26.1 Invertebrate Evolution and Diversity Lesson Overview 26.1 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?

Origins of the Invertebrates When did the first animals evolve?

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

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 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.

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. Cambrian fossils show that over a period of 10–15 million years, animals evolved complex body plans, including specialized cells, tissues, and organs.

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.

The Cambrian Explosion Some early Cambrian fossils represent extinct groups so peculiar that no one knows what to make of them.

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.

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 What does the cladogram of invertebrates illustrate?

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.

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.

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.

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.

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.

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.