How did the dinosaurs become extinct?
Welcome to the Natural History of Dinosaurs.
My name is Benjamin Burger.
IÕm a paleontologist at Utah State University
teaching in the heart of UtahÕs dinosaur
country in Vernal.
This is the last video lecture on the Natural
History of Dinosaurs, and it is all about
the extinction of the dinosaurs, the non-avian
dinosaurs.
So what killed the dinosaurs 66 million years
ago?
There have been hundreds of explanations put
for over the years.
Hypotheses have included diseases, the dinosaurs
become over specialized and too big, mammals
out-competed the dinosaurs, there was a change
in plants, climate change, marine regressions,
were the oceans retreated, and interstellar
dust clouds and cosmic radiation.
Or even dark matter.
But most of these ideas lack much evidence.
In this video we will look for evidence.
Particularly evidence gathered over the last
30 years evidence for two primary causes for
the extinction of the dinosaurs.
First, an asteroid impact, and second volcanic
eruptions.
Before we examine these catastrophic causes
of extinction, we need to define what we mean
by mass extinction, and just regular old background
extinction.
The earth has gone through 5 mass extinction
events, and the mass extinction at 66 million
years ago, known as the K-T extinction event,
K for Cretaceous and T for the older Tertiary
term for the Paleogene and Neogene Periods.
Mass extinctions are when the total extinction
rate is much higher than normal.
That is natural selection, changing environments
and the survival of the fittest, drive species
to extinction, and thatÕs called background
extinction.
But during certain short intervals of time,
extinction rates go through the roof, and
it is these events that many species become
extinct over a very short duration of time.
So periods of high extinction rates are called
mass extinction events, while low extinction
rates are called background extinction.
LetÕs look at what the Earth looked like
66 million years ago.
The large interior seaway in North America
had retreated, and the continents were roughly
in the same places as today.
India still had not reached Asia, as it drifted
northward to form the Tibetan Plateau, and
the highest peaks today.
India was besieged by volcanos, which poured
forth large amounts of lava, that today form
basalt layers of rocks, called the Deccan
Traps.
These lava flows date to around 65 million
years ago, about the same time period that
dinosaurs disappeared.
There has been some conjecture that these
large volcanic eruptions in India during this
time caused the extinction of the dinosaurs,
by releasing large qualities of carbon dioxide
into the atmosphere.
Indeed, there are indications that carbon
dioxide in the atmosphere was much higher
near the K-T event, and that climate may have
been changing, by getting hotter.
But could these volcanoes have been responsible
for the extinction of a group of animals,
and many others living things in the oceans
to go extinct after a successful 180 million
years of living on the Earth?
A different hypothesis was put forth by a
man named Walter Alvarez.
Walter Alvarez was a petroleum geologist working
in North Africa, who also had an interest
in archeology, and loved visiting ruins in
Italy during his time off from his job searching
for oil.
On one trip he met up with some Italian micro-invertebrate
paleontologist who showed him the K-T boundary
exposed in some limestone.
Below the boundary the organisms were dominated
by large foramiferia, but above the boundary,
only tiny foramiferia remained in the limestone.
There was a thin layer of clay at the transition
between the Cretaceous and Paleogene rocks.
This thin layer of clay Walter Alvarez sampled.
He taught his father might be interested in
it.
His father, Luis Alvarez, was nobel-winning
physicist working in California.
He had access to new scientific tools for
understanding the makeup materials.
Luis Alvarez working on some experiments that
could analysis the clay for its mineralology
and chemical makeup.
So he sent off the samples to his father to
look at.
WalterÕs father, Luis Alvarez, was working
with Frank Asaro and Helen Michel at the Berkeley
Lab who were using a new machine called a
Neutron activation analysis, which bombards
a substance with neutrons to make the atoms
in the material radioactive, then measures
the spectra of emissions of the sample to
identify elements in the sample.
It can detect up to 74 elements, most of them
are rare elements, like gold, silver, argon,
but does include common elements like potassium
and silica.
This means that the researchers can get a
spectra of concentrations of these elements,
including the rare elements.
The researchers analyzed the clay that Walter
had collected, and they found large amounts
of Iridium in the sample.
Other samples were sent from below and above
the clay layer, which returned more normal
concentrations of Iridium.
What is Irdium?
Iridium is a rare metal, element 77, which
is very rare.
Gold is 40 times more abundant and platinum
is 10 times more abundant.
It is so rare that it does not make a currency,
because there is just not enough of it.
If you were to look at the amount of elements
found in rocks on EarthÕs crust, you see
that Iridium is near the bottom of the list,
right near Osmium.
The reason for this has to do with several
factors, but the major one is that Iridium
sank into the interior of the Earth during
the early formation of the Earth.
So the crust of the Earth is depleted of its
Iridium.
One place that you can find Iridium is in
rocks that never underwent this melting and
depletion Ðand that is meteorites.
Meteorites contain the original ingredients
of the Earth, which includes a larger concentration
of Iridium.
The researchers at the Berkley lab, concluded
that the clay layer that Walter sent them
was filled with Iridium from a massive meteorite
that struck the Earth 66 million years ago.
Since their original paper, hundreds of additional
sites around the world have demonstrated a
spike in the amount of Iridum at the boundary
between the Mesozoic and Cenozoic.
Did this meteor impact kill the dinosaurs?
