Phanerozoic Eonothem
The Paleozoic Erathem
Biological Principles

Types of marine environments

The Cambrian Explosion

What is the Cambrian Explosion?

Paleontological Principles

Divisions of time


Fossil types

The first metazoans 

The first shelly faunas















Chronostratigraphic  Division of the Paleozoic Eratherm
Eonothem Phanerozoic
Erathem Paleozoic
Permian System


Devonian System
Silurian System
Ordovician System
Cambrian System


Phanerozoic Eonothem:

This Eonothem is often referred to as the tine of "Visible Life". Organisms with skeletons or hard shells appeared by the first time in the geological record. The Phanerozoic Eonothem spans from 543 mya  through today. The Phanerozoic is divided into three Erathems, from older to younger: Paleozoic, Mesozoic, and Cenozoic.

ThePaleozoic Erathem: 543 to 248  mya

The Paleozoic is delimited by two of the most important events in the history of life. At its beginning, members of Eukarya (multicelled animals) underwent a dramatic "explosion" in diversity, and almost all living animal phyla appeared within a few millions of years. At the other end of the Paleozoic, the largest mass extinction in history wiped out approximately 90% of all marine animal species. The causes of both these events are still not fully understood and the subject of much research and controversy. Roughly halfway in between, animals, fungi, and plants alike colonized the land, the insects took to the air, and the limestone shown in this picture was deposited near Burlington, Missouri
The Paleozoic took up over half of the Phanerozoic, approximately 300 million years. During the Paleozoic there were six major continental land masses; each of these consisted of different parts of the modern continents. For instance, at the beginning of the Paleozoic, today's western coast of North America ran east-west along the equator, while Africa was at the South Pole. These Paleozoic continents experienced tremendous mountain building along their margins, and numerous incursions and retreats of shallow seas across their interiors. Large limestone outcrops, like the one shown above, are evidence of these periodic incursions of continental seas. 

Many Paleozoic rocks are economically important. For example, much of the limestone quarried for building and industrial purposes, as well as the coal deposits of Western Europe and the eastern United States, were formed during the Paleozoic. 

The Lower Paleozoic = Cambrian + Ordovician Systems
The lower Paleozoic is an informal division of the Paleozoic Erathem which includes the lower two systems: the Cambrian and the Ordovician


 Summary of Lower Paleozoic Events:

 1.- Interval of time included between 543 to 438 mya

 2.- First skeletal fossils

 3.- Occurrence of the Tommotian Fauna

 4.- Development of Archaeocyathid reefs

 5.- Trilobites dominate since the upper part of the Lower Cambrian

 6.- Radiation of large animals

 7.- Mass extinction of trilobites at the end of the Cambrian

 8.- Marine sediments are weakly burrowed in the Lower Cambrian

 9.- Marine sediments are heavily burrowed in the Ordovician

 10.- First jawless fishes appeared in the middle Ordovician

 11.- Evolutionary radiation at the base of the middle Ordovician

 12.- Mass extinction of warm-water taxa

 13.- Continental glaciation at the end of the Ordovician

 14.- The landmasses (continents) in the Upper Cambrian are: Gondwanaland, Laurentia, Baltica, and Siberia

 15.- Major plate tectonic reconfiguration in the middle part of the Upper Ordovician

 16.- Predators become very common, e.g.. nautiloids.

 17.- Earliest land plants.

The Cambrian System: It derives its name from the ancient Roman name for Wales (see map below)

Biological Principles: The Evolutionary Processes of Phanerozoic Biota ||The concept of extinction  || Cambrian Explosion: What is the Cambrian Explosion?

