Arthropods
Meaning “jointed foot,” get their name because they have a segmented, jointed external skeleton made primarily of chitin, which is often further stiffened with calcium carbonate. As arthropods grow, they must break apart and remove this old skeleton and form a new one by molting. Different arthropod groups have appendages that have been modified for different purposes, such as sensory functions, respiration, feeding, and locomotion. Arthropods include the most numerous and diverse group of animals today, the insects, as well as shrimp, spiders, millipedes, and barnacles. Also included are many extinct forms, like trilobites and eurypterids.

• Chelicerates - Chelicerates are named for the pair of feeding appendages in front of the mouth, called chelicerae. Members of this group lack antennae and have a body divided into two distinct portions: prosoma (head region) and opisthosoma (main body). Spiders, ticks, scorpions, and horseshoe crabs are familiar chelicerates. Less familiar chelicerates include the sea spiders (pycnogonids) and the extinct relatives of horseshoe crabs called sea scorpions (eurypterids). Eurypterids first appear in the Ordovician and were extinct by the Permian, while most of the chelicerates began to appear in the Devonian and are still around today.
• Crustaceans - Crustaceans include a diversity of marine, freshwater, and terrestrial animals. All crustaceans have a head with five pairs of appendages, two of which are antennae. But the number of body segments, appendages on the thorax and abdomen, and presence of a tail vary between groups. Many microscopic crustaceans, like krill and brine shrimp, are members of the marine plankton, an important food source for other animals in the sea. Shrimp, lobsters, crabs, crayfish, and barnacles are crustaceans, as are the pill bugs found on the forest floor or city sidewalk.
• Trilobites - Trilobites are perhaps the most famous and easily recognized fossil invertebrate group. The name comes from the length wise division of the body into three lobes—two side (pleural) lobes separated by one central (axial) lobe. All trilobites had antennae and legs with two branches, one used for locomotion and the other for respiration. Some had compound eyes, like a modern fly. Trilobites were strictly marine animals, but they were very diverse, living in both shallow and deep waters.
• Insects - Insects generally have a body divided into three segments: head, thorax, and abdomen. They belong to the group Hexapoda, meaning “six-footed,” because they have six legs on the thorax. Also included here are three groups of wingless hexapods, Collembola, Protrura, and Diplura, because they are close relatives of insects, and their relationships are still being studied. Some paleontologists consider Collembola (springtails), whose fossils have been found in Devonian rocks, to be the oldest known insects. Members of the other groups first appear in the Carboniferous.
• Other Arthropods - This group consists of the arthropods that are not members of the Chelicerates, Crustaceans, Trilobites, or Insects. Members of this group first appear in the Cambrian and are still present today. They include:
  • Velvet worms (Onychophora) and their extinct relatives from the Cambrian lack exoskeletons and are noted for their many thick, stubby legs and segmented heads.
  • The millipedes and centipedes (Myriapods, meaning “10,000 legs”) have bodies with many segments and many legs.
    Some Cambrian fossils show similarities with arthropods, but their relationships are uncertain.

Brachiopods
Brachiopods are marine animals that have a calcium carbonate or calcium phosphate shell made of two valves, which usually differ in shape and size. Brachiopods are closely related to the bryozoans. Both groups possess a unique feeding and respiratory organ called a lophophore, which looks a bit like a rubber band covered with tiny hairs. Some brachiopods can cement themselves directly to hard surfaces; most have a stalk, called a pedicle, which they use to attach to a surface or to anchor themselves in the sediment of the sea floor. Brachiopods are very common fossils in Paleozoic rocks from many marine environments.

Bryozoans
Bryozoans are also known as “moss animals” for their superficial resemblance to mosses. What we see as “a bryozoan” is really a colony made of many little units, each made up of the animal and its house. Bryozoans are closely related to the brachiopods. Both groups possess a unique feeding and respiratory organ called a lophophore, which looks a bit like a rubber band covered with tiny hairs. By moving the hairs, they generate a current that brings water and food particles to the animal. Most bryozoans live attached to a hard surface, such as mussels, rocks, wood, sea turtle shells, or large algae. Many Paleozoic bryozoans built strong colony skeletons of calcium carbonate and are common fossils.

