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Precambrian Period:
Hadean Eon: 4.5-3.8 Billion Years Ago
Archean Eon: 3.8-2.5 Billion Years Ago
Proterozoic Eon: 2.5 Billion to 542 Million Years Ago

Our museum is designed to explore permineralized plant material through geologic time. The evolution of plants that we recognize as trees starts in the Devonian, thus we focus our attention on the Phanerozoic eon, which includes the Paleozoic, Mesozoic and Cenozoic eras. Before starting our adventure in the Cambrian, let’s note some important fossil evidence found in the Archean and Proterozoic eons, once known as the Precambrian. The Precambrian also includes the Hadean eon, which is represented by meteorites and dates from 4.5 to 3.8 billion years. The rock record for earth starts at around 3.8 billion years in the Archean eon.

Evidence of Life

The Archean eon stretches from 3.8 Ga to 2.5 Ga. The symbol Ga is used to represent a giggannum, a unit equal to 10 raised to the 9th power or one billion years. The symbol Ma is used to denote a megaanum, a unit equal to 10 raised to the 6th power or 1 million years. Evidence for the first life and photosynthesis comes from multiple lines of evidence, which include: organic chemical signatures, fossils, and rock structures representing biogenic activity.


Isotopic Ratios

Organic chemical signatures or biomarkers are chemical fossils that may be in the form of isotopic ratios or molecular fossils. The first chemical evidence of photosynthesis, in the form of C-12 to C-13 ratios, can be found in Archean rock of 3.8 Ga from Isua, Greenland (Kenrick & Davis, 2004, pp 10-11; Johnson & Stucky, 1995, p 22). The process of photosynthesis preferentially utilizes C-12 over C-13 when removing CO2 from the air to synthesize carbohydrates, creating ratios of these carbon isotopes that differ from normal background ratios. Thus, carbon compounds processed by photosynthetic organisms are enriched with C-12 (Rich & Fenton, 1996, p 91). The enrichment of C-12 in rocks is a test for the presence of life. Carbon isotope ratios consistent with presence of cyanobacteria are widespread in rock dated at 3.5 Ga (Johnson & Stucky, 1995, p. 22). One problem with the evidence above is the fact that chemical pathways in non-photosynthetic autotrophs and nonautotrophs can produce C-12 enrichment (Blankenship, Sadekar, & Raymond, 2007, pp 22-23).


Stromatolites & Oncolites

The most common fossil structures attributed to cyanobacteria are stromatolites and oncolites (Stinchcomb, 2008, p. 16). The earliest evidence for life from structural fossils comes from rock outcrops in Australia (dated at 3.5 Ga) and Africa (dated at 3.2-3.4 Ga). Both rock outcrops contain cherts with stromatolite structures. Living stromatolites are communities of cyanobacteria and other microbes, which form a mat-like structure. As filaments of this structure become inundated with sediment the living material migrates to the top to form a new photosynthetic mat-like layer. In this way a mound-shaped structure made of alternating layers of organic matter and sediment is created. Structures resembling prokaryotes are associated with both locations. However, nonbiological processes can produce similar structures and the biogenic origin of these stromatolites is debated (Nudds & Selden, 2008, p. 23). There are examples of stromatolites from the Proterozoic eon (2.5 Ga-542 Ma) that contain microfossils similar to modern forms. Stromatolites are also found in geologic environments consistent with biologic activity (Kenrick & Davis, 2004, p. 14). Oncolites are similar to stromatolites, but form as spherical concretions formed by layers of calcium carbonate.



No consensus is to be found concerning the definition of what constitutes a reef. For the purpose of our museum we will use a definition modified from Rachael Wood. Reefs form as a result of organic activity, developing due to the aggregation of sessile organisms living on the sea-floor, with a resulting higher concentration of carbonate production than the surrounding sea sediments (Wood, 1998, p. 182). Reef structures often serve as a habitat for a community of organisms. Reef structures eventually build rock, often in the form of large carbonate platforms. Stromatolites formed the first reef structures, the oldest of which are late Archean (Webb, 2001, p. 172).


