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Animal Evolution: Test Your Understanding
Teacher's Guide
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When did the earliest animals—the Ediacaran fauna—first appear in Earth's history?
Ediacaran fossils first appear in marine sediments from nearly 650 million years ago.
What is the main reason why some currently living animal phyla do not have complete fossil records?
The structures of most soft-bodied animals do not have enough minerals to leave fossil evidence.
Soft tissue does not calcify, making it difficult to preserve over millions of years.
What is the importance of a true body cavity (a coelom) in animal evolution?
A true body cavity provides a sequestered space for reproductive and other organs. Scientists theorized
that flatworms were among the first animals to have a distinct gut—internal tissues layers that led to
a body cavity for organs. Scientists group both extinct and living animals on the basis of whether they
have a body cavity.
Critical Thinking: Which features of a newly discovered invertebrate should a scientist examine
to determine whether it is closely related to vertebrates?
The invertebrate chordates most closely related to vertebrates share certain features of early develop-
ment, such as a notochord, a hollow dorsal nerve cord, and pharyngeal slits. The scientist should look
for these features.
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