Chordates and Hemichordates
The group “Chordata” includes all vertebrates, but because vertebrates are such a large group in their own right, we discuss them separately. We limit our discussion here to the chordates without a backbone, namely the tunicates and lancelets. Fossil chordates can be found in rocks as old as the Cambrian; however, because they have few hard parts, their fossils are rare throughout the Phanerozoic. The Hemichordates, meaning “half-chordate,” share many of the characteristics of chordates, but differ in some aspects of development and morphology. Graptolites are the best-known group of fossil hemichordates. They first appear in the Ordovician, and later go extinct in the Devonian.

Cnidarians
The name Cnidaria comes from the Greek word “cnidos,” for “stinging nettle.” All cnidarians have specialized cells that can inject poison into their prey or even hapless passers-by. With no hard parts, they are not preserved often, and their fossils may be difficult to recognize.

• Anthozoans - This group includes the sea anemones, sea pens, and true corals, which form large reefs today.
• Scyophozoans - These are the true jellyfish.
• Hydrozoans - These may look like true corals or true jellyfish, depending on the species and stage of their lifecycle.
• Cubozoans - This group, which includes the deadly box jelly, resemble true jellyfish but differ in some of their characteristics.

Echinoderms
Echinoderms, meaning “spiny-skinned,” include sand dollars, sea urchins, and sea stars, as well as less familiar creatures, such as sea cucumbers, crinoids, blastoids, and cystoids. This diverse group is entirely marine and is characterized by the presence of five-fold symmetry. Echinoderms have a specialized system of internal canals that circulates water through the body and services numerous, tiny appendages, called tube feet. They use their tube feet to move around or to capture food.

• Seastars - Two different groups of echinoderms are often lumped together as “sea stars:”
  • True sea stars (Asteroidea) have tube feet used for locomotion and capturing prey. The asteroids are the oldest members of this group, appearing in the Ordovician.
  • Brittle stars (Ophiuroidea) move by wriggling their slender, snaky arms. Brittle stars have a distinctive central disk-shaped body.
• Echinoids - Sand dollars, sea urchins, and heart urchins make up the group of echinoderms called echinoids. They have a rigid, round, oval, or heart-shaped skeleton, called a test, that is composed of tightly interlocking plates of calcium carbonate. Living echinoids are covered with spines, although the spines on sand dollars are short, giving the animal almost a furry appearance. Sea urchin spines are usually long and prominent, and in some species may contain toxins for defense. The spines usually fall off soon after death, thus fossil urchins commonly are found without spines attached.
• Crinoids - These animals are relatively rare in modern seas, but were prominent members of many Paleozoic marine communities. There are two forms, stalked and unstalked, both of which feed by catching floating particles with their tube feet and passing them down to their mouths.
  • The “sea lilies,” , or stalked crinoids, have flexible stalks made of numerous small disks of calcium carbonate, and a sac-like body with feathery arms that are used to trap food.
  • The “feather stars” lack a stalk and a distinct body. They swim around reefs by flexing their feathery arms.
• Other Echinoderms - This group includes some living forms (holothurians) and some extinct forms that are very distinct from the Seastars, Echinoids, and Crinoids. Holothurians, or sea cucumbers, lack the typical calcium carbonate skeleton of most echinoids; their skeletal elements have been reduced to microscopic plates called spicules. The extinct forms, such as edrioasteroids, cystoids, blastoids, and helicoplacoids, appear to have been early evolutionary “experiments” on the basic echinoderm body plan. First and last occurrences of members of this group vary. For example, edrioastroids first appeared in the Cambrian and went extinct in the Carboniferous; holothurians first appeared in the Silurian and are still present today.

Ediacaran Organisms
The Ediacaran organisms comprise a group of Late Precambrian fossils named for the place where they were first found and described, the Ediacara Hills of Australia. Some of these fossils show similarities to invertebrates like cnidarians, echinoderms, worms, or arthropods. Others are unlike any fossil invertebrate, and their relationships to other animals are uncertain. These strange fossils may represent animals that were evolving on their own, apart from the metazoan groups we know today. Ediacaran fossils have also been found in other places around the world, including Russia, Scandinavia, England, Ireland, Newfoundland, United States, and Mexico.

