Last night in the museum's hall
The fossils gathered for a ball
There were no drums or saxophones,
But just the clatter of their bones,
A rolling, rattling, carefree circus
Of mammoth polkas and mazurkas.
Pterodactyls and brontosauruses
Sang ghostly prehistoric choruses.
Amid the mastodonic wassail
I caught the eye of one small fossil.
Cheer up, sad world, he said, and winked-
It's kind of fun to be extinct.
--Ogden Nash
Types of extinction:
Psuedoextinction
Terminal
extinction
Magnitudes of extinction
Background extinction
Mass extinction
-the Big Five
-the Cretaceous-Tertiary
Effects of extinction
Ninety-nine percent of all species that ever
lived are now extinct, or, as Dave Raup puts it, to a first approximation,
all species are extinct.
The point here is that extinction is a very
important evolutionary process, and, as a matter of fact, an evolutionary
process that is best studied by reference to the fossil record. This is because we have, in fact no
information on "natural" extinctions today. All of the extinctions currently going on are extinctions in
which humans have played (or are playing) a key role.
1.
Pseudoextinction, or phyletic extinction: where one species evolves into another species. While the ancestor can be said to have gone
extinct, the evolutionary lineage does, of course, continue. There is no loss of species.
2. True
extinction, or terminal extinction:
where a species lineage becomes extinct. Here, there is a loss of species
The question to ask at this point is: what
proportion of extinctions seen in the fossil record are pseudoextinctions and
what proportion are true extinctions?
The answer is - nobody really knows.
In a few studies that have been done on this (and it's difficult), about
20% of the extinctions were judged to have been of the pseudoextinction type
and the rest, 80%, were true extinctions.
It's an open question.
How are extinctions measured?
What we'd like to be able to do is simply count
up the number of species that make their last appearance in some time
interval. That would give us an
extinction rate: the number of species per million years, for example.
In sad fact, we have named and described only a
very small sample of the number of species actually in the fossil record. As a consequence, most paleontologists work
at a higher taxonomic level, usually the family level (species, genus, family). So, in practice, extinctions are commonly
measured as the number of families that make their last appearance in some time
interval, so the measure of extinction is number (or %) of families becoming
extinct per million years.
The best record that we have is the marine
record. Fossils there are relatively
abundant and the time control is pretty good.
The record of terrestrial vertebrates is not as good, and that of
terrestrial plants even worse.
What the figure of extinction rate on the
handout suggests is two somewhat different magnitudes. Background extinction and mass extinction.
In fact, there is no sharp difference between background extinctions and mass
extinction, one does grade into another, but the distinction is useful and is
commonly made.
1.
Background extinction (on handout)
Note that the extinction rate here never falls
to zero; there is always some extinction going on. But most of the time it's going on at a relatively low rate, say
about 5 families per million years.
This may be the sort of extinction that happens thanks to relatively
small scale environmental changes, disease, predation, psudoextinction,
competition and the like.
Consider an interesting
feature of this graph, focusing now just on the background extinctions. There is a regular trend toward lower levels
of background extinction through geologic time.
Possible interpretations:
a. Species are becoming better adapted through
geologic time. Evolution is building a
better organism.
b. The earth's environment is becoming more
benign. Temperatures may not flucutate
as much, volcanoes may not go off so often, glaciations may be less
frequent. NO evidence for any of this.
c. Change in the species/family ratio. Recall that what is plotted here is the rate of extinction of families. What needs to be done to cause the extinction of a family is, of course, the extinction of all the species in the family. Families with lots of species, like the Muridae, a family of rodents, are difficult to make extinct - just `cause there's lots of them. How many species would need to go extinct to cause the extinction of the family Hominidae? Just one.
So, if there's been a
change in the number of species per family through geologic time, such that
Cambrian families had relatively few species and Tertiary families had, on
average, lots of species, family extinction rates would be high in the Cambrian
and relatively low in the Tertiary.
There is some evidence
for an increase in the number of species per family during geologic time.
2. Mass
extinction. Notice how there are five groups of points
on the graph that stand well-above the background level of extinction.
These are the mass
extinctions: extinctions in which extinction rates exceed background rates by a
factor of two or so (double background), on the average of about 10
families/million years.
These extinctions
affected a broad range or organisms (i.e., not confined to one taxonomic group
or another or one ecological group or another).
These extinctions may
have been of relatively short duration.
That is, they may have been catastrophic, with intervals of time
anywhere from a million years down to one bad weekend. This is still a controversial topic. Were mass extinctions sudden or gradual?
These five groups of points in the marine record have come to be known as the "Big Five". A quick look before I look at one of the big five in detail.
1. Late Ordovician 19.3 families/million
years. 12 percent diversity drop (if
recovery is quick, there won't be much drop, so the drop is not a good measure
of extinction intensity).
Affected: trilobites, brachiopods, echinoderms.
Cause: associated in
time with a glaciation and with a sea level drop.
2. Late Devonian. Approx 10 families/my, 14% diversity drop.
Affected: brachiopods, tabulate and rugose corals,
sponges, early fish, conodonts.
