The Earth Through Time, 8e

Eighth Edition
by Harold L. Levin

Chapter 12 - page 4

Life of the Paleozoic

Phylum Brachiopoda

Brachiopods or lamp shells

Name: Name means "arm" (brachio) + "foot" (pod).

Chief characteristics: Bivalved (two shells), each with bilateral symmetry. The plane of symmetry passes through the center of each shell or valve.

The two valves differ in size and shape in most. Sometimes the larger valve will have an opening near the hinge line through which the pedicle extended in life.

Soft parts include a lophophore consisting of coiled tentacles with cilia. The lophophore circulates water between the two valves, distributing oxygen and flushing out carbon dioxide. Water movements caused by the lophophore also transport food particles toward the mouth.

Geologic range: Lower Cambrian to Recent. Very abundant during the Paleozoic. A few species (belonging to only three families) remain today.

Mode of life: Inhabitants of shallow marine environments; they generally live attached in a fixed position on the seafloor. Inarticulate brachiopods are known to live in burrows in the sediment. Brachiopods are filter feeders.

Living positions of brachiopods
Living positions of brachiopods.
A = Articulate brachiopod attached to the seafloor by its pedicle.
B = Interior of brachiopod valve showing lophophore.
C = Inarticulate brachiopod, Lingula, which lives within a tube or burrow in seafloor sediment.

A. CLASS INARTICULATA - The Inarticulate Brachiopods

            Primitive brachiopods with phosphatic or chitinous valves; no hinge. Spoon-shaped valves held together with muscles and soft parts.
           Lingula is a well known inarticulate brachiopod.

            Geologic range: Lower Cambrian to Recent

            fossil inarticulate brachiopod
            Fossil inarticulate brachiopod from the Cambrian. Maximum length = 2.2 cm.

B. CLASS ARTICULATA - The Articulate Brachiopods

            Brachiopods with calcareous valves attached together with a hinge consisting of teeth and sockets.
            Some of the more common articulate brachiopods are Pentamerus, Rafinesquina, Atrypa, Leptaena, and Spirifer.

            Geologic range: Lower Cambrian to Recent
            Spiny brachiopods (called productids) are characteristic of the Carboniferous and Permian.

            Ordovician strophomenid articulate brachiopod
            Ordovician strophomenid articulate brachiopod, Rafinesquina

            Paleozoic articulate brachiopods
            Miscellaneous Paleozoic articulate brachiopods.
            Photo courtesy of Pamela Gore.

Phylum Mollusca

(Clams, oysters, snails, slugs, Nautilus, squid, octopus, cuttlefish)

Name: Mollusca means " soft bodied".

Chief characteristics: Soft body enclosed within a calcium carbonate shell (a few, like slugs and the octopus, have no shell).
Muscular part of body of clams and snails and some other groups of molluscs is called the foot.

Geologic range: Cambrian to Recent.

Mode of life: Marine, freshwater, or terrestrial.
They may: swim, float or drift, burrow into mud or sand, bore into wood or rock, attach themselves to rocks, or crawl.

Classification of Molluscs

Placophorans are primitive molluscs with multiple paired gills, and a " foot" like that in snails. This group includes both the monoplacophorans and the polyplacophorans (chitons).


Name: Monoplacophora means "single plate-bearer".

Chief characteristics: Single shell resembling a flattened cone or cap. Soft part anatomy shows pseudo-segmented arrangement of gills, muscles, and other organs. Suggests that the primitive mollusc was a segmented animal. Segmentation was lost secondarily. Monoplacophorans are regarded as ancestral to bivalves, gastropods, and cephalopods.

Geologic range: Cambrian-Recent, but only known as fossils from Cambrian to Devonian. Living monoplacophorans found in deep water off Costa Rica in 1952 and named Neopilina. Considered to be a "living fossil" .

            Fossil monoplacophoran
            Fossil monoplacophoran, Pilina, from the Silurian.


(Chitons or amphineurans)

Name: Polyplacophora means " many plate-bearer" .

Chief characteristics: Chitons have 8 overlapping plates covering an ovoid, flattened body.

Geologic range: Cambrian to Recent.

            Living chitons
            Living chitons in Bermuda.
            Photo courtesy of Pamela Gore.

            Diagram showing the anatomy of a chiton.


(Clams, oysters, scallops, mussels, rudists)

Name: Bivalvia means " two" (bi) + " shells" (valvia).

