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Lesson Overview
Invertebrate Evolution and Diversity
Lesson Overview
26.1 Invertebrate Evolution
and Diversity
Lesson Overview
Invertebrate Evolution and Diversity
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?
Lesson Overview
Invertebrate Evolution and Diversity
Origins of the Invertebrates
When did the first animals evolve?
Lesson Overview
Invertebrate Evolution and Diversity
Origins of the Invertebrates
When did the first animals evolve?
Fossil evidence indicates that the first animals began evolving long before
the Cambrian Explosion.
Lesson Overview
Invertebrate Evolution and Diversity
Origins of the Invertebrates
For roughly 3 billion years after the first prokaryotic cells evolved, all
prokaryotes and eukaryotes were single-celled.
1. Animals evolved from ancestors they shared with organisms
called choanoflagellates, which are single-celled eukaryotes that
sometimes grow in colonies.
Choanoflagellates share several characteristics with sponges, the
simplest multicellular animals.
Lesson Overview
Invertebrate Evolution and Diversity
Traces of Early Animals
2. Our oldest evidence of multicellular life comes from
microscopic fossils that are roughly 600 million years old. 1st
animals were tiny and soft-bodied, so few fossilized exist.
Recent studies have uncovered incredibly well preserved fossils
of eggs and embryos that are 565-million-years-old.
Lesson Overview
Invertebrate Evolution and Diversity
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.
Lesson Overview
Invertebrate Evolution and Diversity
The Ediacaran Fauna
Some important 3. 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.
Lesson Overview
Invertebrate Evolution and Diversity
The Cambrian Explosion
The Cambrian Period began about 542 million years ago.
Two major Cambrian fossil sites are in Chengjiang, China, and in the
Burgess Shale of Canada.
4. Cambrian fossils show that over of 10–15 million years,
animals evolved complex body plans, specialized cells, tissues,
and organs.
5. The Cambrain explosion is thought to have begain after the
development of 3 germ layers.
Lesson Overview
Invertebrate Evolution and Diversity
The Cambrian Explosion
6. A number of Cambrian fossils have been identified as ancient
members of modern invertebrate phyla, such as the fossil of
arthropod Marrella shown.
Lesson Overview
Invertebrate Evolution and Diversity
The Cambrian Explosion
Some early Cambrian fossils represent extinct groups so peculiar that
no one knows what to make of them.
Lesson Overview
Invertebrate Evolution and Diversity
The Cambrian Explosion
By the 7. end of the Cambrian Period, all the basic body plans of
modern phyla had been established.
8. Later evolutionary changes, which produced the more familiar
body structures of modern animals, involved variations on these
basic body plans.
Lesson Overview
Invertebrate Evolution and Diversity
Modern Invertebrate Diversity
9. 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.
Lesson Overview
Invertebrate Evolution and Diversity
Cladogram of Invertebrates
What does the cladogram of invertebrates illustrate?
Lesson Overview
Invertebrate Evolution and Diversity
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.
Lesson Overview
Invertebrate Evolution and Diversity
Cladogram of Invertebrates
This cladogram of invertebrates shows current hypotheses of evolutionary
relationships among modern 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.
Lesson Overview
Invertebrate Evolution and Diversity
Sponges
10. 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.
Lesson Overview
Invertebrate Evolution and Diversity
Cnidarians
11. 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.
They are the simplest animals to have body symmetry.
Lesson Overview
Invertebrate Evolution and Diversity
Arthropods
12. Phylum: Arthropoda (arthron = “joint,” podos = “foot”)—includes
spiders, centipedes, insects, and crustaceans
Arthropods have bodies divided into segments, a tough
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.
Lesson Overview
Invertebrate Evolution and Diversity
Nematodes (Roundworms)
13. Phylum: Nematoda (roundworms)
Nematodes are unsegmented worms with pseudocoeloms,
specialized tissues and organ systems, and digestive tracts
with a mouth and an anus.
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, not worms.
Lesson Overview
Invertebrate Evolution and Diversity
Lesson Overview
Invertebrate Evolution and Diversity
Flatworms
14. Phylum: Platyhelminthes (flatworms)
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.
Lesson Overview
Invertebrate Evolution and Diversity
Lesson Overview
Invertebrate Evolution and Diversity
Annelids
15. 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.
Lesson Overview
Invertebrate Evolution and Diversity
Mollusks
16. 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, called a
trochophore, shows they are closely related to
annelids
Lesson Overview
Invertebrate Evolution and Diversity
Echinoderms
17. 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 waterfilled tubes that include suction-cuplike tube feet, which are used
for walking and gripping prey.
Most exhibit five-part radial symmetry and are more closely
related to chordates b/c they are deuterostomes.