Paleoecology: || Types of marine environments

Significant biological events in the Cambrian: The Burgess Shale Fauna  || The Tommotian Fauna  || First Reefs: Archaeocyathids

Geological Principles: Paleogeography || Landmasses: Gondwanaland || Baltica  || Siberia  || Laurentia

 Paleontological   Principles: Divisions of time || Lagerstätten ||
Fossil types: The first metazoans || The first shelly faunas || Cambrian macrofaunas
The Invertebrates:  Poriferaopoda || Mollusca || Brachiopoda || Onychophora || Tardigrada || Arthropoda || Priapulida || Echinodremata || Annelida || Hemichordata || Chordata

The Cambrian is characterized by: (1) the first abundant record of marine life; (2) "The Age of Trilobites" 543 to 500 mya- (3) The Cambrian Explosion of life occurs; (4) all existent phyla develop; (5) Many marine invertebrates (marine animals with mineralized shells: shell-fish, echinoderms, trilobites, brachiopods, mollusks, primitive graptolites). First vertebrates. Earliest primitive fish; (6) mild climate; (7) the supercontinentRodinia began to break into smaller continents (no correspondence to modern-day land masses); (8) Mass extinction of trilobites and nautiloids at end of Cambrian (50% of all animal families went extinct) probably due to glaciation.

The Cambrian System marks an important point in the history of life on earth; it is the time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the "Cambrian Explosion", because of the relatively short time over which this diversity of forms appears. It was once thought that the Cambrian rocks contained the first and oldest fossil animals, but these are now to be found in the earlier Vendian strata.

From Precambrian to the Cambrian these events occurred: (1) The earliest morphological evidence for life at 3.5 billion years, which correspond to fossils of stromatolites (colonies of cyanobacteria) and single, undifferentiated cells, or Prokaryotes; (2) For 1.6 billion years these simple cells were the only kind of living organism, until the arrival of Eukaryotes, or single cells with differentiated nuclei and cell organelles. Although representing a large leap in complexity, the Eukaryotes were still only single cells or cell aggregates; (3) It took another 1.4 billion years before complex, multicellular life made an appearance in the form of the Ediacaran faunas; (4) the true Cambrian animals appeared about 550 million years ago followed, that is, the metazoans or large animals with complex body plans, is termed the Cambrian explosion; (5) In addition to the apparent phyla 'explosion' the Cambrian also sees the advent of a modern type of community structure, with organisms being adapted for a large range of life strategies. These include mobile benthic, epifaunal and burrowing faunas all interacting with each other in complex communities. Predation as a means of acquiring food also makes an appearance at this time. What are even more unusual are the Cambrian organisms which display seemingly bizarre body plans and appendages, and which taxonomically are difficult to assign to any modern phyla (see Burgess Shale type faunas).
In summary, the Cambrian fossil record indicates a distinct development from simple organisms to organisms comparable in morphology and organization to the present-day animals. This rapid phylogenetic development started in the latest Proterozoic and was more-or-less finished at the end of the Early Cambrian. The development of faunas in the Cambrian is documented by faunal assemblages represented by (1) the Ediacara fauna, (2) the first complex trace fossils, (3) the earliest shelly faunas, and (4) the onset of the typical Cambrian macrofaunas. It is amazing that this rapid evolution took place in an interval of less than 25 m.y., and the evolution from the first hard-part animals to the presence of most of the present-day phyla was restricted to an interval of probably less than 10 m.yMulticellular life evolved at an incredible supersonic speed, and for this reason this part of organismal evolution is termed the "Cambrian Explosion", or "Evolution's Big Bang."
Cambrian Macrofaunas

The Marine Environment in the Cambrian

The Ordovician System: 505 to 438  mya .

