1. 26.1 Invertebrate Evolution and Diversity
Lesson Overview
26.1 Invertebrate Evolution and Diversity

2. THINK ABOUT IT
Many modern multicellular phyla first appeared during a period called the “Cambrian Explosion,” between 530 and 515 million years ago.
How did so many kinds of animals evolve so quickly? What simpler forms could they have evolved from?

3. Origins of the Invertebrates
When did the first animals evolve?
Fossil evidence indicates that the first animals began evolving long before the
Cambrian Explosion.

4. Origins of the Invertebrates
For roughly 3 billion years after the first prokaryotic cells evolved, all
prokaryotes and eukaryotes were single-celled.
Data supports the hypothesis that animals evolved from ancestors they
shared with organisms called
choanoflagellates, single-celled
eukaryotes that sometimes grow
in colonies.
Choanoflagellates share several
characteristics with sponges,
the simplest multicellular animals.

5. Traces of Early Animals
Our oldest evidence of multicellular life comes from microscopic
fossils that are roughly 600 million years old.
The first animals were tiny and soft-bodied, so few fossilized bodies
exist.
Recent studies have uncovered incredibly well preserved fossils of
eggs and embryos that are 565-million-years-old.

6. Traces of Early Animals
Other fossils from this time period have been tentatively identified as parts of sponges and animals similar to jellyfish.
Paleontologists have also identified what are called “trace fossils,” tracks and burrows made by animals whose body parts weren’t fossilized.

7. The Ediacaran Fauna
Some important discoveries about invertebrate life before the Cambrian
Period come from fossils in the Ediacara Hills of Australia.
Strange fossils, which date from
roughly 565 to about 544 million
years ago, show body plans that
are different from those of anything
alive today.
Many of the organisms were flat
and lived on the bottom of
shallow seas.
They show little evidence of cell, tissue, or organ specialization, and no
organization into a front and back end.

8. The Cambrian Explosion
The Cambrian Period began about 542 million years ago.
Cambrian fossils show that over a period of 10–15 million years, animals evolved complex body plans, including specialized cells, tissues, and organs.
Many had body symmetry; segmentation, a front and back end; and appendages, structures such as legs or antennae protruding out the body.

9. The Cambrian Explosion
Two major Cambrian fossil sites are in Chengjiang, China,

10. The Cambrian Explosion
and in the Burgess Shale of Canada.

11. The Cambrian Explosion
A number of Cambrian fossils have been identified as ancient members of modern invertebrate phyla, such as the fossil of arthropod Marrella shown.

12. The Cambrian Explosion
Some early Cambrian fossils represent extinct groups so peculiar that no
one knows what to make of them.
Other Cambrian animals appear to be early chordates.
By the end of the Cambrian Period, all the basic body plans of modern
phyla had been established.
Later evolutionary changes, which produced the more familiar body
structures of modern animals, involved variations on these basic body
plans.

13. Modern Invertebrate Diversity
Today, invertebrates are the most abundant animals on Earth.
Invertebrates live in nearly every ecosystem, participate in nearly every
food web, and vastly outnumber so-called “higher animals,” such as
reptiles and mammals.

14. Cladogram of Invertebrates
What does the cladogram of invertebrates illustrate?
The cladogram of invertebrates presents current hypotheses about evolutionary relationships among major groups of modern invertebrates. It also indicates the sequence in which some important features evolved.

15. Cladogram of Invertebrates
Groups shown close together are more closely related than are groups shown farther apart. The sequence in which some important features evolved is also shown.

16. Sponges

17. Sponges Phylum: Porifera (“pore bearers”)
Sponges are the most ancient members of
the kingdom Animalia.
They are multicellular, heterotrophic, lack cell
walls, and contain a few specialized cells.

18. Cnidarians

19. Cnidarians
Phylum: Cnidaria—includes jellyfishes, sea fans, sea anemones, hydras, and corals.
Cnidarians are aquatic, soft-bodied, carnivorous, radially symmetrical animals with stinging tentacles arranged in circles around their mouths.
Some, like corals, have skeletons. They are the simplest animals to have body symmetry. Some live independently while others live in colonies.

20. Arthropods

21. Arthropods
Phylum: Arthropoda (arthron = “joint,” podos = “foot”)— includes spiders, centipedes, insects, and crustaceans.
Arthropods have bodies divided into segments, a tough external skeleton called an exoskeleton, cephalization, and jointed appendages, which are structures such as legs and antennae that extend from the body wall.
Arthropods appeared in the sea about 600 million years ago and have since colonized freshwater habitats, land, and air.

22. Nematodes (Roundworms)

23. Nematodes (Roundworms)
Phylum: Nematoda
Nematodes are unsegmented worms with pseudocoeloms, specialized
tissues and organ systems, and digestive tracts with two openings—a
mouth and an anus. Some are free-living and inhabit soil or various aquatic
habitats.Others are parasites that infect a wide range of plants and animals,
including humans.
Nematodes were once thought to
be closely related to flatworms,
annelids, and mollusks but have
been found to be more closely
related to the arthropods.

24. Flatworms

25. Flatworms Phylum: Platyhelminthes
Flatworms are soft, unsegmented, flattened worms that have tissues and internal
organ systems.
They are the simplest animals to
have three embryonic germ layers,
bilateral symmetry, and cephalization.
Flatworms do not have coeloms.
Most flatworms are no more than a few
millimeters thick.

26. Annelids

27. Annelids
Phylum: Annelida (annellus = “little ring”)—includes earthworms, some marine worms, and leeches
Annelids are worms with segmented bodies and a true coelom lined with tissue derived from mesoderm.
The name Annelidea is derived from
the Latin annellus, which means “little
ring.” This refers to the ringlike
appearance of the body segments
of annelids

28. Mollusks

29. Mollusks Phylum: Mollusca—includes snails,
slugs, clams, squids, and octopi.
Mollusks are soft-bodied animals that
have an internal or external shell.
They have true coeloms surrounded by mesoderm and complex organ systems.
Many mollusks have a free-swimming larva, or immature stage, called a
trochophore.

30. Echinoderms

31. Echinoderms
Phylum: Echinodermata (echino = “spiny,” dermis = “skin”)—includes sea stars, sea
urchins, and sand dollars
Echinoderms have spiny skin and an internal skeleton.
They also have a water vascular system—a network of water-filled tubes that
include suction-cuplike tube feet, which are used for walking and gripping prey.
Most exhibit five-part radial symmetry. The larva exhibits bilateral symmetry.

32. Echinoderms
The skin of an echinoderm is stretched over an internal skeleton of calcium carbonate plates.
Although radial symmetry is characteristic of simpler animals such as cnidarians, echinoderms are more closely related to humans and other chordates because they are deuterostomes.