Evolution: Palaeontology

Basic Ideas: Cambrian Explosion

Content

This article explains what is meant by the “Cambrian Explosion”.  It also explains and evaluates conjectures about its causes.

Sudden onset of various fossilized animal phyla

What caused the Cambrian explosion?

Origin of vertebrates

Summary

Sudden onset of various fossilized animal phyla

One of the most pronounced anomalies in the fossil record can be found at the transition from the Precambrian to the Cambrian (For an overview see Figure 31). Cambrian sediments (=stratified rocks), contain a highly diversified fauna that occurs so suddenly and is so varied that we speak of the “Cambrian explosion” (Valentine 2004) or the “big bang” in paleontology (see Figure 24 and Figure 186).

Living organisms from all known phyla (=major groupings in the classification of living things), which have hard parts, are found as fossils of the Cambrian period (many already in the Lower Cambrian). These include sponges (Porifera), coelenterates (Coelenterata), annelids (Annelida), Brachiopod (Brachiopoda), arthropods (Arthropoda), mollusks (Mollusca), echinoderms (Echinodermata) and chordates (chordates, including the first vertebrates and jawless fish). The fossil evidence of these phyla in the Cambrian is so prolific that one can clearly differentiate the distinguishable sub-groups (classes).  This fossil record is also widely distributed around the world.  In contrast, the upper-most rock layers of the Precambrian contain very little fossil evidence, limited to a few multicellular organisms such as coelenterates and sponges (see below).

Classification of living organisms: People have divided all living organisms into two kingdoms at the highest level: the animal and plant kingdom. The animal kingdom is divided into phyla where the differences between animals are the greatest. One can say that the phyla represent major construction plan differences. The phyla are further divided into classes (see Figure 44), for example, the chordates include the classes of bony fishes, cartilaginous fishes, amphibians, reptiles, birds and mammals, as well as groups with no spine but only a cranium where the notochord (= embryonic elastic supporting axis) is gone in the adult. (In Cladistics (= methods of taxonomy where organisms are arranged into a branching diagram based on similar physical, synapomorphic, characteristics), these subdivisions are done differently, but this is not our main point here.) To summarize, the divisions of the classification hierarchy from most general to most specific is as follows: Domain, Kingdom, Phylum, Class, Order, Family, Genus and Species.

For example, Geyer (1998) writes of the "abrupt appearance of Metazoa groups, whose extensive diversity remains enigmatic. Even in the Lower Cambrian, or at least by the late Cambrian, there appear almost all of today's phyla.  In addition, those metazoans that do not fit easily in the classification are also present." In comparison to the animal groups represented today there are numerous forms whose taxonomic position is unclear or disputed due to the great richness of the Cambrian animal forms. Gould (1991) describes, among the fossils of the Burgess Shale in British Columbia (Canada), 20 new phyla with large building-plan differences as “wild miraculous animals”. The highly diverse arthropods he calls “unique arthropods”, with a “maximum of anatomically efficient ways”. In his opinion, the Burgess fossils surpass probably the entire spectrum of invertebrate life in today's oceans (see Figure 187). Other paleontologists are more cautious. They believe that the time did not include such a wide range of animals as Gould accepts and they are critical of the establishment of so many new phyla. However one decides to classify these animals, their classification does not detract from the amazing disparity of Cambrian animal forms.

So it appears that the main differences between the blueprints of all animals were already present in the beginning of the documented fossil record of multicellular organisms. Darwin had already noted the diversity of the Cambrian fossils as a problem for his theory. In a recent monograph, Valentine notes (2004), that this striking discontinuity has since been confirmed by many researches. Among all the Cambrian and Precambrian fossils, there are hardly any forms that can be cited to link different phyla, and even of the few life forms that are present, none of them are suitable as evolutionary transitional forms. The reason for this is that the single unifying characteristic of these few life forms are present in species with such a complex array of other characteristics that would exclude them as transitional species.

Valentine (2004, p. 31, 35) notes that there is no phylum for which the predecessor life form is known.  Likewise, there is no known source for the formation of all the known classes of invertebrates (see Figure 44 for the taxonomic units). Some Precambrian fossils are so very different from Cambrian fossils, that their origin is also a mystery.  Changes after the Cambrian (ie, the differentiation among the phyla or classes), are also very significant, but smaller in scale than those postulated between phyla that have no fossil evidence for evolution before the Cambrian. “What occurred after that in terms of evolutionary transformations were, for all the variety of forms, basically just variations of the established basic plans that were established during the Cambrian Revolution” (Seilacher 1992, p. 19).

