1. Apply Concepts Design a “new” invertebrate. Create and illustration on which you point out its body plan features. Then show its place on the cladogram of invertebrates and write a caption explaining how its features helped you decide where it belongs. CH 26 ANIMAL EVOLUTION AND DIVERSITY 26.1 Invertebrate Evolution and Diversity Fossil evidence indicates that the first animals began evolving long before the Cambrian Explosion. Origins of the Invertebrates 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 Similar to sponges. Traces of Early Animals Oldest evidence of multicellular life comes from microscopic fossils that are roughly 600 million years old First animals were tiny and soft-bodied, so few fossilized bodies exist. Ediacaran Fauna Before the Cambrian Period From the Ediacara Hills of Australia Fossils 565 to about 544 million years old Flat and lived on the bottom of shallow seas Little evidence of cell, tissue, or organ specialization, and no organization into a front and back end. Cambrian Explosion Cambrian Period began about 542 million years ago Burgess Shale of Canada Fossils show that over a period of 10–15 million years, animals evolved complex body plans, including specialized cells, tissues, and organs. 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. Invertebrates are the most abundant animals on Earth. Sponges Phylum: Porifera Most ancient members of the kingdom Animalia Multicellular, heterotrophic, lack cell walls, and contain a few specialized cells. Cnidarians Phylum: Cnidaria Includes jellyfishes, sea fans, sea anemones, hydras, and corals Aquatic, soft-bodied, carnivorous, radially symmetrical animals with stinging tentacles arranged in circles around their mouths. Arthropods Phylum: Arthropoda Includes spiders, centipedes, insects, and crustaceans Bodies divided into segments, a tough external skeleton called an exoskeleton, cephalization, and jointed appendages (legs and antennae) Appeared in the sea about 600 million years ago. Nematodes (Roundworms) Phylum: Nematoda Unsegmented worm with pseudocoeloms, specialized tissues and organ systems, and digestive tracts with two openings More closely related to the arthropods than flatworms. Flatworms Phylum: Platyhelminthes Soft, unsegmented, flattened worms that have tissues and internal organ systems Simplest animals to have three embryonic germ layers, bilateral symmetry, and cephalization No coelms. Annelids Phylum: Annelida Includes earthworms, some marine worms, and leeches Worms with segmented bodies and a true coelom. Mollusks Phylum: Mollusca Includes snails, slugs, clams, squids, and octopi Soft-bodied animals that have an internal or external shell Have true coeloms and complex organ systems May have a trochophore Immature free-swimming larva stage. Echinoderms Phylum: Echinodermata Includes sea stars, sea urchins, and sand dollars Have spiny skin and an internal skeleton Have a water vascular system that include suctioncuplike tube feet Deuterostomes.