Download answers to review questions – chapter 33

ANSWERS TO REVIEW QUESTIONS – CHAPTER 32
1. Why are fossils mostly found in sedimentary rocks? (pp. 762–763)
Fossils are the preserved remains of organisms or traces of them, such as dinosaur footprints or the
organic compounds produced by them. Hard parts of fossils, such as teeth and bone of animals, or
leaves and woody parts of plants, are most often fossilised. A fossil forms when a dead organism is
buried in sand, silt or clay and the nature of the burial prevents bacterial decay. As sedimentary rocks
form from material in which the dead organisms are buried, physical compaction and chemical changes
force water from them and may alter their shape and composition. For example, fossils that form in
shales are subjected to very high compression and tend to be flattened, while the microscopic spaces in
the structure of some organisms can become filled with minerals, forming fossils like petrified wood.
Fossils may also form by entrapment, such as insects found in amber, or by carbonisation. Examples of
fossil types are trace fossil, chemical fossils, fossil moulds and fossil casts.
2.
Geologists and paleontologists use ‘a relative geologic time scale’ when rocks cannot be
dated directly. What is this relative time scale based on? (pp. 763–765)
Prior to accurate dating techniques, geological time scale for the evolution of life was based on a
relative measure. In sedimentary strata, fossils lower in the profile were considered to be from an older
geological period relative to those higher in the profile. Absolute measures of geologic age became
available with the discovery of radioactivity. The modern geologic time scale incorporates data from
actual age measurements plus sequences of relative age. Actual age measurements of rocks can be
determined from radiometric dating. Measurements of magnetic reversals in rocks can also provide
estimates of age in some strata.
3.
Geologic features that help us to recognise past movements of continents include magnetic
reversals, deep-sea trenches, oceanic ridges and hot spots. Briefly explain each of these
terms. (pp. 766–768)
Magnetic reversals—occasionally the earth’s magnetic field will change in polarity from normal
(present-day orientation) to reversed (magnetic north becomes south and south becomes north). This
change in orientation is called magnetic reversal and particular orientations may last thousands or
millions of years.
Deep-sea trenches—form where subduction results in a section of the sea-floor descending back into
the mantle. Continents are of a lower density than the crust so they do not readily descend into the
mantle.
Oceanic ridges—occur where upwells of lava from the mantle form new crust, forcing basaltic rocks
apart. This is the process of sea-floor spreading.
Hot spots—are immobile points at the surface of the mantle where a column of hot upwelling
asthenosphere rises. When a plate moves over a hot spot, volcanic islands may form.
4.
Briefly explain how the earth’s crustal plates move and what happens at their boundaries.
(pp. 766–768)
Plate tectonics recognises that the earth’s crust and upper mantle are divided into a number of plates
and that these plates move relative to one another. New crust on the sea-floor forms from lava upwells
at oceanic ridges. As the sea-floor spreads either side of the ridge, the continents, which have a lower
density, are carried away from the ridge.
As new crust is formed, old crust must disappear at some point. Deep-sea trenches occur at one plate
boundary where denser sea-floor crust descends back into the mantle at a subduction zone. The less
dense continents tend to remain at the surface. The oceanic trenches in areas of subduction mark sites
of volcanoes and earthquakes.
Plate boundaries may also scrape past one another, deforming the earth’s crust. These boundaries are
the site of earthquakes. A third type of boundary occurs where two plates collide forming mountain
belts. The Himalayas and Andes are examples of mountain belts that formed from such collisions.
5. What is a stromatolite and how does it form? (Box 32.3)
Stromatolites are the hard, dome-shaped rocks formed by a continuing process of sediment trapping,
mineral precipitation and hardening that is facilitated by living cyanobacteria. These cyanobacteria live
as mats on the surface of the dome. They are motile and, as sediment precipitates on the dome’s
surface, the cyanobacteria move through the fine sediment that accumulates to ensure that they are at
the top of the growing stromatolite. The oldest fossil evidence of stromatolites is from 3.3 to 3.5 billion
years ago.
The fossil record indicates that between 2.5 billion and 540 million years ago stromatolites were
abundant and diverse, the result of a large diversity of prokaryotes that formed them. The decline in
stromatolites coincided with the evolution and radiation of marine multicellular organisms.
6.
