11.9.5 Animal Diversity: The Cambrian Explosion and the Move to Land

Question 1

Animal Diversity: The Cambrian Explosion and the Move to Land

Answer 1

• Using anatomical and embryological evidence, we can group the animal kingdom into 35 phyla.
• It is believed that most of these phyla evolved during the Cambrian explosion.
• Key adaptations by terrestrial organisms include a means of preventing desiccation of the:
· body
· gametes
· embryo

Question 2

note

Answer 2

• The illustration shows the evolutionary relationships among some of the major extant animal phyla. The animal kingdom is grouped into 35 phyla, and it is believed that most of the phyla evolved during the Cambrian explosion.
• The Cambrian period marks the beginning of the Paleozoic era, which began about 570 million years ago and lasted 222 million years. The era includes five other periods: Ordovician, Silurian, Devonian, Carboniferous, and Permian.
• Fossils from the Burgess Shale (a fossil bed in British
  Columbia) of the Cambrian period reveal such a rich diversity of life that the period is called the Cambrian explosion. All of the modern animal phyla and some extinct groups were formed during this time.
• During the Cambrian, the ocean floor was covered with corals, sea lilies, sponges, snails, squidlike cephalopods, trilobites, at least one kind of chordate, and other marine animals. Members of the genus Hallucinogenia, odd-looking marine animals, are now extinct.
• Scientists still debate possible causes of the
  Cambrian explosion. Oxygen concentrations probably
  played a key role, increasing over a critical biological
  threshold (10% of present-day oxygen, or higher) in the late in the Precambrian era.
• Geologists believe that the continents were gradually flooded during the Cambrian period. Adaptations of animals pertained to survival in aquatic environments. The sea continued to cover the continents through the Silurian and receded somewhat over subsequent periods.
• Members of the phyla Chordata and Arthropoda were among the first animals to adapt to terrestrial conditions that began to emerge in the Silurian period.
• Survival on land versus water requires adaptations that
  prevent water loss from gametes (sex cells), the developing embryo, and the adult body. The chart on the left shows some key adaptations of amphibians and reptiles.
• The Carboniferous period is also called the age of
  amphibians, because they were the only vertebrates on land during this time. They thrived in swampy conditions, the prevalent terrestrial habitat during this period. Modern amphibians still have a close association with water. Their skin requires moisture for respiration. Fertilization is external and must take place in an aqueous medium. The zygote must develop in water.
• Reptiles evolved from an amphibian ancestor. The reptilian body plan is adapted for terrestrial conditions. Scales provide protection from desiccation. Fertilization is internal, providing the aquatic medium necessary and decreasing the animals’ dependence on water. Development of the embryo takes place in the amniotic egg, which is adapted to dry conditions.

Question 3

Almost all animal phyla alive today were formed during what time period?

Answer 3

• Cambrian

Question 4

True or false?
One theory for the rapid diversification of animal phyla during the Cambrian period is an increase in atmospheric oxygen concentrations.

Answer 4

• true

Question 5

What is the significance of the Burgess shale?

Answer 5

• It shows a great number of fossilized organisms from the Cambrian period.

Question 6

Which of the following is not an obstacle to a water-dwelling species moving to land?

Answer 6

• development of vertebrae

Question 7

Reptiles were able to live fully on land because of

Answer 7

• internal fertilization.
• tough skin.
• amniote eggs.
• all of the above

Question 8

The Cambrian explosion took place over the course of

Answer 8

• thousands of centuries.

Question 9

Which of the following was the chief problem facing organisms as they made the move to land?

Answer 9

• drying out

Question 10

Which of the following did not exist in Precambrian times?

Answer 10

• hagfish

Question 11

Which types of chordates were the first to move from life in the water to life on land?

Answer 11

• amphibians

Question 12

What allows reptiles’ bodies to exist on land without drying out?

Answer 12

• Scales