A SURVEY OF THE FOSSIL RECORD

FOSSILS
Do you recognize your ancestors in these photographs?  Probably not, but these relatively unspectacular 
creatures are close relatives.  
Pikaia is known from the fossil record of Medial Cambrian time (about 525 million years ago).  These 
  fossils occur at a geologically famous locality in British Columbia, Canada that was first discovered 
 almost a century ago by a paleontologist named Charles Doolittle Walcott.  The rocks containing
 these, and many other unusual fossils, are known as the Burgess Shale.  
There exists considerable similarity in size and other anatomical characteristics such as a dorsal nerve 
chord, between fossilized specimens of Pikaia and the living lancet Amphioxus (classified as a primitive
 chordate).  It would seem, therefore,  that our phylum (Chordata) has a long geologic history.  
These specimens don't look much like dinosaurs, great white sharks, humans or even your pet tortoise,  
do they?  Well, we had to start somewhere.
  
The information that follows in this web page does not cover chordates, but it does touch upon most of the other major groups of organisms that have left a fossil record on Earth.  Enjoy yourself!

The purpose of this survey is to provide an introduction to some of the major groups of marine-dwelling organisms (excluding the chordates) and terrestrial plants that make up the largest percentage of the fossil record.  This survey is intended primarily to be used as a review or as an adjunct to various geology courses offered at San Diego State University.  These courses include Historical Geology (Geology 105), Fossils: Life Through Time (Geology 302), and Paleontology & Biostratigraphy (Geology 537).

Life on Earth, representing the biosphere, is now known to have left a fossil record that extends back to possibly 3.8 billion years ago.  Actual remains of cell-like structures, which can be described as "body fossils", are known from rocks as old as 3.5 billion years.  The older record, dating back to 3.8 billion years is, however, based mainly on chemical trace fossil evidence.  Perhaps life existed on this planet at an even earlier date then this hard to relate to ancient time, but, unfortunately, there is virtually no remaining rock record in which to find the evidence. 

For most of this enormous span of nearly 4 billion years, however, the fossil record consists only of microscopic-sized fossils of cellular bacteria and the various types of sedimentary structures that they form.  Only for approximately the past 1.5 billion years does the rock record provide recognizable evidence of more complex cells, and only for the past 600-650 million years does it provide evidence of multicellular organisms.   Thus, it has been only in the last 8% of the Earth's history - since the beginning of Phanerozoic time (or the beginning of the Cambrian Period, approximately 544 million years ago) - that fossils of the kinds of animals and plants that are a bit more familiar to us today become relatively abundant in the sedimentary rock record.  Interesting, that for most of Earth's history, this planet was essentially a bacterial world.  In fact, although there are now lots of other kinds of creatures, none could exist without the presence of bacteria; the earth is really still a bacterial world!

So, what is a fossil?  A simple definition used by most geologists is: The remains or traces of once living organisms that are preserved in the rock record, and that are at least 10,000 years old.  Thus, fossils include such objects as bones, shells, teeth, and traces of living things such as impressions, tracks, trails, burrows and other evidence.  The vast majority of these fossils are preserved in the sedimentary rock record, especially those rocks that formed in oceanic environments such as beaches, bays, lagoons, shallow seas and other areas.  Less commonly, but sometimes very spectacularly, we find fossils in in rocks that were deposited in terrestrial settings.   One example would be the large number of dinosaur bones found in flood plain and lake deposits within what is now Dinosaur National Monument in north-eastern Utah.

As a guide to this trip through the fossil record, the Geologic Time Scale is provided below.  This scale can be used as a reference to the time intervals used throughout this survey.  Note that there are actually two time scales.  One is a relative time scale and consists of a series of names.  The other is a numerical time scale and consists of actual dates.  Of note is that each of these scales was developed at different times.  The relative scale was primarily a product of geologic research during the 1800s and the numerical scale was developed by physicists in collaboration with geologists, primarily during the last two thirds of the 1900s.  It is a testament to the validity of our concepts of the age of the Earth and scientific method that both of these scales compliment each other.

MODERN GEOLOGIC TIME SCALE
Most of the names for the relative time intervals were established during the 19th century, and have remained 
consistent since the last half of that century, except for some newer additions (for example, Ediacaran, which 
was proposed in 1981, and has not yet been recognized by all geologists).  The numerical boundaries for the 
beginning of each time interval were established with increasing accuracy during the last half of the 20th Century, 
and they are indicated in millions of years.  As more information is continuously obtained from radiometric 
dating of rocks around the world these numbers will continue to undergo refinements.  Development of these 
two time scales (relative and numerical) represents a significant human endeavor, and these scales represent 
a major contribution of the science of geology to human knowledge.
 

All living and fossil forms of life are considered to represent the Earth's biosphere.  To date, biologists have collected and named over 2 million species of living organisms (and consider that millions more actually exist).   Paleontologists, on the other hand, have managed to find and name only about 250,000 species from the fossil record.  These two figures provide quite a contrast to one other, and underscore the fact that the fossil record represents but a small fraction of the kinds of organisms that have lived on this planet throughout geologic time.  A thought to ponder here is just how many kinds of organisms have actually lived on Earth throughout it's long geologic history?  Numbers such as 100s of millions or even billions have been suggested, but no one really knows.

Significant problems of organization arise when attempts are made to catalogue and classify all of these living and fossil forms, and we currently use a classification scheme that has a number of major categories and sub-categories.   The major categories of this scheme, and an example for each category, are listed in the table below.   We will use these taxonomic ranks in the discussions of the various groups of fossils.  There are numerous other categories, such as Subphylum or Superfamily, and the system has become relatively complex.   Scientists who study and classify organisms are known as Taxonomists or Systematists, and this branch of science got its start around the middle of the 1700s.

Three domains are recognized.  Two of them, Archeae (including Archaebacteria) and Procaria (including Monera) contain prokaryotic organisms. The third domain is the Eucarya (including the remaining four kingdoms), which includes all eucaryotic organisms. The six kingdoms of organisms that are currently recognized are listed in the chart below.  They are divided into two main groups:  Those having non-nucleated cells, the prokaryotes; and those having nucleated cells, the eucaryotes.  From a biologic standpoint there exists a profound, fundamental difference between prokaryotic organisms and eucaryotic organisms.

At the next level below kingdom in the classification scheme, a combined total of slightly less than 100 phyla (for prokaryotes, protists, fungi and animals) and divisions (for plants) have been recognized.   In addition, there are a few phyla that are known only from the fossil record and are assumed to be extinct.  Of these nearly 100 living and extinct taxonomic groups, only a small portion make up the bulk of the fossil record, and they are found mainly in the Kingdoms Monera, Protoctista, Plantae and Animalia.  The most common of these phyla are listed below, and these are described in more detail in the following sections..

