The Cambrian Revolution and Animal Organization.

Main Points:

  1. Massive increase in animal taxa evident in Cambrian.
  2. Nearly all phyla present by close of Cambrian.
  3. Fossil-Lagerstatte, assemblages of extraordinarily preserved fossil organisms, occur in Cambrian.
  4. Burgess Shale and Chenjiang compression fossils, and Siberian and Chinese phosphatized microfossils are key examples of Fossil-Lagerstatte.
  5. Traditional view (favored here) is that most forms represent extant taxa.
  6. Steven J. Gould's view, expressed in book, "Wonderful Life", is that most taxa DO NOT belong to modern phyla, but represent "failed experiments".
  7. In any case the role of contingency in evolution is obviously very important.
  8. Animal phyla, each have their own basic plans, but several higher level groupings are evident.
  9. Diversity of animal life through the Paleozoic shows exponential increase in early Paleozoic, then leaving off through late Paleozoic.
  10. Trophic Escalation is characteristic of evolution, as seen in the Cambrian onward.
  11. Biogeochemical Effects of trophic escalation in early Paleozoic involves mostly aeration of sediments (Agronomic Revolution) and proliferation of biomineralization, which takes over from stromatolites and inorganic precipitation.

The Cambrian Explosion

Transition into Cambrian from the Precambrian is marked by appearance of enigmatic tiny "shelly fauna". Relationships of mostly tiny fragmentary bits not known.

Within Lower Cambrian and then in Middle Cambrian strata, there are a series of fossil deposits in British Columbia and China (and elsewhere) that have extraordinary preservation of soft bodied as well as hard bodied animals. These extraordinary cases of fossil preservation are termed "Fossil-Lagerstatte".

It is from these Fossil-Lagerstatte that we know that all of the major phyla of animals had appeared by end of the Cambrian. The massive increase in the appearance of different phyla and the generally exponential increase in animal taxa, especially shelly forms through the Cambrian, is called the "Cambrian Explosion" As far as we know this major breakthrough occurred only within the Kingdom Animalia

The Animalia

The Animalia are multicellular, hetertrophic, diploid organisms that develop from two different gametes (i. e., sperm and egg).

There are about 34 phyla within the Animalia. Each has its own basic body plan, although most, at least belong to larger monophyletic groups.

One of the most wonderful things about the Animalia is the enormous variety of body plans that are present. The basics include variations of symmetry (e. g., bilaterally symmetical around a plane or radially symmetrical around a line), the number and developmental origin of tissue layers (e. g., ectoderm, mesoderm, endoderm), and the levels or organization (e. g., tissues, organs, brain, heart, etc.)

These variations in body plan largely determine the physical constraints under which the different kinds of animals live. For example, only the coelomate organisms have attained large size, although some phyla of non-coelomates can make colonies of very large size (e. g. corals within the Cnidaria) while others can get fairly long although very thin (e. g. some flatworms).

Thus most non-colonial non-coelamates tend to be very small (such as nematodes) and live between grains of mud or sand, or as parasites - low-Reynolds number lifes.

So that you can develop some appreciation of this structural diversity, we have produced a gallery if the phyla of the Animalia that will give you an overview of their structure.

Please note that this information his here largely for your esthetic enlightenment. However, I do expect you to understand some of the ways animal phyla differ (as outlined above). We have also provided you with an attempt at a cladogram of the Animalia, below. It is important to stress that the relationships of many animal phyla, and the basic geometry of a cladogram such as this, is far from certain. Morphologists have long debated the relationships of the animal phyla, which is made more difficult by vastly different body plans, and the possibility of structural simplification though time (as in parasites). However, recent advances in molecular biology, especially gene sequencing, are bringing entirely new data sets to bear on these controversial relationships.

|----------------------------------------------- Animalia---------------------------------------------|
    |----------------------------------Eumetazoa ----------------------------------|
|--------------"Diploblastica"--------------| |------------------------Bilateria-----------------------|
Porifera Placozoa Cnidaria Ctenophora Acoela Deuterostomia Ecdysozoa Lophotrochozoa
List of phyla of uncertain relationships: Acanthocephala, Chaetognatha, Cycliophora, Echiura, Entoprocta, Gnathostomulida, Loricifera, Mesozoa, Phoronida, Pogonophora, Rotifera, and Sipuncula. The Cycliophora, Acanthocephala, and Rotifera at least seem to belong to a monophyletic group, but its placement is uncertain. 

An excellent discussion of the relationships and origin of animals can be found at

Animal diversity during the rest of the Phanerozoic.

With the "Cambrian Explosion" in animal diversity came the appearance and diversification of animal hard parts such as shells and spines, usually hardened by some kind of mineral such as calcium carbonate (CaCO3) or calcium phosphate (CaPO4). With the appearance of these hard parts, the average quality of the fossil record (not counting the Fossil-Lagerstatte) improves dramatically. In addition, the proportion of sedimentary rocks largely composed of animal parts, such as limestone composed of shells, also increases dramatically.

Animal diversity continued to increase exponentially through the early Paleozoic leveling off in the Ordovician. It stayed more or less level, with some dramatic, although brief drops comprising modest mass-extinctions, until the close of the Paleozoic, when animal diversity experience its greatest mass extinction of all time, the Permo-Triassic mass extinction event (more in the next lecture). From that time until now, animal diversity has been on a dramatic increase with two additional mass extinctions, those at the the Triassic-Jurassic and Cretaceous-Tertiary boundaries.

A diagram of the number of "shelly" animal families is shown below.


Balavoine, G. and Adoutte, A., 1995, One or three Cambrian radiations. Science, v. 280, p. 397-398.

Gould, S. J., 1989, Wonderful Life, W. W. Norton & Company, New York, 347 p.

Janvier, P., 1999, Catching the first fish. Nature, v. 402, p. 21-22.

Knoll, A. H. and Carroll, S. B., 1999, Early animal evolution: emerging views from comparative biology and geology. Science, v. 284, p. 2129-2137. For those who which to learn more detail about the origin and relationships of animals, this is a superb and very recent review.

Margulis, L. and Schwartz, K. V., 1982, Five Kingdoms (2nd ed.), New York, W. H. Freeman & Company, 376 p.

Miller, A. L., 1998, Biotic transitions in global marine diversity. Science, v. 281, p. 1157-1160.

Ruiz-Trillo, I, Riutort, M., Littlewood, T. J., Herniou, E. A., and Baguna, J., 1999, Acoel flatworms: earliest extant bilaterian metazoans, not members of Platyhelminthes. Science, v. 283, p. 1919-1923.

Shu, D-G., Conway Morris, S., Zhang, X-L., Hu, S-X., Chen, L., Zhu, M., and Chen, L-Z., 1999, Lower Cambrian vertebrates from South China. Nature, v. 402, p. 42-46.

Winnepenninckx, B. M. H., Backeljau, T., and Kristensen, R. M., 1998, Relationships of the New phylum Cyliophora. Nature, v. 398, p. 636-638.

Wray, G. A., Levinton, J. S., and Shapiro, L. H., 1996, Molecular Evidence for Deep Precambrian Divergences Among Metazoan Phyla. Science, v. 274, p. 568-573.

Updated March 10, 2005
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