Page 63 - Introduction to Paleobiology and The Fossil Record

50 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 2.7 Latitudinal variation in diversity through time

Today the tropics are teeming with diverse life built around a number of so-called hotspots, small
areas that have especially high numbers of species. But is this a modern phenomenon? Recent
research suggests that latitudinal gradients have intensified dramatically during the past 65 myr and

that biotic radiations in the tropics are based on relatively few species-rich groups in both marine
and terrestrial environments (Crame 2001). Part of this may have been driven by evolutionary esca-
lation, part by changing climates. In evolution, sometimes predators and prey evolve rapidly in
concert – the predators may adopt ever-more deadly means of attacking their prey, but the prey
evolves ever-better means of defense. This kind of escalation, or arms race, has happened in
many circumstances (see p. 102), and may have happened in tropical oceans through the past 15 myr.
Further, global climate change during this same period probably helped to partition the tropics into
a series of diversity hotspots, such as the Indo West Pacific (IWP) center. It is hard to be sure that

such hotspots in the geological past will be preserved. How we perceive past diversity may be
very much dependent on whether we have or have not properly sampled these hotspots through
time.

Other latitudinal diversity gradients tend to confirm current trends. For example, in a study
covering the past 100 myr, Markwick (1998) found that crocodilians used to have a wider
latitudinal spread than they do today. Modern crocodilians are known primarily from a narrow
tropical belt covering the southern United States down to central Brazil, Africa, India and Australasia.
Abundant crocodilian fossils from the Cretaceous and Tertiary are known from northern parts
of North America and Europe, but the richest finds lie around the paleoequator. So, the

tropical, warm-weather part of the world used to be twice as wide as it is today and, in general,
global climates have cooled through the last 100 myr. Nevertheless crocodilians are, and were, most
abundant round the equator, and their diversity declines the farther one goes away from the
tropics.

from Gondwana and has nothing to do with within orogenic belts? The Banda Arcs are
the geological history of North America until part of a much younger mountain belt, devel-
later in the Ordovician. This is, however, only oped during the Neogene and Quaternary
one school of thought. New structural data along the continental margin of northern Aus-
suggest the Highland Border Complex was tralia (Harper 1998). A precise stratigraphy
part of the Dalradian and, indeed, was always based on foraminiferans has allowed the
intimately linked to the Laurentian craton movement of far-traveled thrust complexes to
(Tanner & Sutherland 2007). Elsewhere in be tracked; thrust sheets were emplaced at
−1
the Caledonides, Harper and Parkes (1989) rates between 62.5 and 125 mm yr whereas
described a series of terranes across Ireland the belt as a whole was uplifted at rates of
−1
based on paleontological data. While some about 15 mm yr .
terranes developed marginal to North America Fossils, surprisingly, can be of great value
and Avalonia (see above), some smaller ter- to structural geologists, not only in under-
ranes in central Ireland almost certainly standing the rates and timing of tectonic
evolved within the Iapetus Ocean itself, with events. Structural geologists study rocks that
their own distinctive faunas. have been folded and faulted, and they want
We can thus reassemble ancient mountain to identify how exactly the rocks have been
belts and trace the origins of their jumbled deformed. If they fi nd a fossil that was origi-
structure using paleontological data, but can nally symmetric, but has since been squeezed,
fossils help us understand the rates of these or stretched, in particular directions, they
tectonic processes, such as plate movements have precise evidence of the magnitude of the
and the transit of individual thrust sheets tectonic forces that have acted. A famous