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NATS 1540 Study Guide - Final Guide: Paleozoic, Cenozoic, Condylarth

Natural Science
Course Code
NATS 1540
Robert Levine
Study Guide

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A Cacophony of Causes
Paleontologists look at mass extinctions as extraordinary events demanding
amazing and wondrous causes. But as with more recent human events, there
are often so many competing geological, climatic, and chemical events that
establishing which represents the cause of the extinction and which the
effects is a real challenge.
Animal fossils only extend through the last 55 million years of the earth’s 4.6
billion year history. During this span, paleontologists have identified six great
mass extinctions:
6th Mass Extinction (END CRETACEOUS)
We will begin by working backwards in time from the best-known mass
extinction, the end-Cretaceous event 65 million years ago when the
environmental effects of the impact of a large, extra-terrestrial object in the
Yucatan peninsula of Mexico wiped out the dinosaurs. In the Late Cretaceous
dinosaurs dominated the land, and after some 150 million years of evolution
had diversified widely. Mammals were widespread, but the great
diversification of modern placental mammals would not occur until after the
extinction. Many different kinds of flowering plants had appeared in only a few
tens of millions of years of the Late Cretaceous. Ocean life bore many
relationships to modern ecosystems, with the critical exceptions that corals
were relatively insignificant and Late Cretaceous reefs were built by massive,
conical bivalves known as rudists, and the coiled ammonite cephalopods
flourished as major predators. The extinction eliminated some 47% of marine
genera and 16% of families, including several microfossil groups, some
bivalves, and the ammonites. Mosasaurs and other marine reptiles became
extinct too. Plants suffered considerable turnover, with one analysis from the
northern Rocky Mountains suggesting a 79% extinction. So much more is
known about this extinction than any of the other crises that it is worth
considering it in greater detail.
The critical feature of the Alverez impact hypothesis was the discovery of a
thirty-fold increase in concentration of the element iridium at the Cretaceous-
Tertiary boundary at Gubbio, Italy, and at other sites.
Whether it was a meteorite or a comet, the object that struck the Yucatan
Peninsula was some 10-15 kilometers in diameter. Since 1980, field and
laboratory studies and computer simulations have revealed much of what
happened 65 million years ago. The impact excavated a cavity about 100 km
in diameter, displacing some 100,000 cubic km of debris, including the melted
rock from the immediate impact, vapor, and solid debris. As the first cavity
collapsed, a crater 170 to 300 km in diameter formed. Beyond the massive
earthquakes, the impact triggered tsunamis that sped out from the Yucatan.
The blast effects incinerated everything within thousands of miles and
extended into the western United States. Rocks blasted out by the impact
heated on re entry through the atmosphere and triggered wildfires on many
continents. The most far-ranging effects were from the rocks vaporized by the
impact. In North America, the impact deposit from the cretaceous-tertiary
event can be readily separated into two layers: a lower layer of material
directly ejected from the impact and an upper layer of atmospheric fallout
from material in the vapor-rich plume that rose far into the atmosphere and