Name the four major geologic eras. What significant biological events characterise each of
them? (pp. 769–777)
Pre-Cambrian—origins of life
Palaeozoic—ancient life
Mesozoic—age of dinosaurs
Cenozoic—the beginning of modern life
Older Pre-Cambrian organisms were prokaryotes. Some of these prokaryotes were photosynthetic
which, in addition to the evolution of first life during this era, is the most significant biological event.
Photosynthesis led to the atmosphere becoming oxygen-rich, as indicated by the formation of banded
iron formations, rich in iron oxide, which range in age from 1.8 to 2.3 billion years.
The first eukaryotes are found in the younger Pre-Cambrian rocks dating about 1.4 billion years ago
(Proterozoic era). These organisms have chloroplasts and resemble green algae. Multicellular softbodied organisms, collectively called the Ediacaran fauna, were present about 570 to 590 years ago and
are the best evidence that multicellular organisms with a larger body size had evolved before the end of
the Pre-Cambrian.
Most phyla of shelled invertebrate animals are present as fossils at the beginning of the Palaeozoic era.
There is a ‘sudden’ appearance of all the basic body plans of metazoans at this time. Marine Palaeozoic
communities depended on phytoplankton by the beginning of the Mesozoic. Fossils of these organisms
resemble modern taxa such as diatoms. Trilobites are the most common marine multicellular animals in
the early Cambrian and by the Ordovician the first vertebrate jawless fish appeared. In the Devonian
there was a rapid evolution of fish, including sharks and bony fish.
The first terrestrial invertebrates appear in the fossil record during the Silurian, while the earliest
known vertebrates are amphibians, fossils of which have been found in Upper Devonian rocks. The
oldest terrestrial plant fossils are of small vascular plants that lacked leaves and roots. These fossils
appear in rocks from the late Silurian. However, during the Carboniferous, forests of large tree-sized
plants with woody stems had evolved.
The Mesozoic era is often described as the ‘Age of Reptiles’. Dinosaurs, which included some of the
largest animals to have evolved, dominated vertebrate fauna during the Jurassic and Cretaceous. Birds
are most likely to have evolved from dinosaur ancestors during this period. With the extinction of the
dinosaurs, there was the opportunity for mammals to fill vacated niches. One of the most significant
biological events in the Mesozoic was the appearance and sudden expansion of flowering plants during
the Cretaceous.
The Cenozoic era (65 million year ago to present) is the era of modern life. Mammals and angiosperms
became abundant, with more and more modern groups becoming recognisable during the era. Hominid
fossils appeared about 4 million years ago.
7.
Match each of the following fossils to its approximate age from a list of ages shown below
(not all the ages shown will match): oldest prokaryotes; first multicellular animals; first
land plants; dominance of flowering plants; dominance of mammals; first hominids. (Check
your answers against Figure 32.5.) (p. 764)
4 billion years—oldest prokaryote from 3.3 to 3.5 billion years ago
2 billion years—no matches
1 billion years—no matches
600 million years—first multicellular animals 570 to 590 million years
400 million years—oldest land plant fossils from the late Silurian age
200 million years—no matches
100 million years—dominance of flowering plants beginning by late Cretaceous
—mammals became more abundant after 65 million years ago
10 million years—Mammalian groups have flourished
4 million years—first hominids
400 000 years—no matches
8.
Briefly describe the Ediacaran fauna, where it is found and of what significance it is. (p.
771)
The Ediacaran fauna are fossils of soft-bodied organisms. They are significant because they suggest
that multicellular animals had evolved in the Pre-Cambrian, some 570 to 590 years ago. The Ediacaran
fossils are found in beds in the Ediacaran Hills of South Australia.
9.
What is a biogeographic region and what is its significance in terms of understanding
evolutionary history? (pp. 777–779)
Biogeographers have divided the earth into several continental and marine biogeographical regions,
characterised by organisms which have simultaneously evolved with the changing geological
landforms and climate. Similar distributions of taxa are explained by present-day environmental factors
as well as previous evolutionary history.
10. Construct a table listing the four major botanical regions of the world, where they occur
and one group of endemic organisms (plant or animal) characteristic of each region. (pp.
777–779)
Botanical region
Location
Characteristic plant/animal
Boreal
Palaeotropical
Neotropical
Europe, Asia and North America
Africa, India, South-East Asia
South America, lower Central
America
Australia, New Guinea, neighbouring
islands in the north and New Zealand
Fir forests
Elephants and rhinoceroses
Cactus
Australian
Kangaroos and wombats