184 Evolution? The Fossils Say YES!
is yes! Microfossils are used routinely for biostratigraphic correlation by thousands
of specialists the world over. This would not be possible unless the sediment record
was good and reliable. We now know (within fairly precise limits) when hundreds
of species of mineralizing plankton arose and became extinct, through a history that
spans over a hundred million years.
—Paul Pearson, “The Glorious Fossil Record”Although the evolution of dinosaurs and humans is a far more glamorous subject, by far
the best fossil record is found in the microscopic fossils left behind by single-celled organ-
isms in the deep sea. These protistans occur by the trillions in many larger oceanic water
masses, and their shells literally carpet the shallow seafloor with hundreds to thousands of
individual specimens in a cubic centimeter of sediment (fig. 8.1). Their density can exceed a
million specimens per cubic meter of sediment and weigh up to 10 grams per cubic meter.
Most of the open ocean floor shallower than 3,000 m is completely covered by “calcareous
ooze” composed of the skeletons of microfossils made of calcium carbonate. The sand of
many tropical beaches is composed almost entirely of the skeletons of microfossils. In a typi-
cal sample of tropical marine sediment, there may be 60 to 70 species. In some groups, such
as the foraminifera, there are over 3,600 described genera and perhaps 60,000 species, mak-
ing them more diverse than any other group of marine animals or plants.
In addition to their great abundance and diversity, microfossils are ideal for evolution-
ary studies for several other reasons. Cores of the sediments covering the deep-sea bottom
have been taken by rotary drilling and by plunging a long tube into the sea bottom (“piston
coring”), and both retrieve an almost continuous record of marine sedimentation over that
part of the ocean floor. Some cores span many millions of years with no breaks or gaps what-
soever. These cores can be precisely dated by methods such as stable isotope analysis and
magnetic stratigraphy, as well as with the biostratigraphy of the microfossil groups them-
selves. Thus we can trace the history of many microfossil lineages through many millions
of years over a single spot in the world, something that is impossible with the much less
complete record of shallow marine invertebrates or land vertebrates. Finally, the biogeogra-
phy of microfossils is relatively simple. Most are confined to a few water masses where the
ocean waters are of a given temperature, and these species range over that entire water mass
(Prothero and Lazarus 1980). Thus a few cores from an area representing a single water mass
will sample all the populations in that water mass, and there will be no small “peripheral
isolate” populations that could be missed. As a result, Prothero and Lazarus (1980) showed
that if we have cores representing most of the world’s major water masses in a given time
interval, we can look at a lineage or group and see practically all there is to see about the
evolution of their skeletons. Prothero and Lazarus (1980) argued that microfossils are our
best “laboratory animal” or “fruit fly” to study evolution in the fossil record.
Naturally, there are a few drawbacks to using microfossils to study evolution. The big-
gest problem is that we still know relatively little about the biology of the living relatives.
Some are difficult to keep alive in the laboratory once they have been caught. Researchers
have studied the few species that can be cultured only over short intervals of time. Besides,
the most valuable information about their biology relates to how they live in the open ocean,
which is hard to simulate in a lab. In addition, microfossils have relatively simple skeletons,
without the many levels of anatomical detail that you find in many macroinvertebrates or
vertebrates. There is good evidence that some forms have evolved more than once through