Midterm 2

Question 1

List the subdivisions of the Linnean classification scheme in their proper order.

Answer 1

Domain, kingdom, phylum, class, order, family, genus, and species

Question 2

Indicate the difference between biological species and paleontological species.

Answer 2

Paleontologists use the same system as biologists in naming and classifying fossil organisms. The fundamental unit of the Linnean system is the species, a group of organisms that look alike and that can produce viable offspring. Since paleontologists cannot determine if fossils could breed when alive, they must rely solely upon similarities in shape and appearance to define species.

Question 3

Explain the concept of natural selection.

Answer 3

Plant and animal distributions reflect a long and complex history during which modern plants and animals arose with modifications from ancestral forms.

Question 4

Briefly discuss evolution in terms of modern genetics.

Answer 4

“To this day, evolution by means of natural selection remains the single best scientific explanation for the nature and distribution of life on Earth. It is the only theory that combines the hard-won, acts listed above into a meaningful whole.”

Question 5

Contrast phyletic gradualism and punctuated equilibrium.

Answer 5

Two competing models explain the mode and tempo of evolution.

Question 6

PHYLETIC GRADUALISM

Answer 6

Evolution proceeds by means of an infinite number of subtle steps. Proposed by Darwin

Question 7

PUNCTUATED EQUILIBRIUM

Answer 7

Evolution consists of fitful, sudden advances that punctuate long periods of negligible evolutionary change. 90* ANGLES

Question 8

What are the differences between Mitosis and meiosis?

Answer 8

Both mitosis and meiosis are methods of cell division. Mitotic division produces daughter cells in which each nucleus contains exactly the same complement of chromosomes as the parent cell. Meiotic division involves two successive cell divisions, resulting in daughter cells having one-half the number of chromosomes as the original cell.

Question 9

What are the differences between Haploid and diploid?

Answer 9

Haploid cells contain only one set of chromosomes; diploid cells contain two sets of paired chromosomes.

Question 10

What are the differences between Gymnosperm and angiosperm?

Answer 10

These are the flowerless and flowering plants, respectively. Gymnosperms, such as cycads and conifers, evolved early and are still present. Angiosperms are more advanced plants in which the seeds are protected by an ovary. Gymnosperms made their appearance in the early Paleozoic. Angiosperms did not evolve until the late Mesozoic.

Question 11

What are the differences between Stratophenetic and cladistics phylogeny?

Answer 11

Stratophenic phylogeny is the traditional method of reconstructing ancestor descendant relationships. Under this model, organisms are arranged in an ancestor-descendant tree chiefly based on geological age, with older forms near the bottom and younger forms at the tips of the branches. Using cladistics, family trees are based upon the state and presence or absence of structural characteristics such as limbs, appendages, and so forth.

Question 12

What are the differences between Domain and Kingdom?

Answer 12

Domains and kingdoms represent major taxonomic subdivisions of the Linnean classification scheme. Everyone is familiar with the kingdoms Plantae and Animalia. With the invention of the microscope, it became clear that some living things were neither plant nor animal; consequently, the kingdoms of Protoctista, Fungi, and Monera were added. Recently, the term domain has been added to encompass two or more kingdoms. The three domains currently recognized are Archaea, Bacteria, and Eukarya. The domain called Eukarya is divided into four kingdoms known as Protoctista, Plantae, Animalia, and Fungi.

Question 13

What is meant by the term adaptation? Cite an example of adaptive radiation:

Answer 13

An adaptation is a modification of an organism that enables it to better live in its current surroundings or to compete in a new environment. Adaptive radiations occur when several organisms adapt simultaneously. These typically occur on the heels of mass extinctions. Examples include the Cambrian explosion, the rise of reptiles to dominance during the Mesozoic, and the early-Cenozoic rise of mammals following the demise of dinosaurs at the end of the Mesozoic.

Question 14

What are peripheral isolates? Why do new species commonly arise from peripheral isolates?

Answer 14

A peripheral isolate is a population that lives on the margins of the species ecological range and is somehow kept in reproductive isolation from the main species population. The peripheral isolate will adapt in response to the less than optimum environmental conditions. Adaptations will be retained and enhanced because of the reproductive isolation. If isolated long enough, the peripheral population may become significantly different from the parent population, giving rise to a new species.

Question 15

What are the contributions of Darwin and Mendel to our modern concept of organic evolution?