Chemical Fossils
Chemicals produced by living organisms can be incorporated into sediments and eventually into rock. Many of these chemicals become altered in known ways and can be stable for billions of years. These chemicals are known as biomarkers and represent molecular fossils (Knoll, summons, Waldbauer, & Zumberge, pp. 134-135). The isotopic ratios discussed above are organic chemical signatures or biomarkers that can also be classified as chemical fossils. The first widely accepted evidence for the occurrence of cyanobacteria comes from 2.7 Ga old biomarkers. Biomarkers indicate a widespread occurrence of cyanobacteria at 1.6 billion years as well as the first appearance of photosynthetic green sulfur bacteria and proteobacteria (Blankenship, Sadekar, & Raymond, 2007, pp. 24-25).


Banded Iron Formations

Banded iron formations, known as BIF's, are ancient sedimentary deposits consisting of alternating layers of iron rich and silica rich layers. BIF's started out as sediments deposited on ancient ocean floors. BIF deposits are huge and can extend for hundreds of kilometers with uninterrupted banding (Perkins, 2009, p. 24).

In 1965 Preston Cloud realized that banded iron formations (BIF) provided clues to Earth’s early atmosphere and oceans. Evidence of low oxygen levels in the atmosphere and high levels of dissolved iron in the oceans is provided by the formation of red beds. Banded iron formations were formed only during a narrow range of time; dating from 2.8 Ga to 1.8 Ga. BIF formation reached a peak at 2.5 Ga and is rare in rocks younger than 2.0 Ga (Cowen, 2005, pp. 25-27). During this time, iron was deposited into the oceans by deep sea hydrothermal vents and terrestrial erosion. Iron dissolves in water that has no oxygen. In the presence of oxygen, iron chemically reacts forming an insoluble iron oxide, which precipitates out of solution. In the organic model for BIF formation oxygen produced from the process of photosynthesis, carried out by cyanobacteria, accounts for the formation of the iron rich bands.

The first photoautotrophs expelled oxygen into the seawater, where it was free to oxidized iron. The precipitates from this oxidation process settled into layers forming the distinctive red formation rich with hematite and magnetite, alternating with silicates, such as jasper and chert. The banding pattern may represent seasonal temperature variations. During the summer photosynthesis and thus oxygen levels would increase, chemically reacting with the dissolved iron and precipitating to form the iron rich sediment layers. During winter as photosynthesis decreased silicate rich layers formed (Perkins, 2009, p. 24).

These banded iron formations are a testament to that fact that biological activity was rusting the planet (Nudds & Selden, 2008, pp. 21-22). Today, the modern steel industry mines the iron laid down by bacteria over 1.8 billion years ago. The steel frames of cars and skyscrapers are made possible due to the biologic activity of Precambrian photosynthetic bacteria (Cowen, 2005, pp. 25-26).


Oxygen in the Atmosphere

BIF's record the transition from an early Earth that had low levels of free oxygen and high amounts of dissolved iron to an Earth with high levels of free oxygen and low levels of dissolved iron. Multiple lines of evidence suggest an atmosphere free of oxygen before and during early BIF formation. Ancient deposits of sand and gravel record that iron and sulfur were in reduced states. Pyrite (iron disulfide), siderite (iron carbonate), and uraninite (uranium dioxide) exist as sedimentary grains in river deposits older than 2.2 Ga. These minerals break down in the presence of oxygen and today are not found among sediments on coastal floodplains (Knoll, 2003, pp. 96 & 97).

The formation of BIF's does indicate that the Earth underwent a global redox reaction. The last of the banded iron formations occurs at around 1.8 Ga. After a staggering 1 billion years of forming red beds around the world there was less free iron to chemically react with the oxygen produced through photosynthesis. At some point during the billion year period of red bed formation the production of oxygen outpaced the production of chemicals that consumed oxygen. Thus, oxygen could flood the atmosphere.

