Page 56 - Introduction to Paleobiology and The Fossil Record

FOSSILS IN TIME AND SPACE 43
may be caused by the formation of an
isthmus.
In some situations, the development of a
barrier for some organisms may provide a
corridor for others. The emergence of the
Isthmus of Panama 3 Ma connected North
and South America, but at the same time it
separated the Atlantic and Pacifi c oceans.
Before this event, South America had been
isolated from North America for most of the
past 70 myr, and was dominated by diverse,
specialized, mammalian faunas consisting of
unique marsupials, edentates, ungulates and
rodents. However, the Isthmus of Panama
provided a land bridge or corridor between
the two continents and many terrestrial and
freshwater taxa were free to move north and
south across the isthmus (Fig. 2.15). The great
American biotic interchange (GABI) allowed
the North American fauna to invade the south
and destabilize many of the continent’s dis-
tinctive mammalian populations (Webb 1991).
South American mammals were equally suc-
cessful in the north and some such as the
armadillo, opossum and porcupine still survive
in North America.
The emergence of the isthmus also caused
changes in the marine faunas of the Carib-
bean. Surprisingly, not many species became
extinct, and there was a diversifi cation of
mollusks (Jackson et al. 1993). The emer-
gence of the terrestrial land bridge and marine
barrier may have initiated the upwelling of
nutrients in the Caribbean area, and this in
turn led to an increase in species diversity.
Valentine (1973) had already drawn attention
to a range of plate tectonic settings, including
the spreading ridges, island arcs, subduction
and fault zones, and the ways they can affect
biological distributions. Thus tectonic fea-
tures such as spreading ridges, transform
faults and subduction zones create barriers
for marine faunas whereas mid-plate island
volcanoes can generate a series of stepping
stones assisting the migration of animals and
plants across great expanses of ocean. But
there may be a more important relationship
between tectonics and provinciality. There is
a striking correlation between provinciality
and continental fragmentation through time.
Intervals when continents were many and dis-
persed apparently were times of increased
provinciality, such as the Ordovician and the
Cretaceous.
Island biogeography: alone and isolated?
Modern oceans are littered with islands. Most
are transitory volcanic chains, developed
above moving hotspots or at mid-oceanic
ridges that will probably be subducted; some,
however, are pieces of continental crust broken
off adjacent continents. These lighter bits of
crust are usually later imprisoned in mountain
chains and can hold important paleontologi-
cal data. The biogeography of modern islands
is complex and it is hard to apply models
based on modern islands to ancient examples
(Box 2.6).
But islands and archipelagos play a number
of biological roles. Most islands are isolated
from the mainland, and they are important
powerhouses of speciation (see p. 119). Some
island chains play an important part in migra-
tions, acting as stepping stones, where species
and their larvae may move, sometimes over
many hundreds or thousands of years, from
one mainland to another. The vertebrate pale-
ontologist Malcolm McKenna introduced
some interesting analogies with ancient ship-
ping. Moving island complexes that can allow
the cross-latitude transfer of evolving animals
and plants may have acted as “Noah’s arks”,
just as Noah’s biblical ship eventually beached
on the summit of Mount Ararat with breeding
pairs of all manner of contemporary life. The
transit of India from Gondwana to Asia,
together with its even-toed artiodactyls and
odd-toed perissodactyls, is a possible example.
In the longer term these complexes may func-
tion as “Viking funeral ships” (originally
bound, of course, for Valhalla with decorated
dead warriors) transporting exotic fossil
assemblages to new locations. The occurrence
of a Gondwanan Cambrian trilobite fauna in
the Meguma Terrane of the Appalachians and
an Ordovician trilobite fauna in Florida from
the same high-latitude province, both now
welded onto the North American continent,
are remarkable examples.
Island biotas (faunas and fl oras) are often
diverse, with many endemic species and com-
monly with evidence that these species came
originally from one or more source conti-
nents. It is fascinating to study such modern
islands and some, such as the Galápagos, or
Aldabra, have become important sites for
biologists to watch “evolution in action”.
It is much harder for paleontologists to