Proposal on Dinosaur Extinction

Chirag ShahViren AminHarit Desai

9/10/97

Abstract

The extinction of the dinosaurs is an import issue that can shed light on the repeatedoccurrences of mass extinctions. It is very important that we don’t blindly accept theoriesbecause they are lavish and extravagant. Our aim is to show that dinosaurs didn’t die out dueone catastrophic celestial event. On the contrary we intend to prove that their extinction was amuch more complex process that took place gradually over millions of years.We propose toshow that this gradual demise was caused by colder climates and accompanying vegetativechanges brought on by the onset of the great tectonic activity of the Cretaceous period.Fossilevidence documents a change in habitat and vegetation occurring worldwide at this time. Wealso show evidence to propose that dinosaurs were cold-blooded and thus further affected by thealterations in the climate. In the fossil record there also lies proof that dinosaur diversitydecreases rapidly over the end of the Cretaceous ending with a gap 2 - 3 m below the iridiumlayer of the asteriodal impact 65 million years ago. We will go on to discredit the asteroid theoryand 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 the cretaceous 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

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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 the Jurassic, 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

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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 KT-

boundary 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

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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 sub-

tropical 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

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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

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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 the Whitemud 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

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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).

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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 the KT-

boundary, 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 from dentine 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.

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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 SeaDrilling Project Sites 536 and 540, previously considered as impactgenerated tsumani deposits of KTB age, appear to be biostratigraphically ofpre-KTB age representing turbidite or gravity-flow deposits. Apaleontological, mineralogical, and geochemical study of KTB channeldeposits in three sections in northeastern Mexico did not confirm thepreviously interpreted tsumani deposits related to a presumed KTB impactat 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,

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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.

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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?” Geological Societyof America 238 (1989): 63-69.

Ruben, John. “Dino-Noses Suggest Cold-Blooded Dinosaurs.” Science 29 Aug. 1996. Oregon StateUniversitv. Online. Netscape Navigator. 5 Dec. 1997.

Sloan, Rigby, Van Valen, and Diane Gabriel. “Gradual Dinosaur Extinction and SimultaneousUngulate Radiation in the Hell Creek Formation.” Science 232 (1986): 629-634.

Srivastava, Satish K. “Palynology of the Cretaceous Tertiary Boundary in the Scollard Formation ofAlberta, 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.

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