Page 218 - Introduction to Paleobiology and The Fossil Record

PROTISTS 205

The world of microbes is more bizarre than can have no idea of the exquisite beauty of
the most contrived science fi ction novel. The microfossils, their tiny shapes showing infi nite
Earth is host to creatures that ingest iron and detail in their sculpture, spines and plate
uranium, thrive in environments akin to patterns. And they are not only beautiful,
boiling sulfuric acid or even live within solid but useful too! Micropaleontology has thus
rock itself (Box 9.1). These amazing organ- attracted the attentions of botanists, zoolo-
isms have a huge variety of shapes, belong to gists, biochemists and microbiologists together
a multitude of groups living in many different with, of course, paleontologists and geolo-
environments while pursuing a wide range of gists. The disparate taxonomic groups included
lifestyles with often apparently alien metabo- as microfossils are, nonetheless, united by
lisms. Microbes such as bacteria and viruses their method of study – all require the use of
are by far the most abundant life forms on the an optical microscope, although more recently
planet, a situation undoubtedly true of the both scanning and transmission electron
geological past. Microfossils are the micro- microscopes have taken microfossil studies to
scopic remains, commonly less than a milli- new, amazing levels. The majority of micro-
meter in size, of either microorganisms or fossils are indeed small and perfectly formed;
the disarticulated or reproductive parts of but they display often the most complex and
larger organisms. They thus include not only intricate of organic morphologies.
microbes themselves but also the microscopic Microfossils thus include material derived
parts of animals and plants. from most of the major groups of life, Bacte-
In his famous book Small is Beautiful, ria, Protozoa, Chromista, Fungi, Plants and
Schumacher argued for small-scale economics Animals, although Fungi are rarely found as
in the world. Among paleontologists, micro- fossils. The broad classifi cation adopted by
paleontologists are obsessed with microscopic most textbooks is both conventional and
fossils. Until you have screwed up your eyes operational: microfossils are usually divided
and peered down a binocular microscope, you into the prokaryotes (mainly bacteria), pro-

Box 9.1 Microbes in extreme environments: the extremophiles

We are aware that microbes are everywhere, but are they as widespread as we believe? Yes, and
probably more so. Scientists have been investigating a range of microbes, the extremophiles (“lovers

of extremes”), that appear to be adapted, with specific enzymes, to some of the most extreme envi-
ronments on Earth. Thus acidophiles (acid environments), alkaliphiles (alkaline environments),
barophiles (high pressure), halophiles (saline environments), mesophiles (moderate temperatures),

thermophiles (high temperatures), psychrofiles (cool temperatures) and xerophiles (arid environ-
ments) have now been identified. Extremophiles are spread across both the prokaryotes and eukary-

otes, although most belong to the Archaea and Bacteria and some scientists have argued they should
be included in a separate domain on the basis of their unique metabolic processes. Thus if modern
microbes can function in both frozen and geothermal habitats, both acid and alkaline ponds and
even deep within the crust, the extreme environments of the Early Precambrian and perhaps even
space were probably not a great challenge to evolving life of this type. Moreover such groups of
organisms could clearly survive the extreme environments of great extinction events. But it remains
a challenge to identify such groups in the fossil record. One group of ingenious algae, the acritarchs
(see p. 216), made it through one of the most extreme series of ice ages our planet has experienced.
The “snowball Earth” hypothesis (see p. 112) suggests that the planet’s oceans froze over during the
Late Proterozoic, with life coming to a virtual standstill. Acritarch diversity was maintained through
the crises (Corsetti et al. 2006). Have we identifi ed a group of extremophiles, or was the climate not
so harsh as suggested by the snowball Earth hypothesis?