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was then carried around the world. Only the upper, fallout layer is found in
Europe, Africa, and the southern hemisphere, although it thins away from the
impact site.
The dust of the vapor cloud obscured the sun and cooled the earth for
months, probably shutting off photosynthesis for a lengthy period, and
causing much of the extinction. Sulfur carbonate rocks in Yucatan would have
been turned into sulfuric acid and then acid rain. How long this destruction
lasted is unclear. Early workers suggested the dust cloud persisted for years,
but later computer simulations showed that as the particles began to stick to
each other they would rain out of the atmosphere more rapidly.
Iridium was soon picked up at many other Cretaceous-Tertiary marine and
terrestrial boundary sections around the world.
The Cretaceous-Tertiary impact is a possibility for the end-Permian event.
The events of the end-cretaceous provide useful comparison for what we
might expect in the Permian.
5th Mass Extinction (END TRIASSIC)
The end-Triassic mass extinction 199 million years ago has been well studied
in Europe, but the extent of the crisis in other parts of the world has been
unclear. A rapid rise in sea level coincides with the elimination of some 53%
of genera and 22% of marine families. Particularly affected were cephalopods
(with octopus and pearly nautilus as modern representatives), brachiopods,
clams, and some snails. Many of the Paleozoic groups that survived the end-
Permian mass extinction finally disappeared. About 12% of vertebrate
families also disappeared, and in the aftermath, dinosaurs took command of
the land. The mass extinction coincided with a series of massive volcanic
eruptions. These areas were then united in the supercontinent of Pangaea,
and the volcanism may be associated with the opening of the Atlantic. The
leading candidate for the trigger of the end-Triassic extinctions is this massive
volcanism, known to geologists as the Central Atlantic Magmatic Province
because it spans the Atlantic margin.
4th Mass Extinction (END PERMIAN)
3rd Mass Extinction (LATE DEVONIAN)
The next mass-extinction in the series is a complex and still poorly
understood episode 376 million years ago during the late Devonian. Some
57% of marine genera and 22% of marine families disappeared during a
series of events. Again, brachiopods experienced considerable extinction as
did trilobites, corals, and other members of reef communities; bivalves and
gastropods suffered relatively minor losses. In Europe, some paleontologists
see evidence for at least three discrete extinction pulses; global studies are
more equivocal. At least one extraterrestrial impact is known from the late
Devonian, but it does not correspond to any extinction horizons. Possible
causes include spread of low oxygen waters, changes in the chemistry of the
oceans, and climate change. But until geologists learn more about the
extinction, it may be premature to consider the causes.
2nd Mass Extinction (END ORDOVICIAN)
The second largest mass extinction brought the Ordovician Period to a close
439 million years ago. The trilobite-dominated communities of the Cambrian
disappeared during the Ordovician Radiation, diluted by the rapid growth of

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the dominant Paleozoic communities with articulate brachiopods, crinoids,
bryozoans, and some Paleozoic corals. These groups lived attached to the
sea floor and filtered food out of the water (there was little, if any life on land
this early in animal history). Two discrete extinction events separated by
perhaps 500,000 to 1 million years extinguished 60% of marine genera and
26% of marine families. Glaciation and global cooling were the most likely
causes of this event. The extinction began with a rapid glaciation and
consequent drop in sea level, and then as the glaciers melted, sea level rose
rapidly and delivered anoxic, or low oxygen, waters into shallow seas and
caused the second pulse of extinction.
1st Mass Extinction (EARLY CAMBRIAN)
The oldest recognized mass extinction in the fossil record occurred shortly
after the Cambrian Radiation, about 512 million years ago, near the end of
the Early Cambrian. Appears to have eliminated 50% of all marine species.
The early Cambrian was a time of rapid overturn in biodiversity, with high
rates of extinction and origination. We don’t know enough about this event to
ascertain the causes.
The K-T Extinction (Cretaceous-Tertiary)
Almost all the large vertebrates on earth, on land, at sea, and in the air
suddenly became extinct about 65 Ma, at the end of the Cretaceous period.
At the same time, most plankton and many tropical invertebrates, especially
reef-dwellers, became extinct, and many land plants were severely affected.
This extinction event marks a major boundary in earth’s history, the K-T or
Cretaceous-Tertiary boundary, and the end of the Mesozoic era. The K-T
extinctions were world-wide, affecting all the major continents and oceans.
There are still arguments about just how short the event was. It was certainly
sudden in geological term and may have been catastrophic by anyone’s
Despite the scale of the extinctions, however, we must not be trapped into
thinking that the K-T boundary marked a disaster for all living things. Most
groups of organisms survived. Insects, mammals, birds, and flowering plants
on land, and fishes, corals, and molluscs in the ocean went on to diversify
tremendously soon after the end of the Cretaceous. The K-T casualties
included most of the large creatures of the time, but also some of the
smallest, in particular the plankton that generate most of the primary
production in the oceans.
A meteorite big enough to be called a small asteroid hit earth precisely at the
time of the K-T extinction. The evidence for the impact was first discovered by
Walter Alvarez and colleagues. The found that rocks laid down precisely at
the K-T boundary contain extraordinary amounts of the metal iridium. It
doesn’t seem to matter whether the boundary rocks were laid down on land
or under the sea. In the Pacific Ocean and the Caribbean the iridium-bearing
clay forms a layer in ocean floor sediments; it is found in continental shelf
deposits in Europe; and in North America, from Canada to New Mexico, it
occurs in coal-bearing rock sequences laid down on floodplains and deltas.
The dating is precise, and the iridium layer has been identified in more than
100 places around the earth. Where the boundary is in marine sediments, the
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