The History of Life on Earth
Midterm exam
Answers to
questions 1 to 6
October 16, 2001
Question 1: On fall break
to get some rest and relaxation, you go out to Nevada for hiking and nightlife.
During lunch, on a hike through some little traveled terrain, you
absentmindedly start breaking apart some thin layers of black shale that litter
the ground. Suddenly, your heart skips a beat.
A small jellyfish, preserved as a flattened carbon film, is
on the piece of shale in your hand. If you bring it to a paleontologist, you
shudder to thing that you will have to relive Biology 303; you know you could
crush it under your boot heel, and no one would ever know. You start to look
around more carefully – dozens of pieces of shale around you have animals
squished as carbon films.
Your conscience gets the better of you and you report it by telephone to a geologist at the University of Nevada. The geologist is skeptical that you found the fossils that you did. She says that while those rocks have been little studied, evidence from rocks stratigraphically above suggest that this black shale is at least 560 million years old. Now you know you’ve made the find of a lifetime.
(Remember, answers go
on answer sheet.)
a) What’s the big deal about finding a small fossil jellyfish of any age?
A: jellyfish have no hard minerlized
parts; soft tissues normally decay quickly after death, so fossils of “soft
parts” are rare
b) Why is the geologist skeptical that you found numerous animal fossils?
A: there are two acceptable answers:
one focuses on finding animal fossils at all in those rock – these rocks are at
the threshold of the very oldest animal fossils known, and none of these
fossils are clearly representatives of later phyla, so the geologist would have
reason to be surprised; the other answer focuses on the “numerous” – soft
bodied fossils are rare, so finding lots of them would also be unusual; the
answer is not that fossils are rare in general; I gave half credit for saying
that fossils are rare from that time – this is partially true, but there are,
in fact, other kinds of fossils such as stromatolites and unicellular fossils
in chert that can be abundant in some circumstances; I also gave half credit
for saying people had looked in the area before [even if the areas is not well
studied] and it is unlikely abundant fossils would have been overlooked
c) Why would this bunch of flattened, carbonized animal fossils be the find of a lifetime?
A: these fossils would be our
earliest “window” on the evolution of soft-bodied animal life, as soft parts can
tell you quite a lot more about what exists and what its anatomy is like than
hard parts generally can; the answer is not simply because the fossils are
rare, as one can imagine lots of rare fossils that would not be very
informative
Question 2: You move to
Cincinnati for a great new job after graduating from Ithaca College. You heard
on your History of Life midterm that the Cincinnati area is superb for fossil
collecting invertebrate marine (sea water) fossils from the Ordovician Period
and decide that you’re just nuts enough to find out a couple collecting
localities. Your boss, also relatively new to the area, wonders about your
critical reasoning facilities when you ask him if he wants to go with you.
Please answer these questions from your boss.
a) How could you get sea water fossils in the middle of the continent?
A: relative sea level can be high
enough to flood the surface of continents [and this has occurred frequently,
and is the basis for much of the sedimentary rocks and fossils at the surface
of the continents]; glaciers are related insofar as melted glaciers do cause
sea level rise, but glaciers did not push the fossil record from elsewhere
nor directly melt into continental seas; there are reasons related to plate
tectonics and the shape of the sea floor that also have a significant influence
on sea level, but the answer is not directly related to Pangea
b) Why isn’t this area covered with abundant fossils of recently dead land animals?
A: on land, in general, organisms do
not get covered with sediment as they would in an aquatic setting; not only do
soft parts degrade, as they also generally do in aquatic settings, but even the
hard parts such as bones weather in the rain and ice, through trampling, etc.;
the answer is not that they get buried, as this is the process that allows
them to be fossilized (fossils can be buried and still be considered “at the
surface,” though I recognize this usage was confusing); the answer also is
not that fossils take a long time to form, though I realize that could have
been confusing since the diectionary definition of a fossil may be evidence of
past life from, say, >10,000 years ago, but we did discuss this issue in
class
c) What kind of vertebrate animal fossils do you think you might find in the Ordovician rocks?
