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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 ExplosionCambrian 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).
Chengjiang
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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