Molluscs
One of the largest and most diverse groups, the Mollusca includes the easily recognizable clams, slugs, snails, and squid, as well as less familiar animals such as chitons, tusk shells, and monoplacophorans. The extinct ammonites and belemnites are also molluscs. All molluscs possess a mantle, a sheath of tissue formed from the body wall. Most have a shell made of calcium carbonate that is secreted by the mantle. Molluscs can be found on land, as well as in freshwater and marine environments, from tide pools to the deep ocean trenches.

• Bivalves - Bivalves are molluscs with a shell made of two interlocking pieces (valves) made of calcium carbonate. They can be found in many different freshwater and marine environments, such as lakes, tide pools, reefs, and hydrothermal vents. Some bivalves attach themselves to hard surfaces (oysters, mussels), a few species can swim (scallops), but most bury or anchor themselves into soft sediment. Partly because they have a hard shell, and partly because they are so numerous, the fossil record of bivalves is fairly good and extends back to the Cambrian.
• Gastropods - Gastropods, meaning “stomach-footed,” differ from all other molluscs by a developmental process called torsion, in which all the internal organs and nervous system are twisted at least 90° from their original positions. Familiar gastropods include limpets and snails and slugs, which lack a shell entirely. Less familiar gastropods include the sea slugs (nudibranchs), which do not have shells, and the sea butterflies (pteropods) and sea hares (heteropods), which may or may not have shells. Gastropods inhabit a wide range of marine, freshwater, and terrestrial environments. Not all gastropods have a great fossil record, but the record of the whole group goes back to the Cambrian.
• Cephalopods - Cephalopods, meaning “head foot,” have a foot that has been modified into flexible arms and a tube (siphon), as well as highly developed eyes and nervous systems. They can move backwards rapidly by expelling water through the siphon. Some members of this group, such as the living Nautilus and the extinct ammonites, have external shells. These shells are divided into chambers, and the cephalopod only lives in the last, largest chamber. Squid, cuttlefish, and extinct forms like belemnites, have internal shells, many of which are also chambered. Octopuses have no shells. Members of this group first appear in the Devonian, and some members are still around today.
• Other Molluscs - There are a number of less familiar groups of molluscs that have a fossil record, such as:
  • Tusk shells (Scaphopoda) live buried in sediment inside their tusk-shaped, conical shells. They first appear in the Cambrian.
  • Chitons (Polyplacophora) have a “shell” made of seven to eight plates and can be seen in tide pools attached to rocks. They first appear in the Cambrian.
  • Limpet-like molluscs (Monoplacophora) have a single cap-shaped shell. They first appear in the Ordovician.

Sponges
Sponges form the group Porifera (Latin for “pore-bearing”) and were named for their unique feeding system, which consists of numerous tiny holes that draw in water and food and a few larger holes that expel waste. Generally amorphous animals, sponges are made up of several different kinds of cells that are connected but function more or less independently of one another. For support and protection, they have tiny spines, called spicules, embedded in between their cells. Although the relationships of sponges remain uncertain, they have traditionally been grouped by the composition and type of their spicules. Calcarea have spicules of calcium carbonate, Demospongia of spongin (a protein) and silica, and Hexactinellida have six-rayed spicules of silica. A few sponges live in freshwater, but the majority live in marine environments, from shallow reefs to deep ocean trenches. The fossil record of sponges is patchy, but it extends into the Late Precambrian.

Other Invertebrates
This group contains a wide variety of animals that have a long, but patchy, fossil record, including nematodes, flatworms, annelids, ctenophores, rotifers, and scolecodonts, among others. Also included here are organisms such as stromatoporoids, archaeocyaths, and receptaculitids that cannot be easily placed in any other invertebrate group. The archaeocyaths first appeared in the Cambrian and were extinct by the end of the period. The remaining taxa appeared later in the Phanerozoic, and some are still around today.

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