Cambrian Explosion

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[edit] Definition

The Cambrian Explosion refers to the evolution of marine life with hard parts during the Cambrian Period (which lasted from 540 to 505 million years ago): it is called an "explosion" because rather more evolution happened in this period than is usual over a mere 35 million years.

[edit] Precambrian fauna

In order to put the Cambrian Explosion into context, we must first look at the Precambrian fauna that preceded it.

Dickinsonia, a soft-bodied bilaterian of the Precambrian period
Dickinsonia, a soft-bodied bilaterian of the Precambrian period
Up until 1947, with the discovery of the Ediacara fauna, it was thought that the Cambrian Explosion marked the origin of multicellular life. It is now known that there was abundant and diverse multicellular life in the Pre-Cambrian, but it was all soft-bodied, and so sites of preservation (lagerstätten, as paleontologists call them) were rare. The Cambrian Explosion, then, is not the origin of multicellular life, but the origin of hard-bodied fauna.

Undoubtedly we do not yet know the full range of Precambrian fauna, but the specimens known[1] include cnidarians (jellyfish and sea-pens) such as Charnia and Charniodiscus, a possible echinoderm[2] (the group including starfish and sea-urchins) displaying the fivefold radial symmetry that characterises this group; organisms resembling soft corals, segmented worms such as Spriggina floundersi, and other segmented bilaterians (creatures with a front, a back, and left-right symmetry) including a number of what appear to be soft-bodied arthropods[3], and what looks suspiciously like a chordate[4] (the group ancestral to us vertebrates) displaying evidence of fins, a notochord (a stiffened rod which is an evolutionary and developmental precursor to the spine), and myotomes (the "herringbone" pattern of muscle bands characteristic of chordates). We should note that while this interpretation has been contested, the people who have actually seen the fossil stand by it[5]. All the Precambrian fauna were of course marine fauna like the Cambrian fauna that succeeded them. They are all very small --- usually a matter of mere centimeters, with the largest, Dickinsonia Rex, reaching a maximum known size of 43 cm; the earliest Precambrian fauna are downright microscopic, being no more than a fifth of a millimeter long[6].

It is instructive to see what is not in the Precambrian fauna. There were no creatures with exoskeletons such as we find in the Cambrian; there are only a few shelly creatures appearing late in the Precambrian, such as Cloudina and Namacalathus[7]; there were no burrowing creatures; and there is nothing that can be identified as a predator. It seems likely that these facts are linked, and that the origin of hard parts and of burrowing behavior in the Cambrian Explosion was an evolutionary response to predation.

The reader should be cautioned that, because of poor preservation and a shortage of material, the identification of Precambrian fauna is often tentative and sometimes controversial. For example, it has only recently been established that Dickinsonia has a gut[8], a question that obviously bears heavily on its classification.

Nonetheless, two facts are clear: first, the Precambrian period had a wide diversity of soft-bodied forms; secondly, many of them are good candidates for ancestors of the creatures with hard parts that succeeded them in the Cambrian.

[edit] Cambrian fauna

In the Early Cambrian period (the Tommotian) we see the advent of creatures that burrowed in the sea-bed, including cnidarians, nematode worms, annelid worms, and arthropods.

Yochelcionella, a member of the Small Shelly Fauna of the Tommotian
Yochelcionella, a member of the Small Shelly Fauna of the Tommotian
We also see the advent of the Small Shelly Fauna (mostly no more than a couple of millimeters in size) some of which are probably ancestral to mollusks, brachiopods, and other modern groups[9]. The Helcionelloida in particular have been identified as basal mollusks[10]: pictured here is Yochelcionella displaying its distinctive "exhaust pipe". More advanced mollusks such as bivalves, gastropods and cephalopods don't appear until later in the Cambrian. Primitive brachiopods ("lamp shells") are also known from the Tommotian; unlike modern lamp shells, they are "inarticulate": that is, they lack a proper hinge between their paired shells.

However, many of the shells in the Small Shelly Fauna are more mysterious: for example, no-one seems to know what sort of creature lived in the champagne-glass shaped shells known as archaeocyathids. In many cases, it is not even known which is the front and which the back end of the creature. Some of the Small Shelly Fauna may be "cataphract armor": that is, armor which doesn't come in continuous sections, like the armor of a lobster, but rather in a set of disconnected plates. This can, of course, only be verified by finding a well-preserved example of the cataphract-armored creature, as is the case with the circular "sieve plates" of Microdictyon (pictured here).

