Spineless Wonders of Evolution 187large. That idea is confirmed by the fact that fusulinids lived in enormous numbers on shal-
low sea bottom (probably shallow enough for light penetration) during the late Paleozoic
(Mississippian to Permian periods, 355–255 million years ago). In many places, huge vol-
umes of limestone are made of nothing but fusulinids, comprising trillions of individuals
(fig. 8.3). Then, at the peak of their success, they were wiped out by the greatest mass extinc-
tion event in earth history, the Permian catastrophe, which extinguished 95 percent of the
marine species on the planet. Whatever the cause of this great mass extinction, the fusulinids
were one of the most conspicuous victims.
The fusulinid shell is shaped like a spindle or a grain of rice (figs. 8.3 and 8.4). As
the shell grew, more and more shell layers were added, spiraling around the long axis of
the spindle, so that when you cut it across the middle, you typically see a spiral pattern.
In cross section, you can see that the spiral layers are supported by a dense network of
smaller walls and chambers, with an intricate, complex structure. These complex wall
structures make each genus and species of fusulinid distinct and easy to recognize for
the specialist. Fusulinids evolved rapidly through the late Paleozoic (fig. 8.4) from simple
forms with only a few chambers like Millerella from the Late Mississippian and earli-
est Pennsylvanian, to a variety of different lineages that become larger with more and
more complex wall structure and interesting variations on the spindle-like symmetry. The
enormous variations within this basic body form are apparent even to the nonspecialist
and provide a dramatic example of evolution within a single lineage. To the specialist, the
different species are so distinctive, and their fossils are so widespread and abundant in
late Paleozoic limestones, that they are the principal method of dating rocks of late Paleo-
zoic age. If you want to know the age of any marine limestone from the Pennsylvanian
FIGURE 8.3. The bottom-dwelling (benthic) foraminifera known as fusulinids were about the size and shape
of a grain of rice and extraordinarily abundant in late Paleozoic limestones, where they may number in the
trillions and make up entire rock units. (Photography by W. Hamilton, courtesy U.S. Geological Survey)