Lesson Overview
Invertebrate Evolution and Diversity
Lesson Overview
26.1 Invertebrate Evolution
and Diversity
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.
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.
Lesson Overview
Invertebrate Evolution and Diversity
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.
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
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 (about 542 million years ago) fossils
show that over a period of 10–15 million years, animals
evolved complex body plans, including specialized cells,
tissues, and organs.
Lesson Overview
Invertebrate Evolution and Diversity
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.
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 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.
Lesson Overview
Invertebrate Evolution and Diversity
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.
Lesson Overview
Invertebrate Evolution and Diversity
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.
Lesson Overview
Invertebrate Evolution and Diversity
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.
Lesson Overview
Invertebrate Evolution and Diversity
Cnidarians
Phylum: Cnidaria—includes
jellyfishes, sea fans, sea
anemones, hydras, and corals
Cnidarians are aquatic, softbodied, carnivorous, radially
symmetrical animals with
stinging tentacles arranged in
circles around their mouths.
They are the simplest animals
to have body symmetry.
Lesson Overview
Arthropods
Invertebrate Evolution and Diversity
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.
Lesson Overview
Invertebrate Evolution and Diversity
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.
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.
Lesson Overview
Invertebrate Evolution and Diversity
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.
Lesson Overview
Invertebrate Evolution and Diversity
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.
Lesson Overview
Mollusks
Invertebrate Evolution and Diversity
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 freeswimming larva, or immature stage,
called a trochophore.
Lesson Overview
Invertebrate Evolution and Diversity
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 and are deuterostomes.