The rock record indicates that Earth has experienced two major increases in oxygen levels. The first rise in oxygen levels is known as the Great Oxidation Event (GOE) and occurred at around 2.4 Ga (Catling, 2008, p. 17). Chemical signatures for oxygen build up in the atmosphere on a global scale occur between 2.4 Ga and 2.3 Ga (Blankenship, Sadekar, & Raymond, 2007, p25). As we have already mentioned oxygen sensitive minerals could exist as eroded grains in sedimentary deposits aged 2.2 Ga and older. As these oxygen sensitive minerals fade from sedimentary deposits sedimentary rock requiring oxygen rose to prominence (Knoll, 2003, p. 97). Red beds, which are terrestrial sedimentary deposits rich in iron oxides, start forming at around 2.3 Ga and become increasingly common after 2.2 Ga (Cowen, 2005, p. 27). Sulfur isotopes in rocks also indicate an increase in the amount of sulfates in ancient sediments, a sure sign that sulfur was being oxidized. Oxygen levels rose from less than 1 ppm to 0.3 to 0.6% during GOE (Catling, 2008, p. 18).

Thus, biological activity started to change an atmosphere made mostly of methane, carbon dioxide, and nitrogen to one that was enriched with oxygen. Methane, produced by Archaea bacteria, acted as a greenhouse gas, warming the Earth during the Archean Eon. Methane breaks down in the presence of free oxygen. So, the build up of oxygen cooled the planet by lowering methane levels. Evidence for large ice ages occur between 2.4 and 2.2 Ga. Atmospheric oxygen also started to form the ozone layer high in the stratosphere. The ozone layer absorbs harmful ultraviolet radiation (UV), critical for evolving life on Earth (Cowen, 2005, pp 27-28).

Cyanobacteria were creating an "oxygen revolution" by converting the early reducing atmosphere into an oxidizing atmosphere. A second spike in oxygen levels took place around 580 Ma. At this time atmospheric oxygen increased up to 15% or more of present levels. This second increase is closely correlated with the first appearance of animals in the fossil record (Catling, 2008, p. 18).

Prokaryotic Structural Fossils

The first universally accepted occurrence of prokaryotic structural fossils comes from the ~2 Ga Gun Flint Formation of Ontario, Canada. This banded iron formation was recently dated at 1.878 Ga. The BIF is made of alternating layers of iron oxides and cherts. The cherts contain stromatolite structures and a variety of microfossils that resemble present day cyanobacteria (autotrophs) and iron and manganese-loving bacteria (chemotrophs). The Gun Flint Formation most likely represents deposition on a continental shelf (Nudds & Selden, 2008, pp. 13-21).


Eukaryotic Structural Fossils

The first undisputed occurrence of fossil eukaryotes comes form the Huntington Formation of Summerset Island, Arctic Canada. The formation is dated at 1.2 Ga. Tidal flat deposits preserve red algae, stromatolite structures, and cyanobacteria (Nudds & Selden, 2008, p.23). The eukaryotic fossils display patterns of thallus organization, cell division, and differentiation that represent bangiophyte red algae (Knoll, Summons, Waldbauer, & Zumberge, 2007, p.150). Chert nodules in the 700-800 million year old Draken Conglomerate Formation of Spitsbergen contain microfossils of cyanobacteria as well as a diverse biota of eukayotes. The Draken Conglomerate Formation represents a tidal flat/lagoon complex (Nudds & Selden, 2008, p. 24). The oldest sponge spicules date to 800 Ma. The first whole sponge dates to 570 Ma (Cooper, 2001, p. 98).


The First Primary Producers

Although eukaryotes are present in the Proterozoic, evidence from fossils and biomarkers indicate that primary production on Earth started with and was dominated by cyanobacteria. Eukaryotes, in the form of green algae, played a limited role in primary production for the last 600 million years of the Proterozoic eon (2.5 Ga-542 Ma) (Knoll, Summons, Waldbauer, & Zumberge, p. 152). Cyanobacteria, green algae, and red algae would continue to be the major primary producers in the oceans during the Paleozoic eon. It could be that cyanobacteria dominated the role of primary producers because ocean conditions favored photoautotrophs capable of nitrogen fixation. To this day cyanobacteria are still important for nitrogen fixation and take on the role of primary producers in some parts of the oceans.