A: the best answer was jawless fish,
but simply “fish” got full credit; it is important to know that amphibians and
reptiles did not show up until later in the Paleozoic
Question 3: While vacationing in Eastern Europe, you note a large coal mining operation. One day while hiking you actually come across some layers of coal. Both above and below the layers of coal are some sandstones with brachiopods and other Paleozoic marine creatures that you learned about in Biology 303. You suddenly realize that with this knowledge alone you have been able to do a little biostratigraphy.
a) Based on your knowledge of life and climates of the Paleozoic Era, which geological period within the Paleozoic Era do you suppose the coal probably comes from?
A: Carboniferous; half credit given
for Devonian or Permian
b) What makes you think so?
A: the best answer was that the Caroniferous is known for its swampy forests that resulted in coal; it could not have been earlier than the development of forests, in the Late Devonian; and we discussed that the interior of Pangea in the Permian was rather arid; I have credit to those who remembered that the “carbon” in Carboniferous refers to the abundance of coal found from that age; I gave some credit in a few cases in the answer instead was based on the abundance of brachiopods
Questions 1 to 3 were
hypothetical. Questions 4 to 6 are based on actual news from the past 5 years.
Question 4: “The White Sea of Russia's menagerie of Ediacaran fauna…. is one of the most diverse fossil assemblages of its kind, but it has never before been assigned an accurate age to secure it to the geological and paleontological record. Mark W. Martin, postdoctoral research associate, and Samuel A. Bowring, professor of Earth, Atmospheric and Planetary Sciences at MIT, and their colleagues used isotope dating of the mineral zircon found in volcanic ash layered with the fossil-bearing rocks to arrive at an age of 555 million years. This new date is important because it is the oldest reliable date for tracks of organisms believed to have been made by worm-like animals [which occur in addition to the Ediacaran fauna fossils]. The ability of an animal to move through sediment on the sea floor implies a certain level of architectural complexity, such as the existence of a rigid body and gut.”[1]
a) Give at least one reason why it might have been such a great challenge to come up with a reliable numerical date for the sedimentary rocks in which these fossils occur.
A: perhaps this question should have asked why it is
difficult in general to get reliable dates for Precambrian sedimentary rocks:
they usually cannot be radiometrically dated directly, so we have to look for
igneous rocks on either side of with the sedimentary rocks we are interested
in; if we don’t have that, we must correlate from one sedimentary section to
another, which is difficult in the Precambrian in which we do not have lots of
fossils for biostratigraphy; I gave some credit to those who suggested
metamorphism might be an issue in rocks of this age, that rocks of this age are
difficult to date precisely, or that sedimentary rocks are formed from grains
of other rocks [and thus the grains reflect the age of the rocks from which
they were eroded]
b) Based on what you’ve heard about the Ediacaran fauna, explain why the tracks would be likely or unlikely to have been created by organisms belonging to the Ediacaran fauna.
A: Ediacaran organisms do not look like the kind of
animals that would leave worm-like tracks and many people have suggested that
these kinds of organisms did not move around
Question 5: For most of this
century, the best picture of Devonian tetrapods[2]
came from a fossil species discovered by Swedish scientists in Greenland in the
1930s. The animal, called Ichthyostega,
had arms, legs, hands, and feet, apparently for walking on land, but it also
had a fishlike tail that could have served the animal only in the water. In the
eyes of the Swedish paleontologists, Ichthyostega
lived its life between two realms, half in and half out of water.
In the late 1980s, [when] a pair
of scientists from the University of Cambridge in England studied the full
skeleton of another Devonian tetrapod, Acanthostega,
in 1989, they found hands and feet attached to an essentially aquatic animal.
What's more, Acanthostega's hands
were some of the oddest known. Instead of having five digits — which was
assumed to be the ancestral pattern among tetrapods — Acanthostega had an overabundance of fingers, eight on each hand.