Answer 15

Darwin mustered a large body of circumstantial evidence suggesting that life evolved in response to its environment through natural selection. He also outlined a methodology for testing evolutionary theory. He was unable to tell us how favorable traits were passed on from one generation to the next. Gregor Mendel started us on the path towards modern genetics with his work on peas and other organisms. Modern genetics has shown how adaptations can be transferred to successive generations.

Question 16

Distinguish between the concepts of phyletic gradualism and punctuated equilibrium. How did Charles Darwin account for the rapid or abrupt appearance of new species? Which then, would be a more appropriate geologic section to study for proof of punctuated equilibrium- a continuous set of cores from the floor of the ocean, or a section on the continent where there has been repeated episodes of uplift and erosion throughout geologic time?

Answer 16

Under phyletic gradualism, evolution occurs progressively, evenly, and slowly. If the fossil record were complete, we would see one species develop into another species through several intermediate steps. According to phyletic gradualism, this transition affects the entire species population. Gaps, or missing links between similar groups are attributed to missing time or imperfect preservation of the fossil record. By contrast, punctuated equilibrium suggests that evolution proceeds fitfully, with long periods of stasis interrupted by brief periods of rapid change. Initially speciation affects only those organisms at the edge of the environmental range of the species and not the species population as a whole. Once the speciation event has occurred, the species remains unchanged until the next period of rapid change. One of the best indicators that validates punctuated equilibrium is the fossil record itself. In sequences of layered, fossil-rich sedimentary rocks, new species often appear suddenly in a specific bed, remain relatively unchanged in higher strata, and then disappear.

Question 17

Primary seismic waves

Answer 17

Speediest of the three kinds of waves; first to appear on the seismograph after there has been an earthquake; longitudinal; called “body waves” because they can penetrate deep into the planet

Question 18

Secondary seismic waves

Answer 18

Travel 1-2 km/second slower than P-waves; more complex motion; can pass through liquid or gas

Question 19

Surface seismic waves

Answer 19

Large motion waves that travel through the outer crust of the Earth, develop when S/P waves disturb the surface of the Earth as they emerge from the interior; last to arrive

Question 20

Briefly describe the properties of the inner core, outer core, mantle, and crust and indicate the role of seismic discontinuities (Mohorovicic and Gutenberg) in locating the boundaries between these layers.

Answer 20

Synthetic and naturally occurring seismic events (earthquakes) produce waves that travel through and along the surface of the Earth. Those waves traveling through the Earth (primary and secondary waves) reveal that the Earth has four major layers: solid inner core, liquid outer core, solid mantle, solid crust. The boundary between the crust and mantle is called the Moho or Mohorovicic discontinuity. Here primary and secondary waves accelerate dramatically as they encounter the denser mantle rocks. Much deeper, at the boundary between the core and mantle, seismic wave velocities decrease markedly as they enter the liquid outer core. This boundary is called theGutenberg discontinuity.

Question 21

Indicate the basic properties that distinguish oceanic crust from continental crust.

Answer 21

Two distinct types of crust occur: thin, basaltic, oceanic crust and thicker, graniticcontinental crust. The oldest known oceanic crust is approximately 300 million years old. The oldest continental rocks thus far discovered are over ten times older (3.8 billion years old)

Question 22

Define fault and indicate how a given fault is classified as normal, reverse, or strike-slip.

Answer 22

Stresses set up at plate boundaries cause crust rocks to deform. This deformation is evidenced by faulting or folding. Faults are classified (based on the relative movement of the footwall and hanging wall blocks) as either normal, reverse, or strike-slip. These faults are characteristic of divergent, convergent, and transform fault-plate margins, respectively.

Question 23

Discuss the differences between anticlines, synclines, domes, and basins.

Answer 23

Down folds are called synclines and basins. Up-arched folds are called anticlines and domes.

Question 24

Explain the types of evidence presented by Alfred Wegener that supports his theory of continental drift.

Answer 24

The modern theory of plate tectonics is actually the offspring of two complementary theories known as continental drift and seafloor spreading. The older theory (continental drift) was formally proposed by Alfred Wegener in his book, On the Origin of Continents and Oceans (1916). Continental drift purports that a single landmass known as Pangea existed at the end of the Paleozoic Era and that the modern continents arose through its subsequent dismemberment. Wegener introduced several lines of evidence to support his claims, which evidence was derived from the continents. His theory was largely ignored because he could not explain how the continents actually moved. Mapping of topographic, magnetic, and seismic characteristics of the ocean floors during and after World War II led to the development the theory of seafloor spreading. This theory suggests that continents drift apart as new ocean crust is introduced between them at midocean ridges.

