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26.1 Invertebrate Evolution and Diversity
Lesson Objectives
Explain what fossil evidence indicates about the timing of the evolution of the first
animals.
Interpret the cladogram of invertebrates.
Lesson Summary
Origins of the Invertebrates It is not known when the first multicellular animals
evolved from single-celled eukaryotes.
Animals probably evolved from ancestors they shared with choanoflagellates. Fossil
evidence indicates that animals began evolving long before the Cambrian Explosion,
which occurred between 530 and 515 million years ago.
Fossils from the Ediacara Hills of Australia date from roughly 565 to 544 million years
ago. Their body plans are different from those of anything alive today. Some seem to be
related to invertebrates such as jellyfishes and worms.
Cambrian fossils (dating back about 542 million years ago) show how animals evolved
complex body plans over a span of 10–15 million years. Many had body symmetry, a
front and back end, specialized cells, and appendages, structures such as legs or antennae
protruding from the body. Some had hard body parts that became fossilized.
By the end of the Cambrian, the basic body plans of the modern phyla were established.
Today, invertebrates are the most abundant animals on Earth.
Cladogram of Invertebrates The cladogram of invertebrates presents current
hypotheses about evolutionary relationships among major groups of modern invertebrates.
The major invertebrate phyla are the sponges, cnidarians, arthropods, nematodes
(roundworms), flatworms, annelids, mollusks, and echinoderms.
• Sponges have pores in their bodies.
• Cnidarians are radially symmetrical animals with stinging tentacles.
• Arthropods have segmented bodies, a hard external skeleton, jointed appendages, and
cephalization.
• Nematodes, or roundworms, are nonsegmented worms with pseudocoeloms. Their
digestive tracts have two openings.
• Platyhelminthes, or flatworms, are the simplest animals to have three germ layers,
bilateral symmetry, and cephalization.
• Annelids are worms with segmented bodies and a true coelom.
Larvae are the immature stages of development in some animals, such as mollusks. Many
mollusks haave a free-swimming larval stage called a trochophore. The trochophore is
also characteristic of many annelids, indicating that annelids and mollusks are closely
related.
• Mollusks are soft-bodied animals that usually have a shell. They also have a true
coelom and complex organ systems.
• Echinoderms have spiny skin and exhibit radial symmetry.
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Origins of the Invertebrates
1. How much time passed between the appearance of the first prokaryotic cells and the
emergence of multicellular organisms?
2. What are choanoflagellates? What is their significance in animal evolution?
3. How old is our oldest evidence of multicellular life?
For Questions 4–9, write the letter of the correct answer on the line at the left.
4. The first animals were tiny and soft-bodied, so
A. no fossilized bodies exist.
B. few fossilized bodies exist.
C. fossilized bodies are plentiful.
D. the only fossils that exist are “trace fossils.”
5. Fossil evidence indicates that the first animals began evolving
A. during the Cambrian Period.
C. after the Cambrian Explosion.
B. before the Cambrian Explosion.
D. after the Cambrian Period.
6. Why are the fossils of the Ediacara Hills of Australia important?
A. Their body plans are different from those of anything alive today.
B. Some had cells, tissues, and specialized organs.
C. Some were differentiated into a front and back end.
D. Some were autotrophic.
7. Over a period of 10–15 million years in the Cambrian Period, animals evolved
A. into eukaryotic, photosynthetic forms.
B. the ability to survive on the bottom of shallow seas.
C. complex body plans, including cells, tissues, and organs.
D. into modern, vertebrate forms.
8. Structures such as legs or antennae that protrude from the body are
A. trace fossils.
B. appendages.
C. shells, skeletons, and other hard body parts.
D. evidence of an extinct phylum.
9. Which animals are the most abundant on Earth?
A. arthropods
C. sponges
B. mollusks
D. invertebrates
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Name
Class
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Cladogram of Invertebrates
10. Write “yes” or “no” to indicate how certain features distinguish each phylum of
multicellular invertebrates. The first row is completed as an example.
Sponges
Tissues
Radial
Bilateral
symmetry
Protostome
development
Deuterostome
development
no
no
no
no
no
Cnidarians
Arthropods
Nematodes
(Roundworms)
Flatworms
Annelids
Mollusks
Echinoderms
Apply the Big idea
11. Describe three evolutionary trends you see in invertebrates.
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