In addition to creating Earth’s oxidizing atmosphere and taking on the role of the first primary producers there is strong evidence that the double membrane bound organelles found in eukaryotes formed as a result of endosymbiotic relationships among prokaryotes. The now well supported Endosymbiotic Theory was proposed by the American Biologist Lynn Margulis in 1966. Multiple lines of evidence support the theory. To name just a few, organelles, such as mitochondria and chloroplasts are the same size as bacteria, retain functional bacteria-like DNA, make some of their own proteins with ribosomes identical to bacteria, possess an inner membrane that is similar in structure to that found in bacteria, and replicate via binary fission independently of the cell. Although not the first to propose this evolutionary explanation it was the hard work and persistence of Margulis and many other scientists that led to the current theory, which is the best plausible explanation for the evolution of eukaryotes. The Endosymbiotic theory was finally popularized in Margulis 1981 book Symbiosis in Cell Evolution.


Ediacaran Fauna

The period of time spanning 670 Ma to about 540 Ma is referred to as Ediacaran or Venadian period and is characterized by some very interesting soft-bodied multicellular organisms. The first embryos of multicellular organisms also occur in Venadian deposits. Cherts and shales in the Doushantuo Formation of Southern China, dated at 600 Ma, contain microfossils of red algae and spheres interpreted as animal embryos (Nudds & Selden, 2008, p. 24). Some of the embryos look very similar to embryos of invertebrate animals with bilateral symmetry (Palmer, 1999, p. 57).


The first multicellular animals are well represented by the Ediacara biota, which consist of soft-bodied marine organisms. Ediacara type fossils are found in deposits ranging from 670 Ma to 540 Ma. Although first found in the Ediacara hills of Australia north of Adelaide similar deposits are now known from the UK, Africa, Canada, U.S., China, etc. (Selden & Nudds, 2004, p. 11).


The Ediacara represents a Fossil-Lagerstatten. Lagerstatten is a German word used in mining that denotes a particularly rich seam (Selden & Nudds, 2004, p. 7). Organisms with hard parts that habitat environments where sedimentary deposition occurs are most likely to form fossils. Fossil deposits with soft-bodied organisms well preserved or with terrestrial animals, such as dinosaurs in the Morrison formation are termed Lagerstatten because they give us a window into past environments seldom preserved in the fossil record. Two types of fossil lagerstatten are recognized. Deposits that contain vast numbers of fossils represent Concentration Lagerstatten. The preservation may not be exceptional, but the great numbers can be very informative. Conservation Lagerstatten contain fossils with soft body preservation, impressions of soft tissue or fossils of well-articulated skeletons without soft tissue preservation. Conservation Lagerstatten allow paleontologists to better understand the morphology and phylogentic relationships of organism, provide knowledge of soft-bodied organisms, and reconstruct paleoecosystems (Nudds & Selden, 2008, pp. 8-9).


Some of the Ediacara biota seem to represent the first occurrences of jellyfish, sea pens, arthropods and flatworms. Other members of this fauna represent extinct groups with no descendants. Some scientists place the Ediacara biota in their own kingdom called the Vendozoa, removing them from Metazoa (Selden & Nudds, 2004, p16).

The Ediacara biota represents a shallow marine environment unlike any known today. These organisms had no mouths or stomachs and bodies with large surface to volume ratios. Nutrients and wastes were probably exchanged with the seawater through diffusion. It is possible that some of the Ediacara fauna obtained food through a symbiotic relationship with algae (Johnson & Stucky, 1995, p. 12).


