Transcript Dinosaur extinction theories

Dinosaur Extinction Theories
The K/T Extinction is one of the greatest
mysteries of paleontology. What caused all nonavian dinosaurs to go extinct at the end of the
Cretaceous Period?
Falsifiability
• There are many
hypotheses about
what caused the
extinction, but
they are not
scientifically valid
if there is no
evidence to either
support or refute
them.
Hay fever killed the dinosaurs.
Flowering plants evolved in the late
Cretaceous period. Dinosaurs could
have died from allergic reactions to
these new plants.
Is this a valid hypothesis?
Hay Fever Theory
• Invalid
• Flowering plants were around for millions of years
before the dinosaurs disappeared. We have no
evidence whatsoever that their pollen or other
pathogenic products killed the dinosaurs, and it is
unlikely that we would find evidence if it existed.
• It still wouldn't explain the massive marine extinction;
there have never been marine angiosperms. Indeed,
around the K-T boundary there is the phenomenon
known as the "fern spike": all land plants except ferns
show a dramatic decline in diversity.
• How could the dinosaurs be wiped out by pollen if a
lot of the pollen producers were gone too?
Sniffles killed the dinosaurs?
• Near the end of the
Cretaceous period, the
continents were shifting,
opening new routes for
dinosaurs to cross into
other areas previously
inaccessible to them. So
the mingling dinosaurs
spread diseases and wiped
each other out.
Is this a valid hypothesis?
Dino Sniffle Theory
• Invalid
• We have no evidence of widespread disease
among the last dinosaurs; they seem quite
normal (we could prove disease in a dinosaur
by the bone pathology — diseased animals
often have deformed, weakened bones).
• This doesn’t explain what happened to the
oceanic and non-dinosaur organisms that also
went extinct.
Dinosaurs got too big.
Dino Crush Theory
• Invalid
• There is no general trend of increasing size
among all dinosaurs.
• None became so large that they couldn't
move; that is an evolutionary impossibility.
• Most dinosaurs were of medium or small size,
even at the end of the Cretaceous.
Mammals are to blame.
• Mammals were outcompeting the dinosaurs
for food, space, or other resources.
Mammal Culprits
• Invalid
• Dinosaurs and mammals evolved together for most of the
Mesozoic era; mammals remained quite small, and only
slowly increased in diversity. If they were outcompeting the
dinosaurs, we would see a trend of decreasing dinosaur
diversity and increasing mammalian diversity. We don't.
• Mammals and dinosaurs probably did not occupy similar
ecological niches; the small mammals could exploit rare food
resources, while the larger dinosaurs could not survive eating
the same things that the mammals did. If they did not occupy
similar niches, they probably did not compete.
• Mammals were not found in the oceans at that time, so the
marine extinction is still not explained.
Mammals ate all of the dinosaurs eggs.
Egg Eaters
• Invalid
• No evidence of mammals eating them.
• But no egg-eaters could eat all of the
dinosaurs' eggs; they would eat themselves
into extinction if they did.
• Mammals couldn't have eaten all of the
marine animals' eggs since there were no
marine mammals at that time.
Cosmic rays killed the dinosaurs.
Cosmic Ray Theory
• Invalid.
• There is no evidence of extraterrestrial events
that were occurring at the K-T boundary that
could have emitted sufficient dangerous
radiation to affect life on Earth significantly.
• There is no evidence for irradiated dinosaurs,
either; this would have to show up in the
bones to be noticed.
With the dismissal of other hypotheses, we are
left with two different schools of thought: the
gradualists and the catastrophists.
Gradualists
• These scientists believe that the fossil record
indicates a gradual decline over 5-10 million
years.
• These scientists believe that plate tectonic
forces caused extensive volcanic activity in
India and perhaps elsewhere that resulted in
dense clouds of soot being released into the
air.
Catastrophists
• These scientists believe
that the fossil record
indicates a sudden
decline that is more
consistent with a
catastrophic event such
as a massive asteroid
impact. This theory was
first proposed in 1980 by
Walter Alvarez.
You will now examine pieces of evidence from
the K/T extinction and sort them into support
for the gradualist theory or the catastrophist
theory. Some may fit in both categories.
When you have finished, decide which
hypothesis you believe is correct and identify
the best supporting evidence for it.
Identify any evidence that refutes your
hypothesis.
Extinction Theories
Gradualist
•
Catastrophist