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The crustaceans appeared at the very beginning of the Cambrian and
soon became the dominant invertebrates. They occupied shallow waters near
the shores of ancient seas, which flooded inland to provide extensive conti-
nental margins. The crustaceans are primarily aquatic arthropods and include
shrimps, lobsters, barnacles, and crabs (Fig. 53). The ostracods, or mussel
shrimps, are small crustaceans found in both marine and freshwater environ-
ments. Their fossils are useful for correlating rocks from the early Paleozoic
onward, which make them particularly important to geologists.
The conodonts are fossilized, tiny, jawlike appendages that commonly
occur in marine rocks ranging from the Cambrian to the Triassic and are
important for dating Paleozoic rocks.They are among the most baffling of all
fossils and have puzzled paleontologists since the 1800s. Paleontologists began
finding these isolated toothlike objects in rocks from the late Cambrian
Figure 51Molds and
shells of mollusks on
highly fossiliferous
sandstone of the Glenns
Ferry Formation on
Deadman Creek, Elmore
County, Idaho.
(Photo by H. E. Malde,
courtesy USGS)77CAMBRIAN INVERTEBRATES