Flummoxed by this, Clack and Coates set upon a recently discovered leg of Ichthyostega to see what it was hiding
at the end of its limbs. They found that Ichthyostega
had seven toes on each foot rather than the five that the Swedish researchers
had assumed.
Clack and Coates realized that Acanthostega and Ichthyostega could not have managed to do much more than flop
around on land. Their upper arm bones, instead of being long and slender, had a
broad, blobby shape ill-suited for walking. Their hind limbs splayed out to the
side and could not have held up the body easily. "If you look at their
skeletal anatomy, they look very aquatic. The limbs are pretty much paddles.
They've got fishlike tails. And we know from the skull that they have all sorts
of adaptations for existing in an aquatic environment," says Coates, now
at University College London.
So fingers, toes, and other
elements of a vertebrate limb evolved before tetrapods spent any quality time
on land.”[3]
This article goes on to discuss other discoveries and how the animals would have used limbs in an aquatic environment.
a) When do you suppose the paleontological events in this story took place relative to the age of the fossils around central New York?
A: These fossils formed at
approximately the same time; to be exact, the fossils in New York are a little
older. It was not acceptable to simply give an age, such as Devonian, as the
question asks for a comparison in age. The answer is not that the
amphibian fossils are younger because the fossils are of more complex animals;
one can, after all, find organisms of every complexity today, including animals
not unlike those found in Devonian seas
b) Why do you suppose the Swedish researchers didn’t know that Ichthyostega had 7 instead of 5 toes, and why did they assume that Ichthyostega’s limbs were suitable for walking? (Answer is related to how paleontologists interpret the past.)
A: The Swedish researchers evidently
didn’t have fossil material of the ends of the limbs, and based their
assumption on their knowledge of modern amphibians, and other known fossil
amphibians.
Question 6: “It has long
been proposed that Devonian ecological crises, and even the mass extinction
itself, were caused by a lack of oxygen in shallow seas, but the reasons for
oxygen depletion have been debated vigorously….. Now preliminary results from a
research group at Northwestern University provide evidence for a different
cause of the oxygen depletion -- an extreme growth of algae in the inner seas.
Graduate student Adam Murphy’s study of a rock core from Western New York State
provides preliminary evidence that excess production of algae led to
consumption of oxygen at the bottom of the sea, resulting in toxic conditions.
When these
algae died and fell to the bottom of the sea, Murphy explained, they were first
consumed by bacteria which are dependent on oxygen from the surface above. The
quantity of algae outpaced the oxygen supply, and bacteria which do not use
oxygen took over. These anaerobic bacteria produce hydrogen sulfide, which is
toxic to all other living things. The research indicates that mud on the sea
floor became toxic, so only these bacteria could live there until the algae
growth finally subsided.
Over time,
large amounts of the dead algal material were buried in the mud on the sea
floor, ultimately causing a shift of carbon from the atmosphere (as carbon
dioxide, one of the principal greenhouse gases) to the sediment (as organic
material). This would have reduced the greenhouse effect and contributed to a
fall in global temperatures.
Lack of oxygen in the environment may have resulted in an ecological "housecleaning," removing less tolerant species, and creating opportunities for different organisms to flourish when reduced algal growth allowed the return of oxygen, Murphy said.”[4]
a)
If it is true that so much organic algae was being sedimented in western New
York at this particular point in time, what color might the rocks be and why?
A: black or dark gray, from the
carbon; coal , for example, is black from carbon.
b) How could Adam Murphy test whether these types of Devonian animals that went extinct would have been very sensitive to a drop in oxygen concentration?
A: He could have done lab
experiments on modern relatives; he also could made some inferences from which
organisms were harder hit by the extinction event. Note that the question
refers to the animals that went extinct, not the algae. Confluence of evidence all by itself is not
an acceptable answer, but is relevant in if you listed more than one kind of
evidence.