Question 25

Explain the concept and applications of paleomagnetism.

Answer 25

The study of magnetism of rocks at their origin. Paleomagnetism (or Palaeomagnetism in the United Kingdom) is the study of the record of the Earth’s magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth’s magnetic field and the past location of tectonic plates. The record of geomagnetic reversals preserved in volcanic and sedimentary rock sequences (magnetostratigraphy) provides a time-scale that is used as a geochronologic tool. Geophysicists who specialize in paleomagnetism are called paleomagnetists. Paleomagnetists led the revival of the theory of continental drift and its transformation into plate tectonics. Apparent polar wander paths provided the first clear geophysical evidence for continental drift, while marine magnetic anomalies did the same for seafloor spreading. Paleomagnetism continues to extend the history of plate tectonics back in time and are applied to the movement of continental fragments, or terranes. Paleomagnetism relied heavily on new developments in rock magnetism, which in turn has provided the foundation for new applications of magnetism. These include biomagnetism, magnetic fabrics (used as strain indicators in rocks and soils), and environmental magnetism.

Question 26

Concisely define the concept of seafloor spreading.

Answer 26

This theory suggests that continents drift apart as new ocean crust is introduced between them at midocean ridges.

Question 27

Describe the three types of plate margins (convergent, divergent, transform fault) and identify what types of geological activities, geological structures (folds and faults), and rocks occur at each.

Answer 27

Plate margins are defined by relative plate motion as the following: (1) convergent(plates collide), (2) divergent (plates pull apart), and (3) transform fault (plates slide horizontally past one another). Most tectonic activity (earthquakes, volcanism, mountain building) takes place at these plate boundaries.

Question 28

List several lines of evidence that support the plate tectonic theory.

Answer 28

Plate motion can be verified through (1) studying chains of volcanic islands and sea mounts related to hot spots, (2) mapping magnetic stripes on the seafloor, (3) radiometrically dating ocean floor basalts, and (4) using of laser beams and lunar reflectors that reveal measurable changes in the relative positions of continents over periods of a few years.

Question 29

Speculate upon the fundamental causes of plate motion.

Answer 29

The propelling mechanism of plate motion is believed to be thermal convection cells that form as mantle material is heated from below, expands, becomes less dense, and rises. On encountering the lithosphere, the flow diverges laterally, dragging along the overlying slab of lithosphere.

Question 30

IF one were able to drill a well from the North Pole to the center of the Earth, what internal zones would be penetrated?

Answer 30

CRUST
MANTLE
OUTER CORE
INNER CORE

Question 31

What are the three major categories of seismic waves? Describe their characteristics?

Answer 31

PRIMARY-COMPRESSIONAL waves that travel at relatively HIGH VELOCITY through Solids and Liquids.
SECONDARY- TRANSVERSE WAVES–SOLIDS ONLY
SURFACE- LARGE MOTION WAVES–travel through the OUTER crust like ripples on a pond.

Question 32

What is a seismic discontinuity? Where are the Gutenberg and Mohoroicic discontinuities located?

Answer 32

A seismic discontinuity is reflected by a sudden decrease or increase in seismic wave velocity.
o 1. Moho. discontinuity is the sudden INCREASE in the velocity of P and S waves at the BASE OF CRUST. It reflects a sudden density increase at the top of the mantle.
o 2. Gutenberg discontinuity is the rapid deceleration of P waves and the halting of S waves at the top of the liquid core.
How do anticlines (and domes) differ from synclines (and basins) with regard to the age relations of rocks exposed across the eriosionally truncated surfaces of these structures?
• Rocks become older as you approach the center of an anticline but become younger as you approach the center of a syncline or basin.

Question 33

What are the principal categories of faults? What kinds of faults might one find in regions subjected to great compressional forces? What kinds of faults result primarily rom tension in the Earth’s crust?