KINGDOM MONERA
     CYANOBACTERIA (Blue-green bacteria in particular) 

KINGDOM PROTOCTISTA
     RHODOPHYTA (Red algae)
     CHLOROPHYTA (Green algae)
     FORAMINIFERA ("Forams")
     RADIOLARIA ("Rads")

KINGDOM PLANTAE
     TRACHEOPHYTA (Vascular plants)

KINGDOM ANIMALIA (Various animal phyla.  Main groups known from the fossil record are listed below)
     PARAZOA (Porifera, the sponges, and Archaeocyathids)
     CNIDARIA or COELENTERATA (Corals, Sea Anemones and Jellyfish)
     ECTOPROCTA or BRYOZOA ("Moss" animals)
     BRACHIOPODA ("Lampshells")
     MOLLUSCA (Gastropods, Bivalves, Cephalopods)
     ARTHROPODA (Trilobites and Crustaceans)
     ECHINODERMATA (Crinoids, Blastoids, Echinoids and Asteroids)
     HEMICHORDATA (Graptolites)
   

EXAMPLES OF MODERN BLUE-GREEN BACTERIA
A.  Cocci (= spherical-shaped) bacteria.   B.  Bacillus (= rod-shaped) bacteria.   C. Spiral 
(= corkscrew-shaped, or filamentous-shaped) bacteria.  All images are scanning electron 
microphotographs and illustrate  cells that are VERY TINY, between 1-20 microns in diameter. 
(From Campbell, 1987, p. 517)

Morphology Of Filamentous, Blue-Green  Bacteria These cells, which are known as Cyanobacteria, are surrounded by an organic sheath (cell wall).  They lack  a cell nucleus or other complex cellular organelles.   Each cell is drawn as it is undergoing asexual cell  division (cellular fission).  Each of these cells is approximately 5 microns in length.  (Modified from Margulis & Schwartz, 1988).

The Kingdom Monera includes very tiny unicellular organisms that are generally 1 to 10 microns in diameter, lack a cell nucleus or other cellular organelles, and reproduce asexually.   Because of these characteristics they are known as prokaryotic (or prokaryotic) organisms, and as a group, they are more commonly known as bacteria.  Modern bacteria exist by obtaining nutrients as parasites or decomposers, or they can manufacture food by various processes such as fermentation and photosynthesis, and are known as autotrophs.  Fossilized remains of spherical and filamentous bacterial cells are known from rocks as old as 3.5 billion years, and are considered to represent the oldest known cellular record of life on Earth.
Geologic Range: ARCHEAN-HOLOCENE.

FOSSIL SPHERICAL AND FILAMENTOUS BACTERIA
Photomicrographs of spherical (left) and filamentous (right) bacteria cells from the Bitter 
Springs Formation (about 900 million years old, from Australia ).  These small cellular 
structures are usually preserved as carbonized films (a type of preservation called carbonization)  
in siliceous (quartz-rich) rocks called chert.  Such cellular structures are known from rocks as 
old as 3.5 billion years, and they appear identical in size and shape with living bacterial cells.  
These fossils are best viewed in thin sections of rock samples.   A thin section is made by attaching 
a sample of rock to a small glass slide and then grinding the rock down to a thickness of 30 microns.   
A rock sample this thin allows transmitted light to pass through it, and thus the sample can be 
studied and photographed under a microscope. 
(Slides courtesy of J. William Schopf, UCLA)

FOSSIL BACTERIAL CELLS
Examples of some of the oldest fossilized bacterial cells include the following:
A.  Spherical cells from a thin section of black chert of the Fig Tree Group (3.1-3.2 B.Y. 
from South Africa).
B.  Filamentous cells from a thin section of black chert of the Warrawoona Group (3.5 B.Y.
 from western Australia).
In both thin sections of these silica-rich rocks the original organic components of the cells have 
been replaced by a carbon film.
(J. W. Schopf, in Levin, 1999, p. 230-231)

One of the most significant groups of bacteria are the Cyanobacteria (=  blue-green bacteria).   Cyanobacteria form spherical or filamentous-shaped cells, and fossils of these cellular structures have been found in rocks possibly as old as 3.5 billion years. This group of bacteria contains chlorophyll molecules and are capable of photosynthesis.  Therefore, we can assume that they were producing oxygen gas as a by-product of photosynthesis early in Earth's history.  In addition, these organisms tend to form mat-like structures of intertwined filaments that grow on the sediment substrate in the intertidal zone and in shallow water environments.  These mats trap fine grained sedimentary particles and cement them together.  When the cells die, the sedimentary particles remain as a thin cemented lamina.  A new layer of cells may grow on the lamina and trap more particles.  If this process continues dome-shaped sedimentary structures may be built up that is called a stromatolites.  If the laminae are ripped up by currents and rolled on the sea floor spherical, laminated sedimentary structures may form that is called an oncolites.  Because both of these structures are formed as a result of organic activity, they are called organo-sedimentary structures.  These structures are known from the rock record as far back as 3.2 billion years (possibly 3.5 B.Y.), and they are known to be still forming today in some environments.

MODERN STROMATOLITES
A, B.  Organo-sedimentary structures (stromatolites) forming in modern environments (intertidal zone and 
at a depth of about 3 meters) at Shark Bay, Australia.  
C.  Cross-section cut through a modern columnar-shaped stromatolite from Shark Bay, Australia 
illustrates one type of domal, laminated form. 
Cyanobacteria can grow and trap sedimentary particles in this location because the water is highly 
saline and prevents invertebrates from living and grazing on the the bacterial mats.  In most other 
modern environments with more normal conditions, invertebrates graze on cyanobacteria and 
prevent organo-sedimentary structures from forming.   Discovery of these modern stromatolites provided 
the evidence that was, and still is, used to interpret an organic source for similar structures found in 
the Archean and Proterozoic rock record.  
(From Scholle, Bebout & Moore, 1983, p. 376 and 377; Lewin, 1982, p. 100

ANCIENT ORGANO-SEDIMENTARY STRUCTURES
A.  Oncolites.  These represent rolled up laminae of sedimentary particles trapped by filamentous cyanobacteria.  They form as water currents (from tides or storms) rip up the laminae and roll them on the bottom.  Faint evidence of the laminar structure is preserved in some of these oncolites.   Within a gray limestone (Chambless Limestone, Cambrian, Mojave Desert, California)   Diameter of largest specimen is about 3 cm.
B.  Stromatolites.  These represent domal structures built up by the stacking of successive laminae of sediment particles trapped by successive layers of filamentous cyanobacteria.   B. Stromatolites in gray dolostone (Goodsprings Dolomite, Ordovician, Nevada) .  About 30 cm tall.  

