary and increased in abundance after the iridium layer showing that climate was changing
prior to the impact of the celestial body (Srivastava, 1994: 143). The study also used a foliar
physiognomic analysis of leaf assemblages across the KT-boundary in the North American
western interior which indicated a higher survival rate among deciduous than ever-green plants
which implies a progressively cooler climate (Srivastava, 1994: 150). It is clearly evident from
the fossil record that the climate was changing prior to the impact of the asteroid and continued
to due so afterwards. This progressively cooling climate gradually affected life globally.
With dinosaurs being cold-blooded and not having any type of homeostatic devices they
rely on climatic temperatures to maintain body temperatures. In research reported in the journal
Science,
sophisticated CAT scans of the nasal bones from three species of dinosaurs provides
some of the first clear, causal evidence that these ancient reptiles were cold-blooded (Ruben,
1996). The newest evidence in this scientific debate evolved from several years of study of the
nasal bones and structure of dinosaurs. In this particular project, those of a duckbill, a
tyrannosaur and an ostrich-like dinosaur were used. According to the researchers, there is a clear
correlation between the volume of nasal passages, the presence of respiratory “turbinate” bones
and warm or cold-bloodedness (Ruben, 1996). Larger nasal structures and turbinate bones serve
to limit bodily heat and water loss in steamy, fast-metabolizing, warm-blooded animals (Ruben,
1996). This mechanism is so essential to the function and survival of warm-blooded creatures
that it has evolved similarly in separate animal classes such as mammals and birds (Ruben,
1996). Metabolic and lung ventilation rates in cold-blooded animals are only about 5 percent of
a similarly sized warm-blooded animal (Ruben, 1996). Scientist concluded dinosaur fossils
examined in this research had no respiratory turbinate bones and comparatively narrow nasal
passages that would not even have been large enough to maintain homeothermic ability (Ruben,
1996). They most closely resembled the nasal passages of modern crocodiles or lizards, which
are cold-blooded reptiles (Ruben, 1996). The nasal structures we saw in dinosaur fossils simply
4
would not have been able to accommodate the higher lung ventilation rates that dinosaurs would
have had they been warm-blooded (Ruben, 1996). Continued findings such as this have greatly
reduced any debate over whether dinosaurs were cold-blooded creatures. Most previous
speculation that they may have been warm-blooded was likely based on less definitive factors
such as posture, lifestyle, growth rates, or nesting behavior (Ruben, 1996). As for these studies
they have no real fossil evidence to persuade a change in current thought.
With dinosaurs being cold-blooded they would have been severely susceptible to climatic
changes. Since the fossil record shows that the climate was changing drastically from a subtropical to a temperate one the dinosaurs would not have been equipped to survive.With no
form of thermoregulation climatic alterations would not allow dinosaurs to maintain the optimum
body temperature necessary to carry out essential bodily functions. Colder weather would have
made dinosaurs more sluggish because it would have reduced blood circulation by making the
blood thicker than usual. This reduced circulation would have greatly affected organs such as
the heart and brain, which need a great amount of blood and oxygen.With the lack of blood an
oxygen organs like these would deteriorate. Essential enzymatic activities would also begin to
cease as the temperature began deviating from the enzyme’s optimal range. Unlike mammals,
which had fur and insulation, dinosaurs lacked any insulation and thus could not have survived at
latitudes higher than sixty degrees during the winters of the late Cretaceous (Lessem, 1992:220).
Of all the dinosaur fossils found only a very few were found in the colder climactic areas.The
few fossils found at these latitudes were most probably from dinosaurs that inhabited this area
for summer occupancy only (Lessem, 1992:220). The new continental climate of the Cretaceous
period, with its seasonal fluctuations and more sharply contrasting day and night temperatures,
would have had lethal consequences for cold-blooded dinosaurs.
Amongst dinosaurs there are two possible types of mechanisms for sex determination in
their offspring. Dinosaur sex can either be determined genetically from heteromorphic sex
chromosomes or by varying incubation temperatures of the eggs.Even though birds are thought
of as descendants of the dinosaurs they have genetically fixed sex determination whereas the
5
dinosaurs do not. This may be due to the fact that temperature-dependent sex determination
(T
( SD) represents a primitive form of sex determination among reptiles and heteromorphic sex
chromosomes are recent evolutionary developments (Paladino, 1989:63). Some lizards, all
crocodilians tested, and most turtles exhibit temperature-dependent sex determination (TSD), in
which the incubation temperature of the eggs determines the sex of hatchlings (Paladino, 1989:
63). In these species, studies by Paladino have shown sex is determined by nest temperature
during a critical period of embryogenesis. In general, a 2 to 4 degree shift in incubation
temperature results in the production of all male or all female hatchlings (Paladino, 1989:64).
