Vocabulary Study 21

The geologic ................ is marked by significant geologic and biological events,

including the origin of Earth about 4.6 billion years ago.

• mud slide

• timescale

• clue

• locale

During a geologically brief 100-million-year period, all modern animal groups evolved. This rapid origin and ................ of animals is often referred to as “the Cambrian explosion.”

• fossilization

• decomposition

• diversification

• complication

..................... of soft-bodied animals is less likely than ....................... of hardbodied animals, but it does occur.

• decomposition

• fossilization

• emergence

• ancestry

The origin of ................... forms of life seems a relatively simple step compared to the origin of life itself.

• multicellular

• marine

• niche

• brief

Conditions that promote fossilization of soft-bodied animals include very rapid covering by sediments that create an environment that discourages ............................

• emergence

• discouragement

• assignment

• decomposition

This 700-million-year-old formation gives few ........... to the origins of modern animals, however, because paleontologists believe it represents an evolutionary experiment that failed.

• ancestors

• descendants

• mites

• clues

Shortly after the Cambrian explosion, ................ rapidly buried thousands of marine animals under conditions that favored fossilization.

• niches

• clues

• mud slides

• locales

These animals include a large swimming predator called Anomalocaris and a soft-bodied animal called Wiwaxia, which ate .................... or algae.

• detritus

• emergence

• descendants

• ancestors

A well-known Burgess Shale animal called Sidneyia is a representative of a previously unknown group of arthropods (a category of animals that includes insects, spiders, ..................., and crabs).

• mites

• clues

• niches

• locales

The Cambrian explosion involved ....... evolutionary diversification, followed by the extinction of many unique animals.

• rapid

• marine

• complicated

• multicellular