Because the organisms that form oncolites and stromatolites are autotrophic photosynthesizers, they must live within the photic zone (water depth of less than 200 m).  The nature of oncolites indicates they result from the action of shallow water currents (tides or storms).  Modern oncolites and stromatolites are all known from intertidal to shallow water environments.   We can conclude that these structures usually indicate intertidal and shallow water environments, and therefore, they provide useful paleo-environmental evidence. 

Although the record of organo-sedimentary structures extends back beyond 3 billion years, they are much less common in rocks that are younger than 544 million years.  This decrease in abundance may reflect the evolutionary rise of  various groups of animals, which are organisms that can graze on these bacterial mats and thus prevent them from forming into larger structures.

To Conclude this discussion of monerans, we have the following tidbits:

• Monerans represent the oldest form of life preserved on Earth; known as body fossils from rocks at least 3.5 billion years old, and as presumed chemical trace fossils back to about 3.8 billion years.

• They are unicellular organisms and represent relatively un-complex, prokaryotes, primarily various types of bacteria.

• Bacteria obtain nutrients as decomposers or as autotrophs and have evolved various metabolic pathways, most of which have been in existence for over 3 billion years..

• Various bacterial groups, such as cyanobacteria, have contributed to the sedimentary rock record by forming organo-sedimentary structures.

• Cyanobacteria have also contributed oxygen gas to the atmosphere and hydrosphere as a byproduct of photosynthesis, and this metabolic process began about 3.5 billion years ago.

• Evidence for the biological production of oxygen gas early in the Earth's history (3.0-3.5 b.y. ago) occurs in  the rock record in the form of Banded Iron Formations (BIFs).   BIF's disappear from the rock record about 1.8-2.0 billion years ago.   
                    
              BANDED IRON FORMATIONS
  Example of a silica-rich rock that contains layers of oxidized iron (red), suggesting that at least small amounts of oxygen gas existed episodically in the atmosphere and hydrosphere.  This sample is about
  5 cm wide. These rocks have a geologic range from about3.2-2.0 billion years ago, and are known to occur on most continents, but then they disappear from the rock record at 2.0 billion years ago.  
  Archean (about 3.0 b.y. old) (Specimen courtesy of
  Dr. Monte Marshall, SDSU)
   

Evidence for the production of abundant oxygen gas, and its accumulation in the atmosphere and hydrosphere later in the Earth's history,  is provided by the rock record of Red Beds.  Red Beds such as those illustrated above appear between 1.8-2.0 billion years ago.

RED BEDS
Layers of sedimentary rocks that are red indicate that they were formed  in the presence of abundant free oxygen gas, which oxidizes (= rusts) any iron minerals  that  are present. "Red Beds" can also occur in various shades of brown, yellow, orange or any other "earthy" tones.  Red beds do not appear in the rock record until about 2.0 billion years ago, and their appearance signifies the time when the Earth's atmosphere and hydrosphere became oxidizing. 
(Mesozoic rocks from Utah)

 

Protoctistans (= protistans) include unicellular, colonial and multicellular organisms having a cell nucleus and cellular organelles.  This type of cellular organization is termed eucaryotic, and the appearance of these types of cells represents a major biological event in the history of life.  Eucaryotic cells are much larger and internally more complex than prokaryotic cells, and are usually over 50 microns in diameter.  Within this kingdom are a variety of microscopic and macroscopic organisms, some that have organelles such as mitochondria in their cells and others that have organelles such as plastids in their cells.   Mitochondria are used in respiration and are found in heterotrophic organisms, whereas plastids are used in photosynthesis and are found in autotrophic organisms.  Some protoctistan groups have a significant fossil record, and those include the foraminifera, radiolarians, various taxa of algae and a few other groups.  The fossil record of protoctistans extends back to Proterozoic time; they first appear in the rock record represented by a group of organisms called acritarchs between 1.5-1.8 billion years ago.

Fossil Acritarch Acritarchs  first appear in the rock record about 1.6 billion years ago.  They probably represent a group of planktic algae, and they are considered the first eucaryotic cells because of their larger size and more complex, ornamented outer wall.  Carbonized cell of Paleozoic  age.  Approximately 0.1 mm in diameter.

A Living Foraminiferan Most of these unicellular organisms (known as forams) have a porous, calcareous test (= shell or skeleton), extended pseudopodia and internal protoplasm.  This specimen is approximately 0.2 mm in diameter and has a planktic mode of life, although many other forams are benthic. (From Campbell, 1987, p. 538)

The two best known groups of sarcodinans are commonly known as Foraminifera and Radiolaria.   These are nucleated (= eucaryotic), unicellular organisms that have pseudopodia for movement and food capture.  Their cells contain mitochondria and therefore they must obtain food externally by capturing prey (= heterotrophic).  Their external skeleton, also called a test, may have an organic composition or may consist of siliceous, or calcareous wall material.  Most sarcodinians live in marine water, but a few have managed to adapt to living in brackish or fresh water.  They may live on the bottom (= benthic) or float in the water column (= planktic).   In open ocean environments, the accumulation of radiolarian or foraminiferan tests can make a significant contribution to the formation of siliceous or calcareous sediments.
Geologic Range: LATEST PROTEROZOIC-HOLOCENE.

Morphology And Examples Of  Foraminiferans And Radiolarians A.  Examples of calcareous Foraminifera.         B.  Examples of siliceous Radiolaria.   There is considerable variation in the shapes of foraminiferan and radiolarian tests, and a large vocabulary of terms is used for their morphologic descriptions.  Classification of these organisms is based on wall structure, symmetry and external ornamentation       of their test.   Various species average about 0.3 mm in diameter, but some foraminifera may be as small as  0.1 mm, and a few "giants" are known to be 2-3 cm in diameter.

Fossil Fusulinid Foraminifera Left.  Specimens of large, benthic foraminifera known as fusulinids.  They had a calcareous skeleton (= test) and were shaped like grains of rice.  Right. Internal structure of fusulinids. This group of foraminiferans existed only  during Late Paleozoic time and are useful index fossils for that interval. Left are Permian from Texas; right are Carboniferous from Nevada. (Left from Levin, 1999, p. 344)

Genus Dentalina

Genus Hantkenina

Fossil Foraminiferans
 Left and Center.  Photomicrographs of elongate benthic (= bottom dwelling)  forams and coiled planktic  
(= floating) forams. Yazoo Formation Cenozoic age, from Alabama. 
 Right.  Photomicrograph of unsorted sample of planktic and benthic forams from the Monterey Formation 
of Miocene age (Cenozoic, Tertiary),  from California.  (From Levin, 1999, p. 142)
 All of these specimens have calcareous tests.