The First Mass Extinction

Evidence of the first mass extinction occurs during the Proterozoic eon. Acritarchs are durable life stages of single-celled algae. Acritarchs appear around 1.4 Ma and become greatly diversified by 700 Ma. Towards the end of the Proterozoic eon at around 650 Ma many Acritarch species meet extinction. The extinction event coinsides with widespread glaciation (Stanley, 1987, p. 52-53).

Towards the end of the Proterozoic eon organisms evolved exoskeletons and shells. The development of these hard structures greatly increased the likelihood of preservation. It is the evolution of these hard parts that defines our next time period, the Cambrian.

Cambrian Period: 542-488.3 Million Years Ago

The Cambrian period extends from 542 to 488.3 million years ago. Adam Sedgwick (1785-1873), an English geologist, mapped marine strata in North Wales, which lay between Precambrian and Silurian strata. Cambrian is derived from Cambria, the Latin name for Wales (Palmer, 1999, p. 58).

The Cambrian Explosion

Cambrian deposits are found on all continents and represent the earliest widespread occurrence of fossils. Many of the major marine groups appear during a 10 million year period, a geologically short time, which has fostered the notion of the Cambrian Explosion. There are approximately 35 extant animal phyla today and most of these phyla along with many that are now extinct appear during this time (Selden & Nudds, 2004, p. 19). However, as evidence for Precambrian animals grows the popular notion of a "Cambrian Explosion" fades.

Organisms with shells and exoskeletons become abundant in the Cambrian and help to explain why fossils are more numerous (Johnson & Stucky, 1995, p. 25). In fact, the abundance of these shells along with their wide spread appearance make correlating rock from different locations based upon fossil content possible (Thompson, 1982, p. 42-43).


Primary Producers & Reefs

Although most of today’s phyla are represented in the Cambrian the world would have looked much different than today. Terrestrial environments were uninhabitable, as land plants had not yet evolved (Waggoner & Collins, 1994 Cambrian Life page). Cyanobacteria and green algae populating the shallow seas took on the role of photosynthesis (Knoll, Summons, Waldbauer, and Zumberge, 2007, p. 148).

The first reef systems built by metazoans or animals appear in the Cambrian. Cambrian reef systems were made of stromatolites, archaeocyathans, and calcimicrobes (Kiessling, 2001, p. 44). Archaeocyathans were small cone-shaped animals possibly related to sponges. Their double-walled calcareous cone structure was perforated with many holes. Calcimicrobe is a term used to describe colonial microbes that secrete calcium carbonate. Calcimicrobes, like Renalcis, acted to bind a framework produced by archaeocyathans (Web, 2001, p. 174). These frameworks built by Archaeocyathans and calcimicrobes represent the first reef systems made, in part, by animals.


Archaeocyathans diversified into many forms, but would go extinct at the end of the lower Cambrian. Calcimicrobes continued to build reefs after the extinction of archaeocyathans. Towards the end of the Cambrian reef building was once again dominated by stromatolites (Web, 2001, p. 174).


Invertebrates & Vertebrates

The late Venadian and earliest stages of the Cambrian (546-520 million years ago) are dominated by small shelly fossils or "little shellies". Little shellies come in a variety of shapes and measure only a few centimeters in length. Little shellies are thought to represent primitive mollusks, sponge spicules, and chain mail armor for worm-like organisms. Most of the small shelly fossils are made of calcium phosphate. Calcium phosphate is the same mineral that makes up the bones of vertebrates. Some scientists believe that the preference of using calcium phosphate over calcium carbonate at this time to construct mineralized skeletons may indicate lower atmospheric oxygen levels. Early Cambrian deposits contain abundant evidence of burrowing indicating the existence of soft-bodied worms with a true coelom. Larger invertebrates with shells and exoskeletons, like brachiopods, trilobites, and archaeocyathans, start to appear at around 530 million years ago. A great increase in animal diversity occurs at around 520 million year due mostly to trilobites (Prothero, 2007, pp. 165-167).