Alligator eggs incubated at 30°C during the second and third week of development produce
females, whereas those incubated at 34˚C produce males (Paladino, 1989:64). The occurrence of
TSD in alligators and other crocodilians, the closet living reptilian relatives of dinosaurs, and the
apparent lack of heteromorphic sex chromosomes in all living crocodilians to date, supports the
notion that TSD may also have occurred among dinosaurs (Paladion, 1989:64). Geologic
evidence indicates that dinosaurs nested in upland sites in western Montana where eggs were
exposed to fluctuating and /or lower temperature as climate deteriorated at the end of the
Cretaceous (Paladino, 1989:64). Production of hatchlings of predominantly one sex would have
drastically altered the population breeding structure and driven these dinosaurs towards
extinction.
It would be difficult to obtain the data to test the TSD extinction hypothesis in light of the
fact that dinosaurs are extinct. However, Paladino believes it is reasonable to accept this
extinction model as a working hypothesis in light of the available data on the distribution of TSD
among living reptiles and the ability to test its conclusions (Paladino, 1989:68). It has also been
argued that a moderate shift in the sex ratio toward an increased production of either sex may not
have resulted in an immediate reduction in reproductive success because there is no evidence to
suggest that dinosaurs were monogamous. However, if all offspring in a dinosaur herd were
produced by a small group of males, they would all be closely related and in future generation
they would be inbred (Paladino, 1989:68). Modern genetics clearly demonstrates the dangers
6
inherent in increased inbreeding, such as the problems associated with increased genetic loading
and decreased allelic variability (Paladino 1989:69). This scenario of genetic loading and
reduced variability could lead to an accumulation of deleterious recessive alleles and an increase
in genetically related diseases. Inbreeding and a decreasing genetic variability of dinosaur
population would have produced a gene pool less able to respond to changing environmental
conditions, habitat alterations, and changes in the food supply at the end of the cretaceous.Thus
this scenario, in conjunction with other environmental factors, could explain gradual dinosaur
extinction without the intrusion of any celestial phenomenon (Paladino, 1989:66).
Fossil evidence analyzed by paleobotanists indicates that the vegetation of the cretaceous
era was undergoing considerable changes. Due to the fact that pollen grains and fungal spores
are so small and plentiful in certain sedimentary rocks, paleobotanists can trace the precise points
at which different plants vanish from the fossil record. The strata in the middle of the Hell Creek
formation shows that pollen diversity was the greatest at this time, with 114 sporopollen taxa
present (Sloan, 1990:632). The sporopollen began to drop off well below the KT boundary; in
the upper 12m of the formation there are only 84 taxa (Sloan, 1990:632). Only 64 taxa were
present at the base of the early Paleocene formation (Sloan, 1990:632). Data from this formation
agrees with the hypothesis of gradual floral change, not with the catastrophic scenario of an
asteroid impact (Sloan, 1990:632). These changes in the flora can be explained by the severe
altering of the regional climate due to the affects of the tectonic shifts occurring at that time.
