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48 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
Iapetus Ocean, all confirming in broad terms Many authors have suggested that chang-
current paleogeographic reconstructions of ing plate confi gurations, oscillating between
this complex ocean system (Harper et al. fragmentation and integration, have affected
1996) (Fig. 2.16e). Finally in this apparent biodiversity through time. For example, the
confusion, some terranes, such as the Argen- huge Early Ordovician radiation of marine
tine Precordillera, have faunas that have even skeletal faunas may be related to the breakup
switched provinces as their terranes drifted of Gondwana, while the end-Permian extinc-
across latitudes (Astini et al. 1995) (Fig. tion event coincides with the construction of
2.16f). But this evidence has been disputed. Pangaea. More recent diversifi cations have
The view of fauna switching is not entirely occurred during the late Mesozoic fragmenta-
supported by a geochronometric study of tion of this supercontinent (Fig. 2.18).
detrital zircons that shows that the Precordil-
lera had an origin in Gondwana, where the
basement rocks that supplied the zircons FOSSILS IN FOLD BELTS
probably occur (Finney 2007). Perhaps on One bad fossil is worth a good working
this occasion the faunal data require an alter- hypothesis.
native explanation.
Careful paleogeographic study has shown Rudolf Trümpy,
that some continents have been put together eminent Alpine geologist
from numerous formerly separated strips of
land. Geological mapping may highlight
major fault zones, lines of disjunction between Fossils from the deformed zones of mountain
unmatched rock units on either side, but it is, belts are rare but important. Relatively few
in fact, the fossils that can pin down where paleontologists study these fossils because
each continental slice, or terrane, came from they are usually poorly preserved, and are
in the first place. A classic example is the metamorphosed and tectonized; fossils in oro-
North American Cordillera, which is a mosaic genic or mountain-building zones are also
of terranes, now plastered onto the west coast rare and difficult to collect from often hazard-
of the continent, but probably originating at ous terrains. Nevertheless, fossils are of fun-
lower latitudes. Paleontologists have recog- damental importance in the formulation of
nized so-called Boreal (northern, low-diver- tectonic models, providing age and geographic
sity) and Tethyan (southern, high-diversity) constraints, although the fossils themselves
faunas of marine invertebrates in the separate are rarely of great morphological signifi cance.
terranes in the Mesozoic. In an east–west tra- The identifi cation of fossiliferous sequences in
verse across the North American Cordillera, thrust belts helped identify large-scale hori-
there is a progressive northward displacement zontal movements of the Earth’s crust in the
of Tethyan-type faunas of Early Jurassic age. Swiss Alps, the Northwest Highlands of Scot-
Some of the more exotic, far-traveled terranes land and in the Scandinavian Caledonides
may have moved over 1300 km (Fig. 2.17). over a century ago (Box 2.8). In many moun-
Biogeography and climatic gradients have tain belts fossil data have provided the only
driven patterns of changing biodiversity. In reliable dates for rock successions; unlike
broad terms, low latitudes support high-diver- radiometric clocks, fossils cannot be reset by
sity faunas, and biodiversity decreases away later thermal and tectonic events.
from the tropics towards the poles. Studies on The Appalachian-Caledonian mountain
modern bivalve, bryozoan, coral and forami- belt, developed during the Early Paleozoic,
niferan faunas show marked increases in contains large pieces of both North America
diversity towards the equator, and since many and Europe, but understanding of its complex
cool-water species breed later in life, polar history and structure is fairly recent. Parts of
and temperate-zone animals are sometimes the belt have been dissected and investigated
larger than their tropical counterparts. But by paleontological data. For example, Charles
this is only plausible if the growth rates Lapworth’s studies on the complex structure
are the same in both regions; they may not and stratigraphy of the Southern Uplands of
be. What is true today is true in the past Scotland in the 1870s were based on recogni-
(Box 2.7). tion of the sequence of graptolite faunas.