What caused the Cambrian explosion?

What caused the Cambrian explosion is not yet clear. The often-expressed view, that evolutionary precursors have not been found yet because of their fragile nature and because of poor conditions for fossilization, is not convincing. There are over 400 known occurrences of very delicate creatures such as microfossils (unicellular organisms, including algae) found in the Precambrian. Another view to be critically evaluated is that the predecessors were very small, perhaps even microscopic and therefore have remained undetected. This assumption is still pure speculation. Valentine (2004, p. 463) notes in this context that even at a smaller scale and after the acquisition of hard parts, there was an explosive appearance of many lower taxonomies.

According to the concept of “unoccupied habitats”, the building plans of many phyla developed quickly because there were many unoccupied ecological niches available. Apart from the fact that this is also mere speculation, it is commonly accepted that there were other phases during the earth’s history when there were many unfilled ecological niches, but there was no resulting evolution or growth of new life forms with new building plans (no new phyla) during those times.

Environmental factors are another reason cited for the explosion. For instance, in the Lower Cambrian, the phosphate rich “small shelly fossils” are found contemporaneous with phosphate rich sediments and then also disappear when the phosphate sediments fade-away. The organisms which follow have carbonate as their primary building material.  In this context it is noteworthy that not only the animal forms but also entire ecosystems appear suddenly in a complex form and disappear just as suddenly.
Finally, in the wake of the discovery of regulatory genes (Hox genes), the idea was expressed that small changes in these regulation genes could produce large morphological changes. This would only be possible if certain structural elements were already present in a latent form. The origin of such living forms is not explained by the Hox genes or their mutations (see Homeobox-Genes and evolution).

Origin of vertebrates

One of the mysteries of the origin of animal phyla is the emergence of animals with an endoskeleton (internal skeleton).  The main question is, how did such internal bones form? There is no explanation found in the fossil remains nor are there any sound theoretical models. The few known fossils of invertebrate chordates give no further clues as to the origin of vertebrates (vertebrates belong to the chordates, see Figure 188).

One must keep in mind what changes are needed in the blueprint of an animal in order to build a system of bones, vertebrae, tendons and associated muscles! These fundamental transformations would have required so many intermediate stages over such a long period of time that you should find some relevant fossils in Precambrian and Cambrian rocks. A perhaps suitable precursor to vertebrates is Pikaia (Figure 189) from the Canadian Burgess Shale (Middle Cambrian). Its body shape is reminiscent of the lancelet (Fig. 188).  The animal has a striped cord along its back, which is interpreted as a notochord, and the typical zig-zag stripes of myotomes (muscle bands).

Only recently, the oldest documented vertebrate fossils were discovered. These were fossils of the jawless fish Myllokunmingia and Haikouichthys (Fig. 190) which appears in the Lower Cambrian, at the famous fossil site Chengjiang (China) and thus in strata older than where the Pikaia was found.

In general, Carroll (1993, p. 16) states of the oldest known vertebrates, that they differ so much from each other, "that their family relationships could only be reconstructed with great difficulty.”  Each of these groups has a mosaic of primitive and derived features, but none approaches the anatomical characteristic pattern that one would expect for a primitive ancestor of the other groups. Evolutionary theory therefore assumed that the bones were acquired independently in several stages and in many different lines of development (convergence, see Similarities in Morphology and Anatomy). Today, due to the lack of fossils, the relationship of vertebrates to other phyla has been reconstructed based on existing life forms.

Summary

In the Lower Cambrian, fossil representatives of all known animal phyla), which have hard parts, occur almost simultaneously in a large variety of forms and over a wide geographical area.  In comparison, the underlying Precambrian rocks contain few multicellular organisms, of which only a few could be interpreted as a precursors to Cambrian forms. The sudden appearance of so many different blueprints (different phyla) at the beginning of the fossil record is also an enigma for evolutionary biologists.

Translator: Olivier Muff, 27.04.2010

Author: Reinhard Junker

 
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