Fossil Radiolarians Photomicrographs of planktic radiolarians.  They have a siliceous skeleton (test), various spines and ornamentation and are approximately 0.1-0.2 mm in diameter.  Left.  Scanning electron micro- graph.  Jurassic from California. (From Levin, 1999, p. 427) Right.  Light microscope photo- graph.  Tertiary, unknown.

LIVING GREEN AND RED ALGAE
A.  Marine dwelling, calcareous green algae (Genus Codium).  (From Thurman & Trujillo, 1999, p. 391)
B.  Marine dwelling, coralline red algae.  (From Campbell, 1987, p. 550)
At least some species of each of these algal groups inhabit shallow marine water within the photic zone
(0-200 m) and secrete calcium carbonate. 

There are numerous groups of algae, of which we will consider the chlorophytes and the rhodophytes.  All algal groups contain the chlorophyll molecule and use photosynthesis for manufacturing food.  Each group can be distinguished on the basis of body shape and cellular pigments, and it is the cellular pigments that provide the source for the group names.  The first group, the  chlorophytes, are commonly known as green algae.  Chlorophytes are unicellular or colonial and inhabit marine, brackish and fresh water environments.   Some marine-dwelling green algae secrete a calcareous outer wall and are known as "calcareous green algae".  These calcium-carbonate secreting algae are significant in the geologic record because the walls break down after the algae die and contribute tiny calcareous fragments that accumulate as sediments on the sea floor.

The second group, the rhodophytes, are commonly known as red algae.  They are unicellular, colonial or multicellular and live only in marine environments.  Some red algae secrete a calcareous outer wall with partitions that crudely resemble the structures formed by corals.  Because of this characteristic they are called "coralline red algae".  As with green algae this calcareous wall material may accumulate on the sea floor.
Geologic Range of Both Groups:  LATE PROTEROZOIC-HOLOCENE.

Morphology Of Red Algae (A) And  Green Algae (B)   These are examples of calcium carbonate secreting algae that form benthic colonies in warm, shallow marine environments within the photic zone.

MODERN CALCAREOUS ALGAE
Individual algal cells are microscopic, perhaps 100 microns (0.1 mm) in diameter, but the structures they 
build are macroscopic.  These organisms inhabit shallow water environments within the photic zone, where 
they prefer warm shallow environments and secrete calcareous material made of calcium carbonate 
(mineral is aragonite).  This mineral breaks down into tiny, aragonite needles, which accumulate as 
calcium carbonate sediment on the sea floor.  
A.  A colony of calcareous, encrusting, coralline red algae, approximately natural size (= rhodophytes).  
B.  A colony of calcareous, green algae (= chlorophytes), approximately natural size.
C.  Sample of modern sediment from a Jamaican beach with fragments of algae and foraminifera.   
Largest fragments are about 2 mm in diameter.  
(See: Sedimentary Rock Record Web Page)

Fossil Algae Fossilized, calcareous green algae(?) from Cambrian rocks in China.  This specimen  is preserved as a carbonized  film on shale. Specimen is approximately 9 cm in height.

Other examples of protoctistans include brown algae (= sea weed or kelp), diatoms, coccolithophorids, ebridians, silicoflagellates and many others.  Only those with a calcareous or siliceous test or shell tend to occur commonly as fossils.  However, many of these taxa provide significant evidence of their existence by forming a significant portion of the sedimentary rock record.  For example, most chalk consists of either calcareous or siliceous mud that results from the compaction of enormous numbers of tiny, microscopic tests of these unicellular organisms. 

Fossil Diatoms Tiny, unicellular organisms with a skeleton (test) made of silicon dioxide.  Diatoms live in fresh, brackish and salt water.  These autotrophic organisms are one of many groups of phytoplankton.  Their geologic range is Cretaceous to Holocene.  These specimens are of Cenozoic age.  They have been described occasionally as "fossil doilies". (From Levin, 1999, p. 507)

The appearance of autotrophic eucaryotes such as acritarchs, various groups of algae, diatoms, and other taxa, beginning about 1.6 billion years ago, and continuing up to the present time, has provided another source of oxygen gas as a byproduct of photosynthesis.  In addition, many of these groups secrete mineralized cell walls (= tests) and these materials may contribute to the formation of sediments and sedimentary rocks.

During Proterozoic time (about 1.6 billion years ago), eucaryotic organisms such as the protoctistans started to contribute their tests to the sedimentary rock record.  Many fine grained siliceous and calcareous sedimentary rocks owe their existence to the accumulations of countless numbers of these organisms on the sea floor.  With the appearance of many new groups of planktic protoctistans (phytoplankton and zooplankton) in Late Mesozoic time (Cretaceous Period), considerably more material was added to the accumulating rock record.  Essentially the tests of these tiny organisms rain down through the water column over thousands and even millions of years and accumulate slowly on the sea floor. Other organisms that form mineralized skeletons, primarily algae and invertebrate animals may also contribute their skeletal remains to the sea floor.  Rocks formed primarily or entirely from these accumulations or skeletal debris are known as biogenic sedimentary rocks.  

Modern Carbonate Sediments Example of a modern coral, Genus  Acropora growing at a depth of 5 m in Eniwetok Lagoon, and contributing  calcium carbonate debris to form  sediments that have partially buried a truck tire within a span of 20 years. (From Scholle, Bebout & Moore, 1983)

BIOGENIC SEDIMENTARY ROCKS
A.  Very fine grained siliceous mudstone consisting mostly of crushed diatoms and other 
plankton, with a bony fish skeleton preserved on the bedding surface.  
Fish is about 15 cm in length.  Green River Formation (Tertiary, Wyoming).
B.  Calcareous chalk made of crushed calcareous microscopic-sized plankton.  
Cretaceous(?), location unknown.  Width of specimen about 9 cm.
C.  Coquina.  This is a rock consisting of broken fragments of calcareous shells that have
been cemented together, and it usually represents a beach deposit.  
Holocene, location unknown.  Larger shell fragments about 1 cm in diameter.