Dinosaur extinction in the Whitemud-Battle formations was initiated with the onset of
climatic cooling and vegetation changes in the area. Thus, the Lancian fauna represents a
declining dinosaur population, which became extinct in about 0.9 Myr. Palynofloral changes
responding to climatic cooling first appeared in theWhitemud formation and continued into the
Scollard formation until after the highest dinosaur record in the area. Therefore, indicating that
climatic cooling, vegetation alterations and dinosaur extinction all coincided in the strata in a
7
Climatic and temperature changes, which are known to have taken place towards the end
of the Cretaceous, had less direct effect on dinosaurs than on vegetation.Rain or dryness makes
vegetation increase or diminish and causes great differences in the soils upon which animals live
and lay their eggs. “Horsetails, cycads, treelike ferns, and conifers that made up the bulk of the
continental plant life until the late Jurassic times gradually gave way to the angiosperms
(flowering plants) which are known to have emerged about the middle of the early Cretaceous;
their pollen has been found in deposits from this period (Michard, 1992:86)“. The modern
deciduous trees, the oaks, elms, planes, maples, and so on, were rapidly spreading at the expense
of the evergreen vegetation, which had long been the staple diet of vegetarian dinosaurs(Lessem,
1992:85). During the second half of the Cretaceous, ceratopsians, ankylosaurs,
pachycephalosaurs, and hadrosaurs filled ecological niches left empty by other herbivorous
dinosaurs such as brachiosaurs and stegosaurs (Michard, 1992: 86). This clearly illustrates that
some dinosaurs were greatly affected by changing vegetation. All the climatic changes caused
by plate tectonics selected against the dinosaurs because they were not as well fit to adapt to
these changes as were the mammals and other smaller vertebrates. The pools of the great
amphibian dinosaurs were being drained and there would be intense competition amongst
herbivores for the remaining pools. Dinosaurs were less fit in comparison to other vertebrates in
their means of defense against predators. Many of the dinosaurs sought protection in water
against the large carnivores and with the opportunity for finding these water holes decreasing,
many dinosaurs were more rapidly falling prey to their predators (Michard, 1992:35). Increasing
competition and less food for all caused a reduction in the herbivores, which inevitably led to a
reduction in the mighty carnivores as well. The changing environment hurtled dinosaurs toward
extinction by placing them in a habitat that was hostile and impossible for them to adapt to.
The gradual demise of the dinosaurs corresponds with the changing vegetation of the
Cretaceous era. Their downfall lasted 7 million years and rapidly accelerated in the final
300,000 years of the Cretaceous. In the last 10 million years of the Cretaceous of Alberta,
Montana, and Wyoming, there is a progressive reduction of dinosaur diversity (Sloan, 1986:632).
8
Peak dinosaur diversity appears in the Hell Creek formation 76 to 73 million years ago with
thirty genera of dinosaurs, the most diverse fauna known to date (Sloan, 1986:632). Reduction
of numbers of dinosaur genera continues in following faunas at 23 genera from the lower part of
the formation and 22 genera from the upper part (Sloan, 1986:632). In the combined Lance-Hell
Creek faunas of the same area the diversity is further reduced to 19 genera (Williams, 1994: 184).
In the uppermost 16m of the Hell Creek formation this trend continues and only 12 genera are
found (Williams, 1994: 184). The decline continues until there is a 2- 3 m gap before the
iridium layer in which very few fossils are found (Williams, 1994: 184).This reduction is not an
artifact of insufficient sampling; the uppermost 30 m of the Hell Creek Formation have been the
most intensely prospected for isolated bones and teeth.
Evidence suggesting that dinosaurs continued decreasing in genera past theKTboundary, thus indicating that the asteroid impact did not cause extinction, are the teeth of seven
species of dinosaurs that have been recovered from Ferguson Ranch locality (Sloan, 1986:633).
These fossils appear in a channel whose top occurs between the upper and lower Z coals
presumably after the deposition of the iridium layer (Sloan, 1986:634). Sloan believes that these
fossils are not reworked for several reasons. These teeth were not transported very far, because
postmortem stream abrasion is minimal (Sloan, 1986:634). “The teeth are all shed teeth, some
with unabraded sharp basal edges resulting fromdentine root resorption before shedding (Sloan,
1986:634).. The likelihood of concentrating these teeth by eroding the flood plain sediments
adjacent to the channel is very low in view of the rare occurrence in the clays of isolated teeth or
other small vertebrate fragments (Sloan, 1986:634). The hollow cavity of the base of the teeth is
unfilled with flood plain silts or clay but instead is empty or filled with channel sands (Sloan,
1986:634). It is even less likely that they were eroded from older channel sands because the
usual occurrence in channel sands is near the base of the sand and this is a very thin channel that
did not cut deeply enough onto lower channels to rework many teeth (Sloan, 1986:634).
Therefore the most reasonable conclusion is that these are indeed early Paleocene dinosaurs.
I
9
One of the predictions of the asteroid theory is that all dinosaurs would have ceased to
exist at the KT-boundary. In order to prove that a catastrophic incident was what vanquished the
dinosaurs it must be proven that the dinosaurs were not in decline but rather that their population
was stable up until the point of the catastrophic event. However, in actuality there is a gap of 2-3
meters between the highest level of intact dinosaur fossils and the iridium layer (Williams,
1994: 184). After this gap and before the iridium layer only a few dinosaur fossils are found.
This gap is contradictory to the predictions made by the asteroid theorists, who as stated above,
require a random distribution of fossils up to the iridium layer.
