Evolution by Donald R. Prothero

Author:Donald R. Prothero
Language: eng
Format: epub
Tags: SCI027000, Science/Life Sciences/Evolution, SCI054000, Science/Paleontology
Publisher: Columbia University Press
Published: 2017-08-22T04:00:00+00:00

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)

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 earliest 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 Paleozoic age. If you want to know the age of any marine limestone from the Pennsylvanian or Permian, ask a fusulinid expert to look at it, and you will get the most precise estimate possible.

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