Spineless Wonders of Evolution 185convergent evolution (Cifelli 1969). In other cases, it is not clear that the skeletal shape is that
well constrained during the life of the organism. Finally, what we do know of the biology
of these organisms suggests that many of them are asexual through at least part of their life
histories and reproduce by cloning, especially when they are trying to multiply quickly to
exploit an abundant food resource. Others may hybridize across lineages (Goll 1976), ending
the reproductive isolation of species that characterizes multicellular animals. Thus we cannot
always be sure that the rules about speciation and evolution developed for sexually reproduc-
ing multicellular animals and plants (such as Mayr’s allopatric speciation model discussed
earlier) can be applied to partially asexual or hybridizing microfossils.
But every research problem has its strengths and limitations, and the strengths of micro-
paleontology are so enormous that it has proven one of the most fruitful areas of research in
all the geologic sciences. Hundreds of micropaleontologists work for oil companies, helping
to precisely date and correlate the formations that they drill through to find oil or help-
ing to determine the depth of the water in which ancient oceanic sediment was deposited.
Other micropaleontologists work mostly as marine geologists and paleoclimatologists,
using microfossils to determine how oceanic currents and climate have changed through
time. A small number of micropaleontologists study the biology and shell chemistry of these
organisms, and even a smaller number are interested in using microfossils as exemplars of
evolution. Despite this trend, however, there are hundreds of well-documented examples of
evolution in the microfossil record, of which we will have space to mention only a few.
Although there are many types of microfossils that could be studied (see Prothero 2013a:
chap. 12), the most important ones include just a few groups. Two are animallike protistans
related to the amoeba, with its oozing, flowing protoplasm—but unlike an amoeba, they have
internal mineralized shells. The most diverse and most widely studied are the Foraminifera
(fig. 8.2A) or “forams” for short, which secrete skeletons of calcium carbonate (the mineral
calcite). Most forams live on or in the sea bottom (benthic), but one family, the Globigerinidae,
are tiny and buoyant and make up a major part of the marine plankton. The second group
of amoeba-like plankton is the Radiolaria (or “rads” in the trade), which secrete skeletons
of opaline silica instead of calcium carbonate. These delicate porous glassy skeletons have
been compared to miniature Christmas ornaments in their beauty and symmetry (fig. 8.2B).
The other important groups of microfossils are actually planktonic plants, members
of the division Chrysophyta, or the golden-brown algae. They include the diatoms, which
secrete their skeletons out of silica and are found in marine and fresh waters all over the
world (fig. 8.2C), and the coccolithophorids, which secrete hundreds of miniscule (a few
microns in diameter) button-shaped plates over their spherical cells (fig. 8.2D). These phyto-
plankton (planktonic algae) are the base of the entire food chain in the world’s oceans, and all
other organisms (from the foraminifera and radiolaria to megascopic predators like crusta-
ceans to fish all the way up to whales) feed directly or indirectly on them. In addition, phy-
toplankton are the single largest producer of the oxygen we breathe (much more important
oxygen producers than land plants), and in many places in the ocean they are so abundant
that they pave the sea bottom with trillions of their shells. In fact, the rock known as chalk
is actually made of millions of skeletons of coccolithophorids and a few foraminifera. Phy-
toplankton are so important to life on this planet that any major crisis in their evolution has
caused mass extinctions all the way up the food chain. To a great extent, none of us would
be here without the oxygen that phytoplankton release, and the food that they provide for
the life of the sea.