The search for an impact structure revealed
a crater on the tip of the Yucatan Peninsula
in Mexico, called the Chicxulub Crater.
The structure was dedicated using geophysical
seismic surveys, made by geologists looking
for oil.
In the cores brought up from drill rigs, geologists
discovered shock quartz, grains of quartz
crystals that have stressed crystalline lattices,
which results from the compression of tremendous
forces.
Other sites have shown that shock quartz grains
were widely distributed during this event,
and found in the clay layer across North America.
The structure of the impact crater showed
that the meteor struck at a low angle into
the Caribbean Sea, estimates based on the
mass balance of Osmium in the clay layer support
an asteroid of 4.1 to 4.4 kilometers in diameter,
while the size of the crater supports a larger
asteroid of 10 to 15 kilometers.
Many models of what happens when the Earth
is hit by an asteroid of this size had been
modeled.
Here we see the first 30 seconds of impact.
On contact the asteroid would push into the
various layers of water and rock with such
speed as to turn the rock into liquid, and
vaporize the ocean waters.
Ejection of liquefied rock and the molten
asteroid itself, with intensely heated water
vapor would soar into the atmosphere.
No matter where you were on the Earth, you
would feel a blast of infra-red radiation,
which would send temperatures up to 60 degrees
Celsius, feeling as if you we trusted into
a hot oven.
The temperature would be hot enough to set
fire to trees and any organic matter around
you.
Everything would be set on fire.
This fire would burn through the rich Cretaceous
forests, and deplete the amount of oxygen
in the atmosphere, which would drop by 1%.
Evidence of a thin layer of charcoal can be
found above the iridium clay zone, supporting
a massive fire and die off of the forests
during this interval.
If you survived the burning landscape, the
skies would be darkened for months.
All those foramiferia in the oceans would
lack sunlight for photosynthesis, and the
larger marine life that depended on the photosynthesizing
plankton, would starve to death during these
months of darkness, and so would any surviving
dinosaur, without plant food.
Ash and other injecta would slowly rain out
of the skies.
Then, with the rain and sunshine, nearly 60
days after the blast, ferns would start to
grow again.
Above the Iridum-clay, and charcoal, there
is a spike of fern spores, similar to the
regrowth of surfaces of land consumed by lava
after a volcanic eruption.
The worst was over.
So which animals survived?
Fish, sharks, amphibians, turtles, crocodiles,
lizards, mammals, and birds.
5 out of 8 of these animals lived in water,
so they may have been insolated form the radiation
and heat, while lizards, mammals and birds
are small creatures, that could have survived
the months of darkness by feeding on seeds
and nuts.
Birds that had beaks adapted to eating seeds
survived, as did mammals that may have stored
food in the form of nuts in burrows and holes.
Lizards did not have to eat as frequently
and with a higher heat tolerance could withstand
the hotter temperatures.
Each group was lucky they had the right lifestyle
to survived such a cataclysm.
But were the dinosaurs already doomed before
the impact of the asteroid?
Where they already bound for extinction?
What is the distance between the K-T boundary,
the Ir spike and the last dinosaur?
Is there any evidence that maybe dinosaurs
survived for a few million years after the
asteroid impact?
One of the most interesting sites, is a fossil
locality in Montana, called Bug Creek.
ItÕs a small site which contains both dinosaur
bones, and Paleocene mammals.
Did these dinosaur bones indicate that dinosaurs
lived into the Cenozoic.
The puzzle was studied by Donald Lofgren.
He thought of two possible scenarioÕs first
was that dinosaur bones were laid down first,
then the surface was eroded down to the exposed
bones, a second filling of sediment deposited
the Paleocene mammal teeth and jaws.
The K-T boundary would be during period of
erosion.
The second scenario is that the dinosaur and
mammal bones were deposited after the erosion
event at the same time, and both after the
K-T boundary.
Hence these dinosaur bones were from a dinosaur
that survived the extinction event.
After careful study, it was found that the
mammals and dinosaur did not live in the same
moment, they were spaced apart by a few thousand
years.
Other sites of dinosaurs mixed within the
Paleocene rocks have been reported, but often
these dinosaur bones are reworked.
Fossils that eroded out of the rock, to be
redeposited in the younger strata.
No one has found a definitive Cenozoic dinosaur.
But were dinosaurs on the way out, anyway.
Some researchers have pointed out that dinosaurs
near the end of the Cretaceous were less diverse
then they were earlier.
These graphs show how morphology was more
similar among ceratopsids and hadrosaurids
during the last three million years, compared
to the earlier periods of the late Cretaceous,
while Coelurosaurs and pachycephalosaurus
show no pattern of similarity.
There were many dinosaurs near the boundary,
and itÕs hard to justify that dinosaurs were
dying out at the end, although, some of the
iconic dinosaurs were larger than they had
previously been, suggesting that they would
have a hard time surviving the blast of a
large asteroid slamming into the Earth.
One thing we do know, is that mammals were
able to survive, and for the next 66 million
years would come to dominance.
This dominance of mammals was due in large
part to the K-T event, in which mammals did
not have to contend with hungry dinosaurs,
and could become larger and more diverse than
ever before.
Alright be sure to Explain what happened after
the K-T boundary, and justify your explanation
with evidence that has been gathered from
scientific observations.
Thank you so much for watching these video
lecture series on dinosaurs.
I hope you enjoyed these, and learned a little
about dinosaurs along the way.
Thanks for watching.
