Page 123 - Introduction to Paleobiology and The Fossil Record

110 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

p. 121). Moreover, it is probable that nar-
rowly-fl uctuating, changing environments
host persistent gradualistic evolution whereas
Extinctions
widely-fluctuating environments host mor-

phological stasis (Sheldon 1996). This resis-
Climate tance to morphological change is clear in a
number of lineages such as Ordovician trilo-
bites and Pliocene mollusks (see p. 123).
PC Cam Ord Sil Dev Carb Perm Tr Jur Cret Cen
Icehouse Greenhouse Ice-house Greenhouse Ice Short-term climatic fl uctuations, for
example those associated with Milankovitch
cycles, can clearly disrupt and promote the
Volcanism Sea level reassembly of both marine and terrestrial
communities. In some cases they can drive
local extinctions and radiations, for example
in the conodont and graptolite faunas of the
Silurian (see p. 434).
Figure 4.26 Climate change through time Climate surely drives larger-scale aspects of
illustrated together with changes in sea level and evolution. For example, the Cambrian explo-
fluctuations in the intensity of volcanicity. (Based sion (see p. 249) – marked by the diversifi ca-

on various sources.) tion of skeletal organisms and the appearance
of reef-building organisms and the fi rst preda-
tors – is associated with increasingly warm
2 On the other hand, greenhouse oceans climates and higher sea levels. On land the
were more stable, and better stratifi ed radiation of early terrestrial tetrapods in the
with surface waters ranging in tempera- Early Carboniferous and the diversifi cation
ture from 12 to 25°C with deep-water of large flying insects in the fi rst extensive

temperatures between 10 and 15°C. Slow forests, in cooler climates and more exposed
bottom currents carried little oxygen and land areas, have been correlated with high
productivity was generally low. levels of atmospheric oxygen (Berner et al.
2000).
Extinctions were associated with the transi- Some of the largest events of all, such as
tions between these oceanic states. the appearance of entire new biotas and grades
In addition to these major climatic fl uctua- of organization, for example the origin of life
tions a series of major extinction events, some itself, the development of photosynthesis and
associated with extraterrestrial causes, clearly the appearance of the metazoans, may also be
prompted major climate change over several associated with climate change. The fi rst two
million years. Such events caused major taxo- events have been associated with a stable
nomic extinctions together with major restruc- Archaean crust and relatively cooler climates,
turing of the marine and terrestrial ecosystems. which are favorable for carbon-based life to
Generally, greenhouse biotas were most sus- evolve. Metazoans appeared and diversifi ed
ceptible to extinction; their species were more after the decay of the near global ice sheets of
specialized and thus more exposed to environ- “snowball Earth” (Box 4.10), whereas skele-
mental change. tal organisms radiated during the greenhouse
climates and higher oxygen levels of the Early
Consequences for evolution Cambrian.
Microevolution is obvious in many fossil lin-
eages (Benton & Pearson 2001) although the
link between speciation events and climatic Biological feedbacks
change is more controversial. Generally, If climate drives evolution, could life itself
marine plankton show gradual evolution drive climate change? Few people doubt that
whereas marine invertebrates and vertebrates humans can affect the climate, and everyone
display a pattern of punctuated equilibria (see is aware of how the industrialized nations are