Page 77 - Global Tectonics

64 CHAPTER 3

stronger competition. Conversely, continental rifting
leads to the isolation of faunas which then follow
their own distinct evolutionary development. For
example, marsupial mammals probably reached Aus-
tralia from South America in the Upper Cretaceous
along an Antarctic migration route (Hallam, 1981)
before the Late Cretaceous marine transgression
removed the land connection between South America
and Antarctica and closed the route for the later
evolving placental mammals. Sea floor spreading then

ensured the isolation of Australia when the sea level
dropped, and the marsupials evolved unchallenged
until the Neogene when the collision of Asia and
New Guinea allowed the colonization of placental
Figure 3.11 Correlation of invertebrate diversity with mammals from Asia.
time and continental distribution. A, earlier Pangea;
B, fragmentation of earlier Pangea producing oceans
preceding Caledonian (1), Appalachian (2), Variscan (3),
and Uralian (4) orogenies; C, suturing during Caledonian 3.6 PALEOMAGNETISM
and Acadian orogenies; D, suturing during Appalachian
and Variscan orogenies; E, suturing of Urals and
reassembly of Pangea; F, opening of Tethys Ocean; 3.6.1 Introduction
G, fragmentation of Pangea. a, Gondwana; b, Laurasia;
c, North America; d, South America; e, Eurasia; f, Africa;
g, Antarctica; h, India; i, Australia (after Valentine & The science of paleomagnetism is concerned with
Moores, 1970, with permission from Nature 228, 657–9. studies of the fossil magnetism that is retained in certain
Copyright 1970 Macmillan Publishers Ltd). rocks. If this magnetism originated at the time the rock
was formed, measurement of its direction can be used
to determine the latitude at which the rock was created.
If this latitude differs from the present latitude at which
species as a result of genetic isolation and the mor- the rock is found, very strong evidence has been fur-
phological divergence of separate faunas. Conse- nished that it has moved over the surface of the Earth.
quently, more species evolve as different types occupy Moreover, if it can be shown that the pattern of move-
similar ecological niches. Figure 3.11, from Valentine ment differs from that of rocks of the same age on a
& Moores (1970), compares the variation in the different continent, relative movement must have
number of fossil invertebrate families existing in the occurred between them. In this way, paleomagnetic
Phanerozoic with the degree of continental fragmen- measurements demonstrated that continental drift has
tation as represented by topological models. The cor- taken place, and provided the first quantitative estimates

relation between number of species and fragmentation of relative continental movements. For fuller accounts
is readily apparent. An example of such divergence of the paleomagnetic method, see Tarling (1983) and
is the evolution of anteating mammals. As the result McElhinny & McFadden (2000).
of evolutionary divergence this specialized mode of
behavior is followed by different orders on separated
continents: the antbears (Edentata) of South America,
the pangolins (Pholidota) of northeast Africa and 3.6.2 Rock magnetism
southeast Asia, the aardvarks (Tubulidentata) of central
and southern Africa, and the spiny anteaters (Mono- Paleomagnetic techniques make use of the phenome-
tremata) of Australia. non that certain minerals are capable of retaining a
Continental suturing leads to the homogenization record of the past direction of the Earth’s magnetic

of faunas by cross-migration (Hallam, 1972) and the field. These minerals are all paramagnetic, that is, they
extinction of any less well-adapted groups which face contain atoms which possess an odd number of elec-