CARBONATE SEDIMENTARY ROCKS
A.  Beds of fine-grained limestone and dolostone of the Waterfowl Formation (Cambrian Period, Canadian
Rocky Mountains).  These rocks consist of calcium carbonate that was derived from the breakdown of
calcareous algae and also shells of invertebrate animals.
B.  Thin beds of fine-grained to chalky limestone exposed at the Cliffs of Dover on the English Channel
(Cretaceous Period).   These rocks consist mostly of the microscopic tests of planktic foraminifera
that accumulated on the sea floor during Cretaceous time.
(From Scholle, Bebout & Moore, 1983 and Berrill, 1966, p. 198)

Examples Of Living Plants Almost everywhere on the Earth's surface there are living plants,  but terrestrial plants are a  relatively recent addition, first appearing a little over 400  million years ago. Left are palm trees growing in Southern California (campus of San Diego State University). Right are giant redwood trees  growing in a humid, temperate  climate of Central California. (Right from Campbell, 1987)

Kingdom Plantae includes multicellular organisms with cells having a nucleus, a cell wall, a cell membrane, and other organelles such as plastids (chloroplasts).  These organisms are autotrophic and obtain food by photosynthesis, a process that may occur in leaves or in stems, and they absorb other nutrients and water from the soil through their root systems.   They also require carbon dioxide.  The most important group of plants in the fossil record are those having an internal system of cellular, tube-like structures such as xylem and phloem.   They are called tracheophytes, or, more commonly, vascular plants.  Most vascular plants live on land and they form the base of the food chain for almost all terrestrial communities.
Geologic Range: ORDOVICIAN(?), SILURIAN-HOLOCENE.

Morphology Of Vascular Plants These examples of a stem, leaf, seed and pollen represent some of the basic structures found in the plant kingdom.  However, many plants do not havewoody tissue and some do not have leaves, seeds or pollen.  Classification of plants is based on characteristics  of the woody tissue, typeof leaves, and types of reproductive structures produced.   The pollen and seed grains illustrated are highly magnified.   In the fossil record, he most  common fossils of  plants are the remains of woody  tissue, leaf impressions,   and spores, pollen and seeds.

Complex terrestrial plants, tracheophytes, are known from fossilized fragments in Ordovician rocks, and are more positively identified in Silurian rocks.  This fossil evidence indicates that before Ordovician time, that is, older than about 500 million years ago, the surface of the Earth was essentially barren of plant life.  It is difficult to visualize an Earth without the profusion of green plants and colorful flowers that today appear almost everywhere.

PRIMITIVE SPORE-BEARING PLANTS
A.  Genus Baragwanathia. One of the oldest plants known from the 
fossil record.  It is a carbonized impression.   Silurian age; locality 
unknown.  Approximately 5 cm in length.
B.  Genus Annularia.  Ancestral plant to modern horsetails (= scouring 
rushes).  Characteristic plant of Carboniferous age; locality unknown.   
Approximately 20 cm in length.

FERNS: MORE COMPLEX SPORE-BEARING PLANTS
A.  Genus Senftenbergia.  Fern-like plant of Carboniferous age.   Locality and size unknown.
B.  Modern sword-ferns.  Geology Park, San Diego State University campus.  Individual fronds 
about 4 cm wide.
These plants show some morphologic advances over the plants illustrated above; they 
have roots, reinforced stems and well developed leaves, but still reproduce using spores.

Fossil Spores Reproductive part of seedless vascular plants such as ferns, horsetails and psilophytes.  They require very wet  conditions to become fertilized and then  germinate.   A. Devonian spore from  Greenland.   B. Carboniferous spore from  Greenland.  Both approximately 60  microns in diameter. (From Centrum. Geological Survey of Denmark)

These specimens of spore-bearing plants illustrated above represent some of the oldest types of terrestrial plants.   Most lacked the familiar and typical plant structures such as roots, woody tissue, leaves, seeds or flowers, and they were, in effect, just at beginning their evolutionary adaptations to a terrestrial mode of existence.  Some of these plant groups, however, even with their relatively simple morphology, have managed to survive until the present time.  In terms of abundance and diversity, these early taxa were supplanted by seed-bearing plants and then flowering plants more recently in geologic time.

Seed-Bearing Plants: Cycads A.  Genus Zamites.  Carbonized frond of a group of plants known as cycads (= sago palm); modern cycads look somewhat like pineapples with palm-like fronds sticking out of the top.  This genus has a geologic range of Triassic - Cretaceous; locality unknown.  Approximately 20 cm in length. B.  Genus Zamia.  Modern cycad, Geology Park, San Diego State University campus.  Approximately 1 meter tall.

Seed-Bearing Gymnosperms I A and B represent specimens of the  Genus Araucarioxylon, of Triassic  age (Chinle Formation, Petrified  Forest, Arizona).  The original woody tissue of this stem has been chemically replaced (= preservation  by replacement) by the action  of  groundwater and is now silica dioxide (a variety of quartz. The original cellular structure of the stem is beautifully preserved and can be seen in this highly magnified slice.    Stem about 15 cm in diameter; thin section about 1.0 cm across.

Seed-bearing Gymnosperms II A.  Genus Ginkgo.  Cross section of stem preserved by replacement (silicified stem).   Note evidence of  growth rings.  This specimen  is from the Pacific Northwest (Cenozoic age).  Stem about 15 cm across. B.   Carbonized Ginkgo leaf impressions of Jurassic age; leaf about 4 cm in width.  C.  Example of modern Ginkgo biloba.  Branch with leaves, from a tree growing on the campus of San   Diego State University.  This genus is an example of a "living fossil".  Thought to be extinct, it was  discovered in the 1800s living  in China, and specimens were smuggled out to other countries.  There is  only one living species, which is known as Ginkgo biloba.

PRESERVATION OF PLANTS
A.  Unusual preservation.  These are small plant leaves preserved as carbonized ash residue in a
 basalt flow of Holocene age from Hawaii.  Leaves are about 1 cm in length.
B.  Carbonized leaves in mudstone.  Lower right (above the letter B) is a fragment of the Genus  
Metasequoia (?), and other leaves are of seed bearing, flowering plants (angiosperms).  
John Day Formation, Tertiary, Oregon.  Metasequoia (?) fragment is about 4 cm tall.
C.  Carbonized, fern-like leaves in carbon-rich, black shale of Carboniferous age from 
Illinois.  Width of specimen about 10 cm.

Aysheaia Very rare carbonized film of a specimen of an onycophoran, a group of organisms that has characteristics of both worms and arthropods.  From the Burgess Shale (Cambrian of British Columbia). Approximately 6 cm in length.

