Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 49

of possible future climatic change. (For an example of a typical summary pollen
diagram see Figure 4.8.)
2.1.6 Speciesasclimateproxies
It is well known that different fauna and flora live under different climatic condi-
tions and we have seen how the adaptations of leaves to different climate conditions
help with their use as indicators of climate. Such adaptations are not just restric-
ted to leaf morphology but (as we have also seen) also physiology, and many other
ways. (Indeed, we are beginning to see these in genetic codes. For example, the
expression of the gene Hsp70 in plants varies linearly with temperature; Kumar and
Wigge, 2010.) The bottom line is that plants and animals evolve to live under certain
(or a limited range of ) environmental conditions. So, as with pollen and spores, it
might seem obvious to use species as climate proxies. Aside from the apparently
obvious connection with climate, another attraction of considering species as palaeo-
climatic indicators is that fossils are frequently all about us in unaccounted millions.
Although large fossils are comparatively rare, small fossils, of a few millimetres or
less in size, are frequently abundant. They come in two principal groups, macro-
fossils and microfossils, depending on whether a microscope is needed to identify
them.
There is a considerable variety of small macrofossils. Usually only the hard parts
of a plant or animal become fossilised. The soft parts decay quite rapidly after death.
Animals are therefore often represented by the bones and teeth of vertebrates, the
shells of molluscs (snails and clams) and the hard chitinous parts of arthropods
(insects, spiders, etc.). These parts are rarely found intact but are fragmented to
some degree. Among the commonest insect fossils are bits of beetles (Coleoptera),
especially their upper wings (the elytra) and the shield covering most of their thorax
(the pronotum).
Plants are commonly represented by fossil leaves, seeds, twigs and other wood
fragments. Fossil charcoal gives evidence of past forest fires. As with animals, the
soft parts of plants usually decay. However, fossil leaves are found if only because a
single tree produces so many leaves, year after year, and that they can be carried some
limited distance. One therefore only needs a small fraction of these to be buried in a
way that maximises the chance of fossilisation for there to be some samples surviving
until today.
Microfossils are far more abundant. The most numerous are fossil spores and pol-
len, discussed in the previous section, but also there are diatoms (a class of eukaryotic
algae of the phylum Bacillariophyta), Foraminifera (which belong to another class of
eukaryotic algae), dinoflagellates, ostracods (a group of small bivalved crustaceans),
chironomids (a family of midge) and stoneworts (part of the Charophyta algae). These
are all important to micro-palaeontologists. Microfossils can be found terrestrially
and in the marine environment (see section 2.2).
As with pollen and spore analysis, there are many problems in using species as
indicators of past climates and past climate change. Not least are problems of dating.
Microfossils are capable of being washed from one place to lie in the sediment in
another. Then there are ecological problems. Species do not migrate and become