Lesson Overview

Lesson OverviewLesson Overview Invertebrate Evolution and DiversityInvertebrate Evolution and Diversity

Lesson OverviewLesson Overview26.1 Invertebrate Evolution 26.1 Invertebrate Evolution

and Diversityand 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.


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.


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.


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.


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.



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.



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.


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.


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.


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.


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.

They are the simplest animals to have body symmetry.


ArthropodsPhylum: 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.


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.


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.


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.


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.


EchinodermsPhylum: 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.

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