Cause: Associated with
black shales - low oxygen conditions?
Some evidence for an impact but it’s controversial.
3. end-Permian. Approx 15 families/my. 52% reduction in family diversity. Maybe as many as 92% of all marine species.
Affected: brachiopods,
corals, echinoderms, trilobites, cephalopods, sponges, fish.... almost every group affected.
Cause: Associated with lowered sea level and
cooling climate but record in vicinity of boundary very poor. Some evidence of an impact. Active Siberian volcanoes too. Controversial.
4. Late Triassic 11 families/my, 12 % diversity
drop.
Affected: bivalves,
cephalopods, conodonts, fish
Cause: Possible impact
record in Canada
5. end-Cretaceous 16 families/my, 11% drop in
diversity.
Affected: dinosaurs, ammonites, marine microplankton,
marine bivalves. (plants not badly affected).
Cause: impact of a comet or asteroid, maybe explosive volcanism,. Still controversial.
The end-Cretaceous, or K/T extinctions:
The extinction of the dinosaurs on land, and the
ammonites and other invertebrates in the oceans about 65 million years ago has
always been a topic of fascination and speculation.
Will Cuppy, in his book “How to become extinct”
says:: "The age of reptiles ended
because it had gone on long enough and it was all a mistake in the first
place".
The hard-core controversy kicked into gear about
twenty years ago when Luis and Walter Alvarez, a father-son team, analyzed some
of the clay collected from the boundary between Cretaceous and Paleogene
(Tertiary) marine rocks in northern Italy, near a small town called Gubbio.
What they found is illustrated on the
handout: very low levels of the element
Iridium up to the boundary, a high concentration in the boundary clay, then a
quick decline to background levels in Tertiary limestones.
It turns out that elements like Ir are not
common in the rocks of the earth's crust:
they are common, however, in meteorites (samples of planetary
interiors).
So, from the evidence of the unusual elements in
the boundary clay they suggested what has come to be called the Impact
hypothesis: namely - that 65
million years ago, the earth was struck by an large asteroid (10 km in
diameter).
Evidence:
High ir levels in boundary sections around the
world
Rapid extinction in marine and terrestrial
sections
Soot in boundary clays (fires)
Shocked quartz in boundary clays (esp in N
America) - impact feature
Glass spherules - from melted rock
Large 65 Ma crater in Mexico’s Yucatan
Peninsula, 300 km diameter
Sedimentary deposits from large tidal waves in
Mexico and Texas
Scenario
The impact pulverizes the asteroid and a good
deal of the crust where the asteroid struck.
The pulverized debris
was injected into the atmosphere, where it blocked sunlight (total darkness for
50 days in one scenario) and rapidly cooled the climate.
The darkness shut off
photosynthesis, killing the short-lived oceanic phytoplankton, cutting off the
food supply of herbivorous dinosaurs.
That, plus the cooling, should have been enough
to cause the extinctions.
Acid rain from nitrous oxides created from
passage of asteroid?
Giant tidal waves,
Global wildfires?
Arguments against:
The impact hypothesis,
while it has lots of support and supporters, is not without its weaknesses and
its opponents.
Counter arguments include:
-The idea that explosive
volcanism could have done the trick, by injecting lots of dust into the upper
atmosphere, etc, etc. And some volcanic
eruptions are relatively enriched with Iridium. The Deccan Traps of India, for example, are a series of volcanic
deposits approximately 65 million years old.
[But greater age range than extinctions, no shocked minerals, spherules,
or soot, though].
-The contention that the
extinctions of dinosaurs, ammonites and the like were gradual affairs, stretched
out over several million years. If they
were gradual, a sudden impact couldn't have caused them. [Weak evidence because poor sampling near
the boundary would produce a record that looked like a gradual extinction.]
No matter how extinctions might be caused, they
have evolutionary consequences - they have strongly affected the history of
life. Consider, for example:
1. Removing incumbents. The K/T extinctions
wiped out the dinosaurs. That seemingly
allowed the diversification of the mammals.
So, if it hadn't been for that volcano or asteroid, we wouldn't be here.
2. Chance effects. The biggest extinction of all, at the end of the Permian, has been estimated to have wiped out 93 percent of all the species then living. A mighty close call. But that's not the point. The point is that those remaining 7 percent are the founding members of the post-Paleozoic biota. With extinction levels so high, it's a good bet that some species survived just because of chance, not because of some superior adaptations. This suggests that chance has played a large role in determining the composition of the earth's biota. Extinction, bad genes or bad luck?
3.
Ecological effects.
Zombies
Dodo - extinct
flightless bird, discovered 1598, extinct by 1681.
Dodo and the Mauritius – islands in the western Indian Ocean.
Calvaria tree also found in Mauritius, but no seedlings
known, only old trees.
Seeds were found to germinate only after the
exterior surfaces were mechanically ground away somewhat, or were passed
through the gut of a turkey.
The extinction of the dodo may lead to the
extinction of the tree.
Calivaria tree is a zombie, the living dead.