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A Brief History of Life

ammonites—Extinct mollusks with a coiled, flat, chambered shell

anthropoids—Often called "higher primates," large, strong, long-lived apes with big brains

Australopithecines—Small-brained but bipedal early hominids that lived in southern and eastern Africa between five and two million years ago and showed similarities to both apes and humans

blastoids—Extinct group of sedentary marine animals that were anchored to the sea floor by a stemlike column of of circular plates

brachiopods—Marine invertebrates with bivalve dorsal and ventral shells (like a clam) and a pair of tentacled, armlike structures on either side of the mouth

bryozoa—Small, aquatic animals that form mosslike colonies

conondonts—Soft-bodied marine animals with sharp calcium teeth used for grasping tiny prey or food particles

crinoids—Marine invertebrates, such as sea lilies and feather stars, with radiating arms and a stalk by which they are connected to the water's surface

crustaceans—Aquatic invertebrates, such as lobsters, crabs, shrimps, and barnacles, that have segmented bodies, jointed legs, and exoskeletons

cycads—Tropical trees resembling palm trees, with crowns of fronds and armorlike trunk patterns

echinoids—Radially symmetrical marine invertebrates that have spiny, calcite plates, such as urchins and sand dollars

eurypterids—Large, segmented aquatic invertebrates (closely related to scorpions) that existed from the Ordovician to the Permian

forams—Unicellular (one-celled) microorganisms with hard, perforated shells through which protrude numerous pseudopodia (leglike structures used for eating and locomotion)

hominids—Erect, bipedal primates whose only living descendant is Homo sapiens

gastropods—Mollusks, such as snails and slugs, that use a single foot to move

nautiloids—Mollusks with spiral shells

pelycosaurs—Reptiles with large, sail-like fins that may have helped to regulate body temperature (also called "sail-back reptiles")

placoderms—Extinct, armored, jawed fishes

protozoans—Single-celled microscopic organisms

scaphopods—Deep-water marine mollusks with tubular, curved shells open at both ends

synapsids—Extinct mammal-like reptiles with strong jaw muscles and large jaws

tetrapods—Any backboned animal with limbs rather than fins

trilobites—Extinct marine arthropods that had segmented, oval exoskeletons divided by grooves into head, thorax, and tail

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