A possible KTB asteroid impact site has been proposed at Chicxulub on the Yucatan
peninsula supported by the occurrence of shocked quartz grains, tektites, and impact glasses in
geological sections of the area (Srivastava, 1994: 152).
However, current bedded volcaniclastic sedimentary rocks at Deep Sea
Drilling Project Sites 536 and 540, previously considered as impact
generated tsumani deposits of KTB age, appear to be biostratigraphically of
pre-KTB age representing turbidite or gravity-flow deposits. A
paleontological, mineralogical, and geochemical study of KTB channel
deposits in three sections in northeastern Mexico did not confirm the
previously interpreted tsumani deposits related to a presumed KTB impact
at Chicxulub on Yucatan. (Srivastava, 1994: 152)
Scientists reviewed all evidences and considered the Chicxulub structure as a volcanic sequence
of late Cretaceous age rather than caused by an impact at the KTB. Furthermore, the terrestrial
palynoflora of the area changed gradually across the KTB within a stratigraphic interval of 375
cm which was deposited in about .13 Myr (Srivastava, 1994: 152). Similarly, there are no sudden
changes in extinction rates of either marine planktonic dinoflagellate taxa or the terrestrial
palynoflora around the Atlantic Ocean (Srivastava, 1994: 152). This evidence further weakens
the instantaneous extinction hypothesis.
Another point decreasing the credence of the asteroid theory is evidence that such an
assumed impact at the K-T boundary was not necessarily unique to the Cretaceous period. At
approximately the end of the Triassic period, an asteroid measuring some 6 miles in diameter
crashed into eastern Quebec, leaving a crater measuring over 40 miles in diameter (Keller,
10
1992:32). Keller and other paleontologists have extensively studied excavation sites near this
impact crater. Fossil evidence did not show drastic decline in dinosaur populations near the time
of this impact nor did it show as great a fauna1 alteration of the landscape in the vicinity of the
crater (Keller, 1992: 32).
After consideration of all of the fossil evidence it seems clear that the leading cause of
dinosaur extinction was not a catastrophic event. This evidence tends to point towards a gradual
demise in dinosaur populations as a result of environmental cooling and accompanying changes
in the predominant vegetative fauna of the time due to tremendous tectonic activity. Evidence
gathered shows the gradual decline of terrestrial dinosaurs. To date, there is not much fossil
evidence regarding marine dinosaurs. and how they met their demise in support or against the
gradual theory. Studies need to be conducted at oceanic sites to obtain conclusive fossil
evidence. Studies also need to be conducted to further explain how and why creatures like the
crocodiles and birds, which are also extremely vulnerable to climatic change, survived. Studies
on these species would probably help further discredit the asteroid theory. There are still many
unanswered questions pertaining to this topic but interest in this topic continues.With the threat
of humanity’s own extinction looming in its future, the idea that these former rulers of the Earth
perished mysteriously holds a strange perhaps even morbid attraction.
11
Works Cited
Colbert, Edwin H. Dinosaurs. New York: E.P. Dutton & Co. Inc., 1961.
Dodson, Peter. Paleobiologv of the Dinosaurs and the Cretaceous. Colorado: Simon& Schuster,
1989.
Keller, Steven B. “Dinosaur Extinctions.” NewScientist 138 (1992): 28-35.
Lessem, Don. Kings of Creation. New York: Simon& Schuster, 1992.
Michard, Jean-Guy. The Reign of the Dinosaurs. New York: Harry N. Abrams Inc., 1992.
Paladino, Frank V. “Temperature-Dependent Sex Determination in Dinosaurs?” GeologicalSociety
of America 238 (1989): 63-69.
Ruben, John. “Dino-Noses Suggest Cold-Blooded Dinosaurs.” Science 29 Aug. 1996. Oregon State
Universitv. Online. Netscape Navigator. 5 Dec. 1997.
Sloan, Rigby, Van Valen, and Diane Gabriel. “Gradual Dinosaur Extinction and Simultaneous
Ungulate Radiation in the Hell Creek Formation.” Science 232 (1986): 629-634.
Srivastava, Satish K. “Palynology of the Cretaceous Tertiary Boundary in theScollard Formation of
Alberta, Canada, and Global KTB Events.” Review of Palaeobotanv and Palynology 83
(1994): 137-158.
Williams, Michael E. “Catastrophic Versus Noncatastrophic Extinction Of The Dinosaurs: Testing,
Falsifiability, and the Burden Of Proof.” The Journal of Paleontology 68 (1994): 183-189.
12