Chief characteristics: Skeleton consists of two calcareous valves connected by a hinge. Bilateral symmetry; plane of symmetry passes between the two valves.

Geologic range: Early Cambrian to Recent

Mode of life: Marine and freshwater. Many species are infaunal burrowers or borers, and others are epifaunal.

            Paleozoic bivalve mollusc
            Paleozoic bivalve molluscs.


(Snails and slugs)

Name: Gastropod means "stomach" (gastro) + "foot" (pod).

Chief characteristics: Asymmetrical, spiral-coiled calcareous shell.

Geologic range: Early Cambrian to Recent.

Mode of life: Marine, freshwater or terestrial.

            Fossil gastropod
            Fossil gastropod.


(Squid, octopus, Nautilus, cuttlefish)

Name: Cephalopod means " head" (kephale) + " foot" (pod).

Chief characteristics: Symmetrical cone-shaped shell with internal partitions called septae (singular = septum). Shell may be straight or coiled in a spiral which lies in a plane. Smooth or contorted sutures visible on the outside of some fossils mark the place where septae join the outer shell.

Geologic range: Late Cambrian to Recent.

Mode of life: Marine only; carnivorous (meat-eating) swimmers.


    The shells of nautiloid cephalopods have smoothly curved septa, which produce simple, straight or curved sutures.
    Geologic range: Cambrian to Recent.

    Nautilus Shell of a nautilus sawed in half to show internal structure
    Photo courtesy of Pamela Gore.
    Shell of a Nautilus sawed in half to show
    internal structure including living chamber
    and septae.
    Photo courtesy of Pamela Gore.

    Fossil of a straight-coned nautiloid
    Fossil of a straight-coned nautiloid. Note gently curving septae and the siphuncle passing lengthwise through the center.
    Photo courtesy of Pamela Gore.


    Ammonoid cephalopods have complex, wrinkled or crenulated septa, which produce angular or dendritic sutures.

    Geologic range: Devonian to Cretaceous - all extinct.

    There are three basic types of sutures in ammonoid shells:

    1. Goniatite or goniatitic (septae have relatively simple, zig-zag undulations)
    2. Ceratite or ceratitic (septae have smooth "hills" alternating with saw-toothed "valleys")
    3. Ammonite or ammonitic (septae are complexly branching and tree-like or dendritic)

    Comparison of the sutures in nautiloid cephalopods with the three types of sutures in ammonoid cephalopods.

    Goniatite ammonoid cephalopod ammonoid cephalopod with ceratite sutures Ammonite cephalopod
    Ammonoid cephalopod with
    goniatitic sutures.
    Ammonoid cephalopod with
    ceratitic sutures.
    Ammonoid cephalopod with
    ammonitic sutures.
    Photo courtesy of Pamela Gore.


    a. ORDER BELEMNOIDEA (belemnites)

    The belemnoids have an internal calcareous shell (which resembles a cigar in size, shape, and color) called a rostrum. The front part of this shell is chambered, as in the nautiloids and ammonoids. The rostrum is made of fibrous calcite, arranged in concentric layers.

    Geologic range: Mississippian to Eocene - all extinct.

    Fossil belemnoid
    Fossil belemnoid in floor tiles of the Jurassic Solnhofen limestone from Germany. Fernbank Museum of Natural History, Atlanta.
    Photo courtesy of Pamela Gore.

    b. ORDER SEPIOIDEA (cuttlefishes)

    Geologic range: Jurassic to Recent

    c. ORDER TEUTHOIDEA (squids)

    Geologic range: Jurassic to Recent

    fossil squid
    Fossil squid, Acanthoteuthis sp., Lower Jurassic (145-140 m.y.), Germany.
    Photo courtesy of Pamela Gore.

    d. ORDER OCTOPODA (octopi)

    Geologic range: Cretaceous to Recent


(Tusk shells or tooth shells)

Chief characteristics: Curved tubular shells open at both ends.

Geologic range: Ordovician to Recent.

Mode of life: Marine.

Dentalium is a common genus.

Scaphopods, Dentalium sexangulare, Pliocene, Piacenzia Blue Clay, Castellarquato, Italy.
Photo courtesy of Pamela Gore.

            Diagram of a scaphopod
            Diagram of a scaphopod.

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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

January 5, 2006