major groups of animals that commonly occur as fossils

chordates

All These have a *spinal chord, "back bone", gills slits, and a bony or cartilaginous tail*

three major groups mollusks & examples of these

In geology, we commonly divide MOLLUSKS into three major groups: *Bivalves, Gastropods, and Cephalopods* Bivalves (*two shells*: *clams, scallops, oysters* Gastropods (*stomach‐foot*): *snails and slugs* Cephalopods (*head‐foot*): *octopus and squid*

Bivalves (mollusk)

The *muscular foot in these is shaped like a blade* and is *used for burrowing* into sediment It can swell into an *anchor for retraction*

cnidarians

The group *includes sea anemones, jellyfish, and all types of corals* These were the *first to have true tissues* All these have a *sac‐like body that is filled with a jelly‐like fluid consisting mostly of water* HAS NO ORGANS These have *stinging cells* on their tentacles The cell releases a small barbed whip loaded with toxins that penetrate the prey *Living portions of coral reefs are polyps* Polyps resemble small sea anemones that inhabit the cavities within the coral skeleton Corals tend to be better preserved as fossils Corals are the *only of this group of animal to have a skeleton made of calcite (seashell material)*

Sponges

These are the *most archaic form of animals* They consist of *simple cells held together by a common animal protein called collagen* In addition to collagen fibers, these are held together by *spiny pieces called spicules* Spicules add rigidity to the this animal's structure These are *hollow with a central cavity* This cavity is lined with cells that are capable of generating a current and trapping food These *feed by filtering seawater* Seawater is sucked in through the sides of the this group of animal and pumped out the top like a chimney

Cephalopods (mollusk)

These have a *muscular foot that has been modified into grasping tentacles* Their foot extends from their head (head‐foot) Unlike other mollusks, the squid and octopus have an internal shell/skeleton of sorts The *skeleton is made of the carbohydrate chitin* Some ancient these had a large spiral shell like the modern shelled Nautilus This *skeleton is made of the mineral calcite*

Gastropods (mollusk)

These have modified their undersides into a *long muscular foot that they use to glide* Hence the name gastropod (stomach‐foot)

Arthropods

These include *crabs, lobsters, shrimp, barnacles, spiders, insects, and trilobites* These have *jointed legs and a segmented body* These have one of four basic body plans with a specific number of legs and segments All These also have an *exoskeleton* that allows for the attachment of muscles This *exoskeleton is essential for movement* This exoskeleton does not grow once built These *must molt this exoskeleton and make a new one when they outgrow it* Barnacles are the only of These to make a shell out of the mineral calcite

echinoderms

These include *starfish, sea urchins, sea Cucumbers, brittle stars, and crinoids* All These have *5‐fold symmetry* The *test (skeleton) of These consists of overlapping calcite* plates connected by muscle All These have *tube feet with suction cups on them that are used for movement* Tube feet can also *grab onto objects and prey*

Mollusks

These include many shell‐building animals Common members include *clams, snails, octopus, and squid* All These share several anatomical features All These have a *muscular foot, as well as a fleshy mantle used for making a shell*

Lophophorates

These include the shell‐building *brachiopods and the bryozoans* These are defined based on the *presence of a structure called a lophophore* A lophophore is a tentacle‐covered feeding organ