Kingdom Animalia includes multicellular organisms with cells having a nucleus (= eucaryotic), a cell membrane, and organelles such as mitochondria.  They must obtain food from external sources (= heterotrophic), and they require oxygen.  Some of the oldest known fossils of these organisms are found in rocks ranging from 540-650 million years old (the Ediacaran Period of the geologic time scale).  These oldest animal fossils are found at nearly 30 locations around the world and are distinctive.   Characteristically, they are preserved as thin impressions on bedding surfaces of fine to medium-grained sedimentary rocks.   In life, these organisms were extremely thin, seem to have lacked any mineralized hard parts or well developed organs or organ systems, and they had a quilted-appearing outer surface.  Some uncertainty exists as to what groups (phyla) of animals these fossils might represent, and if they were ancestral to the multitude of multicellular animals found in younger rocks.  Collectively they are known as the Ediacaran Fauna.
Geologic Range:  EDIACARAN-HOLOCENE.

Examples Of The Ediacaran Fauna A. Genus Spriggina.   B. Genus Dickinsonia.  These bodies of these specimens are preserved as impressions in fine grained sandstone of the Pound Quartzite (Ediacaran age, Australia).   Similar appearing, soft-bodied fossils are known from over two dozen localities worldwide.  These organisms were all small, had thin bodies with a quilted outer surface, and lacked any internal or external hard parts.  Specimens about 4 to5 cm in length.

Living In The Ediacaran: A Diorama Reconstruction of a portion of a very shallow sea-floor that existed during Ediacaran time, approximately 600 million years ago.  This  reconstruction is based on fossils found from various geographic locations.  Note the  relatively low diversity and simplicity of the  organisms present and the current ripples  that occur on the fine grained, sandy  substrate.  (Diorama in the U.S. Natural History Museum)

Living Sponges    Two examples of sponge colonies.  The orange animals crawling on the sponges at left are brittle.  starfish.  Larger opening at top of sponge (= osculum) is about 15 cm across.     Cayman Islands (left); Belize (right).  (From Lewin, 1982, p. 88-89; and D. Huntley, San Diego State University)

Poriferans, or pore-bearers, are commonly known as sponges, and they represent a group of very simple, perhaps the simplist, multicellular animals.  They have a two layered body wall that is perforated by numerous holes (pores), and they lack tissues and organs; thus they represent what is called the parazoan grade of development. Water enters pores in the sides and passes through the canals into the central body cavity and then is pumped out through a large opening at the top called the osculum.  So basically, sponges operate like simple water pumps.  As water passes through, the cells in the canals absorb oxygen and trap food particles.  Many sponges secrete microscopic-sized, mineralized structures called spicules that occur between the two layers of the body wall.  In the rock record, preserved sponge body fossils are relatively uncommon, but their spicules occur more commonly.
Geologic Range:  EDIACARAN-HOLOCENE.

Morphology Of Sponges And Sponge Spicules.  Classification of sponges is based on organization of pores and internal canals and by mor-  phology and composition of spicules.  Spicules are usually microscopic, roughly on the order    of 1 mm in length. Spicules may consist of organic material, but more commonly have a calcar- eous or siliceous composition. Spicules are classified on the basis of their composition and shape. (Right modified from Boardman, Cheetham and Rowell, 1987, p. 117)

FOSSIL SPONGES
A. Example of the Genus Hydnoceras, preserved as a lattice-like impression of the body in
 fine-grained limestone of Devonian age.  Specimen is about 10 cm in height.
B.  Example of the Genus Raphidonema, preserved with its original calcite skeleton in 
sandstone of Mesozoic age.  Specimen  is about 7 cm in height.  (B. From Fortey, 1982, p. 16)

Fossil Archaeocyathids Archaeocyathids are an extinct group  of sponge-like animals that had a  two-layered body wall of calcium  carbonate.  They are known only  from Cambrian rocks.   These specimens (A) are from  dolostones of the Poleta Formation  (Cambrian from California).  Large  specimen near top about 2 cm across.

HYDROZOAN	SCYPHOZOAN
ANTHOZOAN	ANTHOZOAN

EXAMPLES OF THE THREE MAIN CLASSES OF MODERN CNIDARIANS
A.  Colony of polyps representing hydrozoans.  These organisms live mostly in fresh water environments.
  Each polyp is approximately 1 cm in diameter.
B.  Jellyfish medusae, representing scyphozoans.  These organisms live mostly in marine environments. 
 These small medusae are approximately 12-15 cm in diameter.
C.  Sea anemones, representing anthozoans, with starfish.  These organisms live exclusively in marine
environments, and are approximately 9 cm in diameter
D.  Colonial coral polyps, representing anthozoans.   These organisms live exclusively in marine
environments.  Individual polyps are about 1 cm in diameter.
Note that all of these organisms exhibit radial symmetry and have tentacles which contain
nematocysts (= stinging cells).
(A, B, and D. From Campbell, Reece & Mitchell, 1999, p. 602;    C.  From Campbell, 1987, p. 601)

Cnidarians, also known as Phylum Coelenterata or Radiata, represent the simplest group of animals that are collectively known as metazoa (or eumetazoa).  Although this phylum is represented in modern environments by hydras, jellyfish, sea anenomes and various types of corals, the fossil record of the group consists mostly of skeleton-secreting corals. Organisms within this phylum usually have radial symmetry, tissues and organs in a simple body cavity, and tentacles that contain stinging cells called nematocysts which are arranged around a mouth.  Most cnidarians are marine organisms and are carnivores, obtaining food by using the stinging cells to capture anything that blunders into their tentacles. 
Geologic Range:  EDIACARAN-HOLOCENE.

The fossil record of cnidarians extends back into Ediacaran time, to approximately 600 million years ago.  Fossilized impressions of jellyfish-like organisms, possibly representing scyphozoans, and a group of anthozoans have been preserved in sandstones of Ediacaran age in many localities around the world (see diagrams at beginning of this chapter on animals), and provide the first record of metazoans.  Think of how difficult it would be to fossilize an organism such as a jellyfish!

In the fossil record the most abundant group of cnidarians are corals of the Class Anthozoa.   Most anthozoans are benthic marine organisms that have an external skeleton of calcium carbonate, which is divided internally by various types of partitions called septa and tabulae.  Three groups of corals are known from the fossil record: rugosans, tabulates and scleractinians.

Phylum: Cnidaria (or Coelenterata)    Class: Hydrozoa (Cretaceous?-Holocene)    Class: Scyphozoa (Ediacaran-Holocene)    Class: Anthozoa (Ordovician-Holocene)       Subclass: Octocorallia (Ediacaran-Holocene)       Subclass: Zoantharia (Ordovician-Holocene)           Order: Rugosa (Ordovician-Triassic)*           Order: Tabulata (Ordovician-Permian)*           Order: Scleractinia (Triassic-Holocene)*

CLASSIFICATION OF PHYLUM CNIDARIA
In the fossil record the most abundant and diverse class of cnidarians are the anthozoans, of which there are 
two subclasses and three main orders.  The diagram at the right illustrates the patterns of septa growth that 
distinguish rugose corals (septa grow in patterns of four) from scleractinian corals (septa grow in patterns 
of six).  Numbers indicate the relative order of septa growth.  It is likely that scleractinians evolved from 
rugosans in the Triassic Period.