Just after the Tommotian, we find the first known bivalves, such as Fordilla: they had small gills and poor hinges compared to modern forms: inefficient as burrowers and filter feeders, they most likely browsed on sea-floor algae.

Marella, the commonest of the Burgess fauna. A mere 2 cm long, it probably filled a similar ecological niche to modern-day krill.
Marella, the commonest of the Burgess fauna. A mere 2 cm long, it probably filled a similar ecological niche to modern-day krill.
The most famous fossils of the Middle Cambrian are those of the Burgess Shale[11], where a series of mudslides resulted in preservation of soft as well as hard parts. Many of the "weird wonders" of the Burgess Shale seem to have left no descendants, but some are basal to groups surviving today: Santacaris, for example, is plainly a chelicerate, though the forelimbs have yet to develop into the large claws (chelicerae) that we associate with the group today. Pikaia is an early chordate, close to the ancestry of vertebrates: for more on this creature, see the section on invertebrate-to-vertebrate evolution in our article on intermediate forms between classes. Other familiar forms such as sponges and priapulid worms are also present.

The Middle Cambrian sees the disappearance of the "cataphract-armored" creatures: it seems likely that though this style of armor prevented early predators from swallowing them whole, it was no match for the specialized feeding organs of such creatures as Opabinia and the fearsome two-foot long Anomalocaris, the largest predator known from the Cambrian period.

In the Late Cambrian we see the advent of the first primitive cephalopods, such as Plectronoceras, still quite similar to basal mollusks. Analysis of the structure of its shell shows that it would have had poor buoyancy: unlike later forms such as ammonites and nautilids, it would have been a poor swimmer, probably browsing on the sea bed.

Some general remarks may be made about the Cambrian fauna. Firstly, the species are relatively unspecialized: with some exceptions, such as Aysheaia, thought to be a specialist browser on sponges, the Cambrian fauna consisted of predators that ate everything that moved, and browsers which ate everything that didn't. The next geological period, the Ordovician, would see a decline in generalists and a rise in specialist species.

Secondly, and similarly, each Cambrian species tends to show little differentiation and specialization within its body plan when compared with later forms. If you look, for example, at the legs of a Cambrian arthropod, you'll find that all its legs look pretty much the same. By contrast, in the Ordovician, you may find in a single species legs adapted for walking, as claws for grasping prey, as feelers (pedipalps) and as paddles for swimming with.

[edit] Creationist misconceptions

We may lay it down as a general rule that if it is possible to get some aspect of biology wrong, creationists will do so. The Cambrian explosion is no exception. The following fairly typical piece of creationist nonsense is taken from a pamphlet produced by the Jehovah's Witnesses:

Thus, at the start of what is called the Cambrian period, the fossil record takes on an unexplained dramatic turn. A great variety of fully developed, complex sea creatures, many with hard outer shells, appear so suddenly that this time is often called the 'explosion' of living things.
Are there any links between the Cambrian period and what went before it? The butterfly, fern, rose and fish appear suddenly and fully developed.

This is wrong in almost every respect:

  • There are no butterflies, ferns, or roses in Cambrian fossils; as far as fish go, there are primitive invertebrate jawless creatures that one might class as "fish", such as Haikouichthys, but no "fully develped" modern fish.
  • The evolution of hard-bodied creatures is "sudden" only in geological terms --- that is, it took a mere thirty-five million years, or, to put it another way, about five thousand times longer than most creationists will admit that the Earth has existed.
  • There are indeed, as we have seen, links between Precambrian and Cambrian fauna, with many good candidates for soft-bodied ancestors of the hard-bodied fauna of the Cambrian.
  • The Cambrian fauna is not "fully developed" by modern standards: we have given the example of chelicerates without chelicerae, brachiopods lacking hinges, cephalopods with poor buoyancy, et cetera.

[edit] Related Articles

[edit] Note on sources

Where not otherwise referenced, statements in this article are usually sourced from Dr. David Norman's book Evolution: The Rise Of The Vertebrates. Other sources include:

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