Answer 33

Normal, reverse, transform. Reverse faults predominate in regions subjected to compressional stresses. Normal faults are formed in response to tensional stresses.
Compile a list of items that Alfred Wegener might have used to convince a skeptic of the validity of his theory of continental drift:

Question 34

List of evidence in support of continental drift:

Answer 34

1. apparent fit in outline of continents bordering the Atlantic Ocean
2. distribution of Late Paleozoic tillites
3. distribution of Permian plants (Glossopteris)
4. and animals (Mesosaurus)
5. similarity in vertical stratigraphic sequence (suggesting nearly identical history) of Carboniferous-Cretaceous rocks in Africa, South America, and India

Question 35

According to plate tectonics, how did the Himalaya Mountains form? The San Andreas fault? The Dead Sea and Red Sea?

Answer 35

Each of these reflects a different type of plate margin interaction. The Himalayas are the result of India colliding with Asia (convergent plate margin). The San Andreas fault is a transform fault (passive margin) between the North American plate, which is moving south, and the Pacific plate, which is moving north. Finally, the Dead Sea and Red Sea occupy depressions resulting from the early stages of rifting (divergent plate margin).

Question 36

According to plate tectonics, where is new material added to the sea floor, and where is older material consumed?

Answer 36

New oceanic crust is generated at spreading centers (midocean ridges) and destroyed at subduction zones.

Question 37

What is remnant magnetism? What is its origin? How is it used in finding ancient pole positions? How has remnant magnetism helped validate the concept of plate tectonics?

Answer 37

Remanent magnetism is magnetic information frozen into iron-bearing igneous or sedimentary rocks as they form. With igneous rocks, as lava begins to cool, magnetic ironoxide minerals form and align their polarity with Earth’s magnetic field. The alignment is retained in the rocks unless they are heated beyond the Curie Point (the temperature at which the minerals are no longer magnetic). Iron-rich grains settling through quiet water may align themselves with Earth’s magnetic field. If undisturbed by burrowing animals, the resulting layers of sediment will retain a magnetic signature. The position of the modern north magnetic pole is indicated by observing a compass needle. As the “compass needles” of ancient rocks were analyzed, they suggested that the magnetic north pole was not always in its present geographic position relative to where the paleomagnetic data had been gathered. This suggests that either the magnetic pole has moved through time or that the continent where the paleomagnetic data was collected had moved. Paleomagnetic data from all the continents indicates that the magnetic pole has not changed and that the continents have drifted, changing their position relative to magnetic north.

Question 38

Define red shift in the context of the big bang theory.

Answer 38

Red shift of galaxies, residual background radiation, and the predominance of hydrogen and helium suggest that our universe originated from the big bang about 15 billion years ago. Red shift indicates that the Universe is expanding, which in reversal, would have caused a huge explosion, which was considered the “Big Bang.”

Question 39

Explain the main idea behind the solar nebula hypothesis.

Answer 39

The solar nebula hypothesis is the most widely accepted account of the solar system’s and Earth’s origin. It suggests that the sun and its coterie of planets condensed from a cloud (nebula) of interstellar dust and gas.

Question 40

Explain the origin and evolution of Earth’s oxygen-rich atmosphere.

Answer 40

Solar winds blew the primordial atmosphere into space. Hence, the modern atmosphere was formed from scratch, largely through volcanic outgassing. In the early Precambrian, this nascent atmosphere was comprised of methane, ammonia, and carbon dioxide. The modern oxygen-rich atmosphere evolved through photochemical dissociation of water vapor and, once life evolved, from photosynthesis.

Question 41

Discuss the duration and order of eons comprising the Precambrian interval.

Answer 41

The Precambrian interval encompasses over 87 percent of Earth’s history (4.6 billion to 544 million years before present) and is divided into three eons: the Hadean (4600 million years??), theArchean (1300 million years)approx 2000 million years), and the Proterozoic. Precambrian rocks generally do not contain abundant fossils, so radiometric dating is the principal means of correlation.

Question 42

Articulate the differences between the terms shield , craton , and platform

Answer 42

Each continent is comprised of three parts: shield, stable platform, and marginal mobile belts. The shield is the flat, geologically stable (few earthquakes and volcanoes) region of the continent where Precambrian igneous and metamorphic rocks are exposed at the surface. The surrounding areas, where Precambrian rocks are covered with a thin blanket of sedimentary strata, are called the stable platforms. Like the shield, the platform is geologically quiet. Together the shield and platform form the craton, or stable interior, of the continent. Tectonic activity in the form of mountain building, volcanic eruptions, and earthquakes is generally restricted to the marginal mobile belts.

Question 43

Explain the origin of the Precambrian provinces of North America shown on figure 6–17 of the text.