As we have already noted archaeocyathans were the first invertebrates to act as reef builders. One of the first animals with eyes (trilobites) and the first predators appear in the Cambrian period. Perhaps predator-prey interactions and the evolution of sight fueled natural selection as the ecosystems of the Cambrian became more complex than that found in the later Proterozoic (Neoproterozoic Era).


Trilobites are one of the most abundant fossils in the Cambrian deposits. Trilobita represents an extinct class of the phylum Arthropoda. The trilobite exoskeleton is divided into three segments both longitudinally and latitudinal. The name trilobite refers to the longitudinal segments. Their exoskeleton is made chitin (a polysaccharide) and arthropodin (a protein). The dorsal side of the trilobite exoskeleton is reinforced with the mineral calcite. The exoskeleton was shed at different stages of growth making trilobite preservation more probable (Nudds & Selden, 2008, p. 57). In fact, many trilobite fossils represent the dorsal, calcite reinforced, part of a shed exoskeleton. Many trilobite species had compound eyes made of calcite lenses. Trilobites evolved and diversified for over 300 million years (Fortey, p. 14).

In addition to trilobites, inartiuclate brachiopods also diversified and became abundant during the Cambrian (Kazlev, 2002). Bryozoans, sponges, echinoderms, gastropods, crinoids, nautiloid cephalopods, conodontophoras, planktonic graptolites and fish (jawless) were rare and not diverse.

The Burgess Shale

The Burgess Shale of British Columbia, Canada provides the clearest window into the Middle Cambrian (505 million years ago) fauna and flora. The Burgess Shale, the most well known Fossil-Lagerstatten, preserves over 120 animals (Johnson & Kirk, 1995, p. 26). Approximately 85% of the genera preserved in the Burgess Shale are soft-bodied animals that are not represented by other Cambrian assemblages (Selden & Nudds, 2004, p 19). This fact underscores the misleading nature of the fossil record and highlights the importance of Fossil-Lagerstatten. The Burgess Shale represents a benthic community living on a muddy seabed at the foot of a submarine cliff in a tropical marine environment (Selden & Nudds, 2004, p. 26).

Many organisms in the Burgess Shale are difficult to classify. Anomalocaris is often referred to as the “monster predator” of the Burgess Shale (Selden & Nudds, 2004, pp. 25-26). The head has two large eyes and two grasping arms. The body is covered with overlapping flap-like structures and the tail adorned with an erect fan-like structure. Its mouth resembled a “pineapple ring” constructed of overlapping plates and serrated prongs possibly good for crushing prey. At 1m, Anomalocaris, was the largest predator of the Burgess Shale (you can click on the image above to learn more). Opabinia had five eyes, a trunk like proboscis/grasping organ, and an overlapping body flaps and fantail reminiscent of Anomalocaris. The segmented body of these organisms may place them in the phylum Arthropoda. The mouth of Anomalocaris could place it in with pseudocoelomate worms. Another possibility is that they represent a transition between Lobopodia and Arthropoda (Kazlev, Anomalocaris page). Ottoia (Phylum Priapulida) was a mud dwelling carnivorous worm with a bulbous proboscis surrounded by hooks and spines (click on picture to the right). Hallucigenia (Phylum Onychophora) is a marine velvet worm that crawled on fleshy limbs scavenging the seafloor. Hallucigenia had protective spines lining its dorsal side. The first chordate is found in the Burgess shale. Pikaia was a 5cm long lancelet-like creature with small tentacles on the anterior end and a fin-like tail on the posterior end (click on the picture to the left). Pikaia’s flattened body was divided into segmented muscle blocks running the length of the body. Embedded between the lateral muscle blocks was a stiff notochord, a prerequisite for making backbones found in vertebrates. The anatomy of Pikaia indicates it is a choradate, the phylum to which all vertebrates belong (Palmer, 1999, p. 66).



