T

his chapter examines the invertebrate life-forms and landforms of the
lower Paleozoic era. The Cambrian period from 545 to 500 million
years ago was named for a mountain range in central Wales, Great Britain
that contained sediments with the earliest known fossils. Nineteenth-century
geologists were often puzzled why ancient rocks were practically devoid of fos-
sils until the Cambrian, when life suddenly sprang up in great abundance the
world over.The base of the Cambrian (Fig. 44) was therefore thought to be the
beginning of life, and all time before then was simply called Precambrian.
The period was generally quiet in terms of geologic processes, with lit-
tle mountain building, volcanic activity, glaciation, and extremes in climate.
The breakup of the late Precambrian supercontinent Rodinia and the flood-
ing of continents with inland seas created abundant, warm, shallow-water
habitats, prompting an explosion of new species. For more than 80 percent of
its history, life remained almost exclusively unicellular.Then less than 600 mil-
lion years ago, multicellular organisms abruptly came onto the scene. Never
before or since had so many novel and unusual organisms existed; surprisingly,
none have any counterpart in today’s living world.

4

CAMBRIAN

INVERTEBRATES

THE AGE OF SHELLY FAUNAS