Answer 43

The Canadian shield has been divided into a number of Precambrian provinces; These boundaries are often marked by abrupt truncations in structural lineations, or they may be represented by bands of severely deformed rocks of former orogenic belts. They are also deformed, metamorphosed, and intruded rocks that mark the location of collision of the various cratonic elements. // They are defined based on differences in rock type, fault patterns, folding direction, and radiometric ages. Each province reflects a period of intense mountain building and erosion during which a new segment of continental crust was added to the margin of ancient North American. The most important thing to remember about the Archean history of North America is this: from a small protocontinent, North America grew to its current size by accretion of new crust to its margins

Question 44

Indicate the significance of Stanley Miller’s experiments to theories about spontaneous generation of life on Earth.

Answer 44

Scientist Stanley Miller performed an experiment to see how laboratory synthesis of amino acids and other similar molecules could be announced. He infused an atmosphere similar to ours into an apparatus, consisting of methane, ammonia, hydrogen and water vapor, and watched as sparks of electricity were discharged into the other glass bowl (condenser). It was shown that similar organic compounds could also be produced from gases, if it has little to no water vapor.
List the major fossils of the Precambrian and explain the differences between prokaryotic and eukaryotic organisms.

Question 45

MAJOR FOSSILS OF THE PRECEMBRIAN:

Answer 45

BANDED IRON, STROMATOLITES, ARCHAEA CELLS, DICKINSONIA, ETC.

Question 46

Differences between prokaryotic and eukaryotic organisms:

Answer 46

Prokaryotic organisms do not have a nucleus and eukaryotic organisms do. Eukaryotic DNA is linear; prokaryotic DNA is circular (it has no ends)

Question 47

List evidence for Earth’s first glacial episode (Gowganda formation).

Answer 47

Glacial sediments of the Gowganda formation indicate that glaciers covered the region north of modern Lake Huron during early Proterozoic time.

Question 48

Discuss the nature and significance of banded iron formation (BIF).

Answer 48

Banded iron formations are marine sedimentary rocks consisting of alternating thin layers of red iron oxides and grayish chert. Proterozoic banded iron formations (BIF s) are typically hundreds of meters thick and can be regionally extensive. They are, in fact, the world’s most important source of iron ore. Approximately 92 percent of BIFs were deposited during the early part of the Proterozic eon between 2.5 and 2 billion years ago. Earlier they did not occur abundantly because oxygen was not yet sufficient in the atmosphere and ocean. Later they did not occur abundantly because by then free iron had been swept from the oceans by the early Proterozoic BIF forming event. They represent a unique interval in the chemical coevolution of Earth’s atmosphere, hydrosphere, and lithosphere.

Question 49

Describe the fossil record of the Proterozoic eon, including proliferation of stromatolites, appearance of eukaryotes, and evolution of the first multicellular animals.

Answer 49

During the Proterozoic, stromatolites proliferated and covered the floors of the extensive cratonic ocean, thereby generating the oxygen that forced the deposition of BIFs. By middle Proterozoic time (1.6 billion years ago) single-celled eukaryotes appeared with their capacity for sexual reproduction. In late Proterozoic time the first multicellular animals (Ediacaran Fauna) evolved.

Question 50

Study figure 7-33 on page 258 of the textbook. Familiarize yourself with the major developments occurring in the atmosphere, lithosphere, and biosphere during the Precambrian time interval.

Answer 50

The earth’s atmosphere and hydrosphere were highly devoid of free oxygen. Then about 2 billion years ago, atmospheric oxygen began to accumulate because of increased plant photosynthethic acrivity. One result of the oxygen buildup was the accumulating on land of considerable amounts of ferric iron oxide, which stained terrestrial sediments a rust-red color. Such rock is now called red beds, and are considered a valid indication of the advent of an oxygenic environment. The oxygen level probably rose slowly and very likely did not approach 10% of present atmospheric levels of free oxygen until the Cambrian period. Proterozoic rocks are evident for different climatic conditions.
• At the dawn of the Proterozoic, life continued to be restricted to simple prokaryotes and cyanobacterial stromatolites. During the Proterozoic, stromatolites proliferated and covered the floors of the extensive cratonic ocean, thereby generating the oxygen that forced the deposition of BIFs. By middle Proterozoic time (1.6 billion years ago) single-celled eukaryotes appeared with their capacity for sexual reproduction. In late Proterozoic time the first multicellular animals (Ediacaran Fauna) evolved.