The Chengjiang Lagerstatten (515-520 Ma) in Southern China helps add to the Burgess Fauna and predates it by 10 to 15 million years (Clowes, 2006, Chengjiang Lagerstatte page). Fossils are exposed in Maotianshan shale Member of the Yuanshan Formation and represent a deltaic environment (Nudds & Selden, 2008, p. 54). The Chengjiang Lagerstatten contains a variety of soft-bodied and biomineralized metazoans that add to our knowledge of the Cambrian explosion. Chengjiang biota is very diverse and includes: Algae, acritarchs, sponges, chancellorids, anemones, ctenophores, hyoliths, brachiopods, paleoscolecids, priapulids, echinoderms, trilobites, primitive chordates, trace fossils, lobopods, arthropods, and many enigmatic forms. This fabulous biota provides paleontologists with a window into Cambrian benthic and pelagic faunas (Hagadorn, 2002, pp. 35-60). Two important fossils from this location establish the first appearance of jawless fish. Today, fish represent the most successful class of vertebrates. Agnathans or jawless fish were the first vertebrates to evolve. These fish had no jaws, scales, or paired fins; their body was ell-like. Their skeletons were made of cartilage. The lamprey-like Haikouichthys ercaicuensis and the hagfish-like Myllokunmingia fengjiaoa are two agnathans found in the Maotianshan shales. Over 40 Cambrian localities with Burgess Shale type biotas have been documented across the world. In general these localities are dominated by nontrilobite arthropods, priapulid worms, sponges, and lobopodians (Hagadorn, 2002, pp 91-92).


Orsten Sweden

Limestone nodules, called orsten, are found within the Alum Shale of southern Sweden. Within the orsten can be found phosphatized and silicified meiofauna representing a flocculent layer community living just above the seafloor. Orsten is the first meiofaunal assemblage found in the fossil record. Eyes, hairs, spines, muscle scars, joints, pores, and soft body parts have been preserved on miniature late Cambrian arthropods. Exoskeletal replacement and or coating by calcium phosphate occur only on specimens less than 2 mm is size (Tang, 2002, pp. 117-121). A process of acid etching recovers these tiny 3-D fossils. Orsten yields the oldest known examples pentastomids (“tongue worms”), which are internal parasites that inhabit the respiratory system of most modern terrestrial vertebrates. Pentastomids found in this deposit must have had a marine host. The first appearance of tardigrades (water bears) also occurs in orsten. The Orsten Lagerstatten helps to deepen our understanding of arthropods as many larval stages are preserved. Among vertebrates, the orsten have broadened our knowledge of conodonts (Tang, 2002, pp. 117-121).


From so Simple a Beginning

In our article on the Precambrian we learned that the first evidence of simple prokaryotic life dates to roughly 3.5 billion years ago. Eukaryotes appear at around 1.2 billion years ago. Multicellular life appears at over 600 million years ago. In this article we see the first evidence of organisms that have the ability to construct mineralized skeletons. The first mineralized skeletons are small structures known as "little shellies" and appear in the Venadian. Little shellies are the dominant mineralized skeletons for around 25 million years and are slowly joined by larger invertebrates with mineralized skeletons at around 530 million years. Thus, there is a sequential pattern in the fossil record from single-celled prokaryotes to eukaryotes to multicellular soft-bodied animals to animals with tiny shells and finally, in the Middle Cambrian, to a variety of large shelled invertebrates. Evidence gathered in the past few decades supporting the sequence and timing of these appearances does not support an instantaneous "Cambrian explosion" (Prothero, 2007, p. 169).

Cambrian Extinctions

Trilobites help to document at least four mass extinctions that occurred during the Cambrian period. Each extinction event reveals a similar pattern. Trilobites and other organisms are diversifying and flourishing when their adaptive radiation is ended. While most genera permanently disappear a few survive. The survivors quickly dominate the environment and a new adaptive radiation begins. The first extinction at the end of the early Cambrian effects olenellid trilobites and archaeocyanthids. The extinctions during the late Cambrian effect trilobites, conodonts, and brachiopods. Evidence exists for lowering sea levels and climactic cooling conditions, which could be the causal factor for these extinctions (Stanley, 1987, pp. 54-62).


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