Three Orders Of Class Anthozoa A.  Rugosa.  Ordovician-Triassic. B.  Tabulata.  Ordovician-Permian. C.  Scleractinia.  Triassic-Holocene. These are the most common types of corals in the fossil record. Specimens slightly less than natural size.

Morphology Of The External  Skeleton Of Corals  Identification of taxonomic groups within the corals is based on variations in skeletal morphology.   In particular, important features are the number and arrangement of the septa, and the development of tabulae.

FOSSIL RUGOSE COALS
A & B.  Rugose corals (commonly called solitary horn corals), illustrating well developed septa,
 from Lower Paleozoic rocks.  These corals grew as individuals on the sea floor.  The original 
outer epitheca has been preserved by recrystallization, but is still calcium carbonate.  Larger 
specimen about 4 cm in diameter
C.  Rugose coral (colonial rugose corals) from Upper Paleozoic rocks preserved in a limestone 
(Bird Springs Formation, Nevada).  This type of rugose coral formed colonies of encrusting 
individuals.  These fossils have also been recrystallized.   Individuals about 1 cm in diameter.
Visualize these organisms growing on the Paleozoic sea floor with nematocyst-bearing tentacles 
extending from between the septa waiting patiently for food to float or swim by and get trapped 
and stung by the nematocysts in the tentacles.
 

Genus Halysites

Genus Favosites

FOSSIL TABULATE CORALS
D.  Genus Halysites.  Top view of a colonial coral that resembles a chain (hence the popular
name of chain coral).  Each node on the chain is an individual coral (called a polyp).  
  Specimen about 10 cm across.
E.  Genus Favosites.  Side and top view of a colonial coral.   Note that the top surface resembles
a honeycomb structure (hence the popular name of honeycomb coral).  Each "honeycomb" is an
individual coral polyp; they grew upward and secreted the horizontal tabulae.   Specimen is 8 cm across.
These specimens are now silicified; the original calcareous skeletal material has been preserved
by mineral replacement.  Both are Silurian age
(Hidden Valley Dolomite, Death Valley, California).

SCLERACTINIAN CORALS
F & G.  Scleractinian corals (hexacorals) that occur in Mesozoic and Cenozoic rocks.  
All scleractinians are colonial, and most live in sub-tropical to tropical regions of the 
oceans within the photic zone.  These two specimens are modern and are original 
calcium carbonate.  Both about natural size.
H.   Scleractinian coral (Genus Flabellum), from Cenozoic rocks.   This is an example of 
a solitary scleractinian.  About natural size.

Some anthozoans are solitary organisms, but others may form colonies of individual polyps cemented together that can grow into large, wave resistant structures called reefs.  Living and fossil anthozoans have contributed to the building and formation of organic reefs since Mid-Paleozoic time.  They form large calcareous colonies of individuals that grow up from the sea floor in warm, nutrient-laden marine water.  A modern example of a large reef containing abundant corals is the Great Barrier Reef of Australia.  Ancient and modern reefs also contain a wide variety of other groups of organisms, and may form with only a few or no corals.  Organic reefs provide habitats for a very diverse range of organisms, and reef structures preserved in the rock record are an important source area for petroleum such as oil and natural gas.

MODERN ORGANIC REEFS
Reef-forming organisms are primarily colonial and include such organisms as calcareous algae, sponges, corals 
as well as many others.  Reefs consisting of a variety of taxa have formed throughout most of Phanerozoic time. 
Top: Small barrier reef surrounding Buck Island in the Caribbean. 
 Bottom: Modern reefs consisting mostly of various species of colonial corals and calcareous algae.
(From Berrill, 1966, p. 161;   Lewin 1982, p. 27;   Scholle, Bebout & Moore, 1983, p. 351; D. Huntley, San Diego 
State University)

Ectoprocts and brachiopods have an internal feeding structure called a lophophore, and, in addition, have similarities in other soft part morphology such as a true body cavity (coelom) and other organs.   These two phyla are sometimes called lophophorates because of these similar morphologic features; therefore they will be discussed together. 

Living Ectoprocts A.  Modern bryozoan colony with individuals having their fan-shaped lophophore extended in feeding  mode.  Because of their superficial resemblance to plants these organisms have been  called "moss animals". Bar scale is approximately 4 mm. B. Group of living brachiopods (called lamp shells) attached to algae covered rock substrate.   Each specimen approximately 3 cm in diameter. (A is from Buchsbaum, 1976, plate 172-3;    B is from  Pinet, 2000,  p. 293)

Ectoprocts (also known as bryozoans) are all benthic, colonial organisms, and most are marine dwelling.  Colonies are made of nearly microscopic-sized individuals known as zooids, and most species secrete a calcareous outer skeleton.  Colonies can form as discrete, branched-shaped or encrusted on the surface or rocks or other organisms.  Often, encrusted colonies can be found on the surface of the fronds of  brown algae (seaweed), look for these on seaweed that has washed up on the beach.    Individual bryozoans are filter feeders, trapping particles on the tentacles of their lophophore.  In the geologic record and in modern environments they have made a significant contribution to the formation of organic reefs.  They have left a modest, but continuous fossil record through most of Phanerozoic time.
Geologic Range:  ORDOVICIAN-HOLOCENE.

Morphology Of Lophophorates Examples of ectoprocts (left and center) and brachiopods (right).  Both groups have a distinctive feeding structure called a lophophore.  Identification of ectoprocts  is based on shape of the aperture, internal structures of the individual zooid, and characteristics  of  the colony.  They are usually studied microscopically.  Identification of brachiopods is  based on  composition, structure and external ornamentation of the bivalved shell.  In this figure  the ectoprocts  are greatly enlarged; an individual zooid is usually no larger than a few mm in length;  individual  brachiopods range from a few mm to almost 10 cm in diameter.