Question 51

What is the source of the Sun’s heat? Given the amount of solar radiation intercepted by the Earth, why is the Earth’s surface not hotter than it is?

Answer 51

The source of the sun’s heat is nuclear fusion. In the core of the sun, hydrogen nuclei are fused together to produce helium. This fusion of nuclei into larger nuclei released tremendous amounts of energy. Three factors play a role in maintaining a surface temperature range of –50 to +60 degrees Celsius. First, only half of Earth receives radiation at any one time, but heat is returned to space over the entire surface. Second, much of the incoming solar radiation is reflected back to space by the outer atmosphere. Finally, another part of the incoming radiation is absorbed by the atmosphere and radiated back to space.

Question 52

What evidence indicates that the universe is expanding?

Answer 52

The spectral signatures of galaxies are red shifted.

Question 53

Distinguish between the terms Precambrian shield, craton, and platform.

Answer 53

All continents are comprised of three parts: shield, platform, and mobile mountain belt. The shield is an extensive area of continent where Precambrian igneous and metamorphic rocks are exposed and have been eroded to a nearly flat surface close to sea level. The platform is similar to the shield except the Precambrian rocks have been covered with layers of sedimentary rock. Together, the platform and shield constitute the craton (stable interior portion of a continent).

Question 54

What geologic evidence suggests that free oxygen was beginning to accumulate in the Earth’s atmosphere about 3 billion years ago?

Answer 54

Banded iron formations are important as a source of iron and as an indicator of atmospheric evolution. Banded iron formations are special sedimentary rocks comprised of alternating layers of iron-rich (red) and ironpoor (gray) layers (figure 7–4 on page 239 of your text). Most rocks of this type are older than 2 billion years. They account for most of the iron ore that is mined in the world today. The fact that the iron in banded iron formation was weakly to strongly oxidized indicates that oxygen was beginning to accumulate in the environment at that time.

Question 55

Discuss the role of symbiosis in the evolution of eukaryotes. What organelles may have originated by symbiosis?

Answer 55

The endosymbiotic theory for the origin of eukaryotes suggests that the nucleus and well-ordered genetic material in a eukaryotic cell may have arisen through ingestion, not digestion, of one prokaryotic cell by another. Through time the ingested cell was modified into the chloroplast and mitochondria of the typical eukaryote (see pages 227–28 of your text).

Question 56

How do eukaryotes differ from prokaryotes? When do eukaryotes appear in the fossil record?

Answer 56

Eukaryotic organisms are those with a well-defined nuclear wall, definite chromosomes, and a capacity for sexual reproduction. By contrast, prokaryotic organisms possess cells with no definite nucleus and lack membrane-bounded organelles. They are capable of only asexual reproduction and are typically smaller. The earliest appeared about 1.6 to 1.4 billion years ago.

Question 57

What are metazoans? What is the earliest known occurrence of abundant metazoans? With regard to their general appearance, what are the three major groups of Ediacaran metazoans?

Answer 57

Metazoans are multicellular animals. The earliest known occurrence is in the Rawnsley Quartzite (late Proterozoic) of Australia. The fauna, called the Ediacaran Fauna, is comprised of the impressions of worm-like; discoidal, jellyfish-like; and frond-shaped animal groups.

Question 58

When did continental glaciation occur during the Proterozoic? What is the evidence that such glaciation occurred? Why is it unlikely that continental glaciers would have formed during the Archean?

Answer 58

In the region north of Lake Huron, there exist conglomerates and laminated mudstones assigned to the Gowganda Formation. The unsorted conglomerates and varved sands and shales are indicative of glacial conditions. The period of glaciation occurred sometime between 2.6 and 2.1 billion years ago. The higher thermal regimes of the Archean would have made sufficient ice accumulation to form glaciers improbable.

Question 59

Stromatolites were exceptionally widespread during the Proterozoic but became relatively sparse thereafter. What other organisms may have contributed to the post-Proterozoic decline of stromatolites?

Answer 59

Stromatolites are cabbage-shaped, laminar, organosedimentary structures formed by the trapping of sedimentary particles and precipitation of calcium carbonate in response to the metabolic activities and growth of matlike colonies of cyanobacteria and other prokaryotes. The evolution of grazers, such as snails, would have cut back the number of stromatolites and limited them to nasty environments where grazers could not live.