The Ordovician is best known for the presence of its diverse marine invertebrates (invertebrates dominate), including graptolites, trilobites, brachiopods, and the conodonts (early vertebrates). A typical marine community consisted of these animals, plus red and green algae, first fishes (primitive fish), cephalopods, corals, crinoids, and gastropods. More recently, there has been found evidence of tetrahedral spores that are similar to those of primitive land plants, suggesting that plants invaded the land at this time.
High sea levels at first, global cooling and  glaciation , and much volcanism. North America under shallow  seas. Ends  in huge extinction due to  glaciation. From the Early to Middle Ordovician, the earth experienced a milder climate in which the weather was warm and the atmosphere contained a lot of moisture. However, when Gondwana finally settled on the South Pole during the Late Ordovician, massive glaciers formed causing shallow seas to drain and sea levels to drop. This likely caused the mass extinctions that characterize the end of the Ordovician, in which 60% of all marine invertebrate genera and 25% of all families went extinct. Silurian System: 438 to 408  mya. First terrestrial plants and animals.
 The first jawed fishes and  uniramians  (like insects, centipedes and millipedes) appeared during the Silurian (over 400 million years ago). First vascular plants (plants with water-conducting tissue as compared with non-vascular plants like mosses) appear on land (Cooksonia  is the first known). High seas  worldwide.Brachiopods , crinoids, corals. Devonian System: "The Age of Fishes" 408 to 360  mya . First amphibians, ammonites, fishes abundant.
Fish and land plants become abundant and diverse. First  tetrapods  appear toward the end of the period. First amphibians appear. First sharks, bony fish, and  ammonoids. Many  coral reefs, brachiopods, crinoids. New insects, like springtails, appeared. Mass extinction (345  mya) wiped out 30% of all animal families) probably due to  glaciation  or meteorite impact. Carboniferous: Wide-spread coal swamps,  foraminiferans, corals, bryozoans, brachiopods,  blastoids , seed ferns,  lycopsids , and other plants. Amphibians become more common. 360 to 280  mya Mississippian Sub-System: 360 to 325  mya: Sharks and amphibians abundant. Large and numerous scale trees and seed ferns. First winged insects. Pennsylvanian Sub-System: 325 to 280  mya. Great coal forests, conifers. First reptiles .First reptiles. Many ferns. The first may flies and cockroaches appear. Permian System: "The Age of Amphibians" 280 to 248  mya.Mass  extinction, most kinds of marine animals, including trilobites.
Southern  glaciation . "The Age of Amphibians" - Amphibians and reptiles dominant. Gymnosperms dominant plant life. The continents merge into a
single super-continent, Pangaea. Phytoplankton and plants oxygenate the Earth's atmosphere to close to modern levels. The first stoneflies, true bugs, beetles, and  caddisflies, The Permian ended with largest mass extinction. Trilobites go extinct, as do 50% of all animal families, 95% of all marine species, and many trees, perhaps caused by  glaciation  or volcanism.


Lagerstätten (sing.  lagerstätte) : are fossil localities which are highly remarkable for either their diversity or quality of preservation; sometimes both. There are two famous Cambrian age  lagerstätten  known to almost everybody with an interest in paleontology   – the Burgess   Shale in Canada, and  Chengjiang  in China. Other very well-known  lagerstätten  include the Green River Formation of Wyoming, USA, which has produced countless fossil fish for the commercial fossil market, the  Solnhofen  Limestone of Germany, famous primarily for the  Archeopteryx  fossils found there.

The Tommotian fauna  This fauna represents the Tommotian Age, which began about 530 million years ago, is a subdivision of the early Cambrian. Named for rock exposures in Siberia, the Tommotian saw the first major radiation of the animals, or metazoans, including the first appearance of a great many mineralized taxa such as brachiopods, trilobites, archaeocyathids, molluscs, echinoderms, and more problematic forms. Soft-bodied members of many other phyla were also appearing and diversifying at this time.

A few million years before the Tommotian, in the Ediacaran (Vendian), the continents had been joined in a single supercontinent called Rodinia (from the Russian word for "homeland", rodina.) As the Cambrian began, Rodinia began to fragment into smaller continents which did not always correspond to the ones we see today. Much later, in the Permian, the continents came back together to form a new supercontinent, called Pangaea.

The Nature of the Tomotian Fauna: A much richer fauna appears abruptly in the middle portion of the Lower Cambrian record. This so-called Tomotian fauna, named for the Tommotian Stage of Early Cambrian time, was first discovered in Siberia. It includes a host of small skeletal elements that cannot be assigned to any living phylum and that show no relation to any group of fossils found in post-Cambrian rocks. The Tommotian fauna also contains the oldest known mem¬bers of a few groups that survive to the present day¬sponges, which are very simple animals monoplacophorans, which were ancestral to all present-day groups of mollusks.