FOSSIL ECTOPROCTS
. B.  Portion of a fan-shaped colony (Genus Fenestella), Upper Paleozoic from Texas.   These are branching
 fronds that were attached to a supporting column or stem and anchored to the sea floor.  Bar scale is 0.5 cm.
C.  Portion of a ramose (= branching) colony (Genus Hornera), Cenozoic from England.  Note tiny 
apertures from which the zooids extended their lophophore.   Bar scale is 2 mm.
(A is from Pinna, 1972, p. 36;   B is from Moody, 1977, p. 34;   C is from Murray, 1985, p. 49)

Brachiopods (more common name is lampshells) are all benthic, solitary organisms, and are all marine dwellers.  Individual brachiopods secrete a shell that is made up of two valves that are joined along a hinge-line and held together with internal musculature.  Some brachiopods have relatively small, thin shells of calcium phosphate or calcium carbonate that are held together by muscles.  These are known as inarticulate brachiopods.  Other brachiopods have thicker, more ornamented shells of calcium carbonate, with a hinge line containing a series of teeth and sockets, and they that are held together by the structure of the hinge line by as well as by internal muscles.  These are known as articulate brachiopods.  Brachiopods were very common in Paleozoic seas, but suffered significant extinctions during the later part of the Paleozoic Era and have become much less common in Mesozoic and Cenozoic oceans.  As with bryozoans, brachiopods use the tentacles of their lophophore to capture food.  Some groups of articulate brachiopods became very diversified and abundant during Paleozoic time.   They have provided useful paleoecologic information and have been used extensively for correlation of sedimentary rocks.
Geologic Range:  CAMBRIAN-HOLOCENE.

Phylum: Brachiopoda (Cambrian-Holocene)             Class: Inarticulata (Cambrian-Holocene)             Order: (two orders recognized)         Class: Articulata (Cambrian-Holocene)             Order: (at least six orders recognized)

CLASSIFICATION AND SYMMETRY OF BRACHIOPODS
Left.  Simple classification of Phylum Brachiopoda.
Right.  Photographs illustrating the difference in symmetry between a bivalve mollusc shell (A) and an
 articulate brachiopod shell (B).   By noting the positions of the red dots you can see that shell A is not
 symmetrical in this view, but shell B is symmetrical.  The symmetry in the brachiopod divides the shell 
into two equal halves, but each valve has a different shape (see photos below of articulate 
brachiopods).  The symmetry in most bivalve molluscs is different; each valve is asymmetrical, but the 
two valves are mirror images of each other (see next section on molluscs).

FOSSIL INARTICULATE BRACHIOPODS
A. Inarticulate, linguloid brachiopods with small, thin shells, preserved in light 
brown siltstone of Mid-Paleozoic age; individuals are about 5 mm in length. 
B.  Inarticulate brachiopods with small, thin, rounded shells having concentric 
ornamentation (= growth lines?), preserved in Lower Paleozoic shale;  
individuals about 1 cm across.

FOSSIL ARTICULATE BRACHIOPODS
A, B, C, D.  Four samples of articulate brachiopods illustrating some of the variations in shell morphology 
of the group.   Sample B also has two small, nearly round, inarticulate brachiopods cemented to the 
upper part of its shell; largest brachiopod specimen is about 5 cm in length. 
E.  Brown, fine-grained sandstone (Devonian, New York) with impressions of "winged" articulate 
brachiopods (spiriferid brachiopods); about 6 cm in length. 
Note that the valves of all specimens are bilaterally symmetrical so that each half of the valve is a 
mirror image of the other half; this is much like the symmetry of the human body.  All of these 
specimens are of Paleozoic age, but brachiopods still exist in modern oceans, although they are
 considerably reduced in numbers.

LIVING MOLLUSCS
A.  Clam of the Class Bivalvia. The structures extending from the valves are the siphons, which are used 
for obtaining food and oxygen. (Pearse & Buchsbaum, 1987, p. 358)
B.  Snail of the Class Gastropoda (gastro = stomach; poda = foot). (Pearse & Buchsbaum, 1987, p. 329)
C.  Snail shell sectioned to illustrate internal twisted structure (= torsion). (Fortey, 1982, fig. 16)
D, E.  The pearly Nautilus of the Class Cephalopoda.  This is the only living species of shell-bearing, 
coiled cephalopod.  Nautilus shell sectioned to illustrate internal structure.  The large chamber is the 
living chamber.   (Lewin1982, p. 117;  Fortey, 1982, p. 72)
F.  Squid of the Class Cephalopoda.  Some species have an internal calcareous shell called a "cuttle 
bone". (Pearse & Buchsbaum, 1987, p.366)

Molluscs are one of the most abundant groups of organisms in the marine and fresh-water environments today.  One group, the snails, have even managed to adapt to a terrestrial mode of life.  This phylum is very diverse, and includes such groups as clams, snails, squids, octopi, and a wide variety of other living and extinct forms.  Many biologists consider octopi to be the smartest invertebrate in modern oceans.   Molluscs have a soft body that in most groups is covered by an external shell of calcium carbonate.  The shell is secreted by a structure called a mantle, which is a fold of soft tissue in the body wall.  They have well developed organs and organ systems within a coelomic cavity.  Some molluscs are herbivorous, some are scavengers, but others, such as squid and octopi, are active predators, and still others are passive filter feeders.    Although a number of classes of molluscs are known from living and fossil specimens, we will consider only three major groups in this survey:  Class Bivalvia; Class Gastropoda; and Class Cephalopoda.
Geologic Range: EDIACARAN-HOLOCENE.

MORPHOLOGIC FEATURES OF THREE MAIN CLASSES OF MOLLUSCS
All molluscs have a characteristic internal, soft-part feature called a mantle, which functions in the secretion of the
 calcium carbonate shell. 
Bivalves (left) are classified on the basis of internal shell characteristics such as the hinge line, teeth and sockets 
and patterns of muscle scars.  This is an interval view of one valve. 
Gastropods (center) are classified on the basis of external shell shape, ornamentation and characteristics of 
the aperture. 
Cephalopods (right) are classified on the basis of shell shape and suture lines (the pattern that septa make where 
they join the outer shell).  This is a cross-section view (see Figure E in living molluscs above).

HOW TO TELL A BIVALVE MOLLUSC FROM AN ARTICULATE BRACHIOPOD
Both taxa are bilaterally symmetrical, but the plane of symmetry is oriented differently.  Note position of red dots.
A.  Bivalve Mollusc.  The plane of symmetry divides the valves of the shell into a 
left valve and a right valve, which are mirror images.
B.  Articulate Brachiopod.  The plane of symmetry divides each valves of the shell 
into a left half and a right half, but the two valves are not mirror images.

LIVING AND FOSSIL CLAMS OF THE CLASS BIVALVIA
A, B, C and D are exterior and interior views illustrating some of the variations in shape of bivalve shells of Cenozoic 
age.  The shapes of the shell are often a good indication of where and how the animal makes its living.  All specimens 
are between 3-5 cm in diameter. 
E. Exterior and interior views of a oyster-like bivalve of the Genus Exogyra (Cretaceous, Texas).  These specimens