Surely ever since the first fossils of obviously extinct animals were found, humankind has
wondered: "Why did they die?" A poignant question, for it has relevance to us if extinct
animals were wiped out by some catastrophe, couldn't that just as easily happen to us?
Could we be found as fossils someday, and would no one know why we died?
History: Until recently, people simply knew that dinosaurs went extinct their
fossils were found throughout the Mesozoic era, but were not
located in the rock layers (strata) of the Cenozoic era.
So, we knew that dinosaurs went extinct some 64-66 million years ago, but that was all. Many wild
ideas about how the dinosaurs were rendered extinct were presented over the years.
1980: Few satisfactory answers to the mystery behind the extinction of
dinosaurs were offered until 1980, when a group of scientists at the University
of California at Berkeley Luis and Walter Alvarez, Frank Asaro, and Helen
Michel proposed a stunning and convincing mechanism for the
"K-T extinction" (meaning the extinction of dinosaurs at the boundary
between the Cretaceous period (K) and the Tertiary period (T)). This hypothesis
is discussed later. Since the Alvarez hypothesis was first proposed, the search
for the "perpetrator" of the K-T extinction has been a thriving area of
scientific research. It incorporates scientists from many different fields
including astrophysics, astronomy, geology, paleontology, ecology, geochemistry,
and so on. The mystery has drawn extensive media coverage over the last
15 years, as you may know; some paleontologists have since lost interest in
the issue, preferring to study how the dinosaurs and their contemporaries lived
rather than why they died.
Mass Extinctions: But before we dive into the complex issue of the K-T
extinction, we need essential background information to understand the basics of the
controversy. The "great dying," as it is sometimes called, is an example of a mass
extinction: an episode in evolutionary history where more than 50% of all known
species living at that time went extinct in a short period of time (less than 2 million years or
Other Mass Extinctions? We know of several mass extinctions in the history of
life; the great dying is not nearly the largest! The largest would be the "Permo-Triassic"
extinction, between the Permian and Triassic periods, of the Paleozoic
and Mesozoic eras. In this obviously catastrophic event, life on Earth nearly was wiped out an
estimated 90% of all species living at that time were extinguished. We are fairly sure that the
extinction was due to many changing global conditions at that time, but even that is not
solved yet. The issue has not received much press because the dinosaurs were not
involved, but another familiar group, the trilobites,
were wiped out among others.
Who Died? How does the K-T extinction compare to this debacle? Well, about
60% of all species that are present below the K-T boundary are not present above the line
that divides the "Age of Dinosaurs" and the "Age of Mammals." In fact, dinosaurs were
not among the most numerous of the casualties the worst hit organisms were those in the
oceans. Large groups of organisms, including some members of Foraminifera,
Echinodermata, Mollusca, and the marine Diapsida
all were devastated by the K-T event. On land, the Dinosauria of course went extinct, along with the Pterosauria. Mammals and most non-
dinosaurian reptiles seemed to be relatively unaffected. The terrestrial plants
suffered to a large extent, except for the ferns, which show an apparently dramatic increase in diversity at
the K-T boundary, a phenomenon known as the fern spike.
Now we're heading into the tough stuff; the reasons why we have
no conclusive answer to the mystery of the K-T event. Several complications that make
work hard for the scientist/detectives trying to crack this case:
The Fossil Record: It's not perfect, as you may know; that's why
paleontologists keep finding new fossils: so much is hidden in the rocks! Most data on the
K-T event comes from North America, which is one of the few areas known that has a
somewhat continuous fossil record (remember, fossils are only formed under certain rare
conditions, and are only found in sedimentary rocks). The infamous Hell Creek locality in
Montana is one such continuous site enclosing the K-T boundary. UCMP researchers have
led and continue to lead expeditions to Hell Creek, gathering fossils from the rich fossil
beds. The secret to the K-T event may lie within our collections; who knows! Anyway, we
don't know much about what was occurring in the rest of the world at the time of the K-T
event. The marine fossil record gives us great hints about what was occurring within the
sea, but how applicable is that to what went on in the terrestrial realm?
The Nature of Extinction: Extinction is not a simple event; it is not
simply the death of all representatives of a group. It is the cessation of the origination of
new species that renders a group extinct; if species are constantly dying off and no new
ones originate through the
process of evolution, then that group will go extinct over time no matter what
happens. New dinosaur species ceased to originate around the K-T boundary; the question
is, were they killed off (implying causation, especially a catastrophe), or were they not
evolving and simply fading away (perhaps implying gradual environmental change)?
Time Resolution: Determining the age of rocks or fossils that are millions of
years old is not easy; carbon dating only has a reasonable resolution when used
with organic material that is less than about 50,000 years old, so it is useless with the 65
million year old K-T material. Other methods of age determination are often less accurate
or less useful in certain situations. So we don't know exactly when the dinosaurs went
extinct, and matching events precisely to give a picture of what was happening at a specific
moment in the Mesozoic is not easy. Thus, the ultimate question of a gradual decline of
dinosaurs vs. a sudden cataclysm is almost intractable without a wealth of good data.
Reconstruction: To truly understand the situation of the dinosaurs around
the K-T boundary, we need to understand the paleoecology of that time on Earth.
Paleoecology is an extension of the discipline of ecology, attempting to understand the
interactions of organisms with their environment, using geological (the rocks tell
you what the soil was like, and thus tell a lot about the abiotic (non-living) environment) and
paleontological (what plants and animals are found as fossils tell you a lot about the
biotic (living) environment) evidence. With the problems of the fossil record and
time resolution, it is difficult to understand the paleoecology of a region at a specific time in
The Signor-Lipps Effect: Proposed by Phil Signor and UCMP's own Jere
Lipps, this concept helps us to understand the limitations of the fossil record. The theory
states that groups of organisms may seem to go extinct in the fossil record before they
actually do; this is an artifact of the fickle nature of the fossil record rather than actual
extinction. Thus, it is possible that some groups of organisms did not go extinct at the K-T
boundary, and also possible that some organisms that seemed to have gone extinct earlier
may have survived up to the boundary, and then gone extinct. This matter further
complicates the important issue of the selectivity of the K-T extinction (discussed
Falsifiability: Sad but true: many hypotheses about dinosaur extinction
sound quite convincing and might even be correct, but, as you know, are not really
science if they cannot be proven or disproved. Even with the best hypothesis, such as
the impact hypothesis, it is very difficult to prove or disprove whether the dinosaurs were
rendered extinct by an event that occurred around the K-T boundary, or whether they were
just weakened (or unaffected) by the event. This is not to say that all extinction hypotheses
are not science; many are excellent examples of good science, but a linkage of direct
causation is a problem. "Why" questions, such as "Why did the dinosaurs die out?" or
"Why did dinosaurs evolve?" are among the most difficult questions in paleontology.
Ultimately, a time machine would be required to see exactly what killed the dinosaurs.
Now that you have a background in the extinction issue, feel free to delve into the modern
arena of scientific examination of the "Mystery of the Great Dying."
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