The Burgess Shale fauna
Strata in western North America have yielded a spectacular fauna of Middle Cambrian soft-bodied animals that invites comparison with the Early Cambrian Chengjiang fauna described earlier. The largest group of species in the North American soft-bodied fauna comes from the Burgess Shale, in the Rocky Mountains of British Columbia (Figure 13-10). Laterin this chapter we will examine the environment in which the Burgess Shale formed, but for now we can simply note that it accumu¬lated in a deep-water setting where soft-bodied animals were buried in the absence of oxygen and bacterial decay. Among the Burgess Shale fossils is a species that represents the Chordata, the phylum to which verte¬brate animals belong. Pikaia, the chordate genus of the Burgess Shale fauna, possessed a notochord-the struc¬ture that, in some Cambrian animal group that may never be singled out, evolved into a backbone. Recall that the lancelet possesses only a noto¬chord today.

Arthropods are the most abundant of the Burgess Shale fossils, and some of them resemble certain of the Chengjiang taxa . Also present are anomalocarids. In addition, both the Chinese and North American fau¬nas include onychophorans. Elongate animals with jointed legs, onychophorans are generally intermediate in form between segmented worms and arthropods. Today members of this group live as pred¬ators on moist forest floors, having somehow invaded the land. Priapulid worms also occur in the Burgess Shale fauna, along with several types of seg¬mented worms. An overall comparison of the Chinese fauna with the younger North American fauna indicates that evolutionary changes between Early and Middle Cambrian time were relatively minor for soft-bodied in¬vertebrate animals.

The earliest vertebrates Conodonts also diversified in the course of the Cambrian Period. Their teeth, which are abundant in the fossil record, reveal nothing of their body form, but the recent discovery of fossils of their soft bodies has shown them to have been small swimming animals; the teeth themselves indicate that conodonts were the earliest known vertebrate animals. Similar small teeth in very early Cambrian faunas may represent conodont ancestors.

Reefs The oldest organic reefs with skeletal frameworks are low mounds that formed in Lower Cambrian time, beginning in the Tommotian. The main builders of these reefs were archaeocyathids, which apparently were suspension feeders that pumped water through holes in their vase-shaped and bowl-shaped skeletons. Archaeocyathids were probably sponges, the simplest of which resemble them in general body plan. Although archaeocyathids were the primary frame builders of Lower Cambrian reefs, organisms of unknown taxonomic relationships actually contributed a larger volume of calcium carbonate to these reefs by encrusting archaeocyathid skeletons and binding them together. At the end of Lower Cambrian time, nearly all archaeocyathids became extinct. From then until mid-Ordovician time, all that remained were small, inconspicuous reeflike structures formed by the en¬crusting organisms that had previously lived with the archaeocyathids.

The Evolutionary Process of Phanerozoic Life
Major groups of animals are already present in the Phanerozoic, at the beginning  of the Cambrian System

The opisthokonts (Greek: ??????- (opisth?-) = "rear, posterior" + ?????? (kontos) = "pole" i.e. flagellum) are a broad group of eukaryotes, including both the animal and fungus kingdoms, together with the phylum Choanozoa of the protist kingdom. Both genetic and ultrastructural studies strongly support that opisthokonts form a monophyletic group. One common characteristic is that flagellate cells, such as most animal sperm and chytrid spores, propel themselves with a single posterior flagellum. This gives the groups its name. In contrast, flagellate cells in other eukaryote groups propel themselves with one or more anterior flagella.



See Biological Principles for a review of these concepts

Schizocoels: A group of animal phyla, including Bryozoa, Brachiopoda, Phoronida, Sipunculoidea, Echiuroidea, Priapuloidea, Mollusca, Annelida, and Arthropoda, all characterized by the appearance of the coelom as a space in the embryonic mesoderm.

Lophophore: A horseshoe-shaped ciliated organ located near the mouth of brachiopods, bryozoans, and phoronids that is used to gather food.

The sponges or poriferans: Po·rif·e·ra (n. pl.)
[NL., fr. L. porus pore + ferre to bear.]
 A grand division of the Invertebrata, including the sponges; -- called also Spongiæ, Spongida, and Spongiozoa. The principal divisions are Calcispongiæ, Keratosa or Fibrospongiæ, and Silicea.
The sponges or poriferans (from Latin porus "pore" and ferre "to bear") are animals of the phylum Porifera. They are primitive, sessile, mostly marine, water dwelling filter feeders that pump water through their bodies to filter out particles of food matter. Sponges represent the simplest of animals. With no true tissues (parazoa), they lack muscles, nerves, and internal organs. Their similarity to colonial choanoflagellates shows the probable evolutionary jump from unicellular to multicellular organisms. There are over 5,000 modern species of sponges known, and they can be found attached to surfaces anywhere from the intertidal zone to as deep as 8,500 m (29,000 feet) or further. Though the fossil record of sponges dates back to the Neoproterozoic Era, new species are still commonly discovered.
Sponges are a diverse group of sometimes common types, with about 5000 species known across the world. Sponges are primarily marine, but around 150 species live in fresh water. Sponges have cellular-level organization, meaning that that their cells are specialized so that different cells perform different functions, but similar cells are not organized into tissues and bodies are a sort of loose aggregation of different kinds of cells. This is the simplest kind of cellular organization found among parazoans.
One of the divisions of phyla of the animal kingdom containing snails, slugs, octopuses, squids, clams, mussels, and oysters; characterized by a shell-secreting organ, the mantle, and a radula, a food-rasping organ located in the forward area of the mouth.
Brachiopods are marine animals that, upon first glance, look like clams. They are actually quite different from clams in their anatomy, and they are not closely related to the molluscs. They are lophophorates, and so are related to the Bryozoa and Phoronida.
Although they seem rare in today's seas, they are actually fairly common. However, they often make their homes in very cold water, either in polar regions or at great depths in the ocean, and thus are not often encountered. There are about 300 living species of brachiopods.
The brachiopods are a large group of solitary and exclusively marine organisms with a very good geologic history throughout most of the Phanerozoic and are among the most successful benthic macroinvertebrates of the Paleozoic. They are typified by two mineralized valves which enclose most of the animal. Like the bryozoans, brachiopods are filter feeders which collect food particles on a ciliated organ called the lophophore. An excellent example of a brachiopod lophophore can be seen in the Recent terebratulid.  Brachiopods differ in many ways from bryozoans (in both soft and hard-part morphology), and are thus considered by most workers as a separate but closely related phylum. However, one of the most distinguishing features of brachiopods is the presence of a pedicle, a fleshy stalk-like structure that aids the animal in burrowing and maintaining stability. The pedicle can be seen in the Recent Lingula.  Currently, brachiopods are divided into two or three major groups. We depart from your text in considering two major groups: Class Inarticulata (including lingulids), and Class Articulata based on the presence or absence of hinge teeth and sockets.

Brachiopods superficially resemble bivalve mollusks in that the animal secretes a bivalved (two-part) shell of calcium carbonate or a combination of calcium phosphate and chitinous organic substance.  However, Bivalve mollusks generally have shells that are equal in size and shape (although mirror images of each other), whereas the two shells of brachiopods are of unequal size (the technical term is inequalvalved).  The valve (shell) that has the attachment for the pedicle is the pedicle valve which is usually the lower and larger valve.  This valve includes the pedicle opening.

Ecology and physiology
A. Marine, mostly within the shelf, but some forms are abyssal.
B. Benthic : sessile or burrowing. Substrates: rock, shells, algal stems, soft sediment.
C. Not colonial but tend to aggregate intraspecifically and interspecifically, which seems to depend on the larval settlement.
D. Inhabit largely the cold waters, but are present in all latitudes. Usually highly endemic.
E. Feeding biology
1. Feed on fine phytoplankton (diatoms), and dissolved and colloidal material.
2. Adjustable ciliary current created by the lophophore ciliation. Distinct inhalant and exhalant apertures. Particles are trapped on the filaments and converge in the groove down to the mouth.  There is mucus, but it seems not to play a major role here.
3. In burrowing species, the mantle setae prevent fouling by the sediment.  Mucus is secreted by the gland zone of the mantle lobes.
4. Some species can reverse the water current when particles accumulate within the lophophore.
5. Ingestion is controlled by peristaltic movements of the esophagus and stomach.
F. Predators : fish, men.
G. Parasites : gregarines.