The Cambrian Explosion
The Cambrian 'explosion' of animal diversity
The Cambrian Period extends from about 541 to 485 million years (Myr) ago, during which almost all of the major animal phyla appear in the fossil record, including:
- Annelida (segmented worms)
- Arthropoda (e.g. trilobites, crustaceans)
- Brachiopoda (‘lamp shells’; superficial resemblance to mollusc bivalves, but have a very different internal structure)
- Chordata (includes vertebrates)
- Cnidaria (e.g. jellyfish, sea anemones, corals) (*)
- Ctenophora (comb jellies)
- Echinodermata (e.g. starfish, sea urchin)
- Mollusca (*)
- Nematoda (round worms)
- Porifera (sponges) (*)
* Although some authors have interpreted some Ediacaran fossils as cnidarians, molluscs or sponges, these are not generally accepted; see below.
The main exceptions are:
- Bryozoa (moss animals)
- Platyhelminthes (flat worms)
- Rotifera (wheel animals)
which are known only in later strata.
Something of the wide variety of body plans can be appreciated from the selection of fossils from the Cambrian period shown on this page.
The sudden appearance of such different animal types clearly poses a challenge to the theory of evolution which predicts that new forms should emerge gradually. It’s not just that the different forms appear suddenly, but if they had evolved from common ancestors then there should be examples of the diverging forms.
Darwin recognised this difficulty to his theory but anticipated that new fossil finds would fill in the gaps. However, over 150 years later, although major new fossiliferous strata have been discovered, and many new fossil forms found, the issue is as stark as ever. In fact in some ways it is more problematic (from an evolutionary point of view) because an even greater diversity of fossils has been found, yet without intermediates.
The Burgess Shale fauna
A prominent example of this are the fossils from the Burgess Shale in the Canadian Rockies which date from the middle of the Cambrian Period, about 510 Myr ago.
They were discovered in the late 19th century by palaeontologist Richard McConnell, but were not well known until the work of Charles Walcott from 1909 onwards. Despite their diverse appearance, Walcott assumed they were early members of animal phyla that were already known, and classified them accordingly. Indeed, as indicated by some of the images on this page, there are fossils of extant phyla in the Burgess Shale. However, when the Burgess Shale fossils were reassessed about 50 years later, it became evident that many had morphological features that were not consistent with known taxa. For example, as well as Anomalocaris and Hallucinogenia whose names speak for themselves, there’s the feather-like Thaumaptilon, and many more such as Chancelloria and Pollingeria.
Important in the current context is that the Burgess Shale fossils have increased the known variety of the Cambrian explosion, yet without predecessors showing how the diversity arose from common ancestors. So these important new discoveries have increased the challenge posed to evolution by the fossil record.
Abrupt diversity within phyla, or parallel innovation
The enigma is further compounded because not only do different phyla appear suddenly, but also, within many phyla, distinct subgroups (which may be subphyla, classes or orders) arise within a short period of time, and without linking intermediates. Notable are the arthropods of which at least 5 distinct subphyla occur in the Cambrian:
- Trilobites are the best known.
- Bradoriida (bivalved arthropods which were quite different from the bivalve molluscs or Brachiopoda).
- Megacheira (arthropods with ‘great appendages’, superficially similar to crustaceans but now recognised as substantially different).
- Marellomorphs - "make up a small group of Palaeozoic arthropods, renowned for their bizarre anatomy" . Although they are a ‘small group’ in terms of number of species, Marella is the most abundant animal fossil (not just the most common arthropod) found in the Burgess Shale. So it may be 'bizarre' but it's not just an atypical outlier.
In evolutionary terms, it would be expected that the essential characteristics of the arthropod phylum would have arisen in one progenitor lineage, from which other arthropod groups would have gradually diversified. Even evolutionary biologists recognise that it is extremely improbable that a similar innovation (with all of the compatible mutations required to produce a novel body plan) should arise independently, especially at about the same time.
It’s worth noting that for the arthropods, because the exoskeleton is hard and cannot expand as the organism grows, it is necessary periodically to shed it and replace with a larger one (i.e. moulting is required). So what would be required are not only the genes to produce their distinctive body-plan including their chitinous exoskeleton, but also those to implement and control moulting (ecdysis). That the early arthropods moulted is illustrated in image (k) on this page.
Not only do these different groups of arthropods appear during the Cambrian, but even within these there are diverse subgroups (e.g. orders or classes), for example:
- Nine orders of trilobites appear in the Cambrian. 
- Similarly, the different classes of crustaceans appearing in the Cambrian are so distinct that some researchers concluded there must have been a 'cryptic Cambrian radiation' of crustaceans before their appearance as fossils. 
- And even the marrellomorphs are subdivided into two groups which many authors do not regard as being related. 
1. Intermediates lacked fossilisable hard parts
The usual - indeed, pervasive - explanation given for the absence of transitional forms in the fossil record is that the intermediates lacked hard parts (e.g. skeletons) so were not susceptible to fossilisation. That is, the diversification of fauna in the Cambrian was not as explosive as it appears: ancestors had been evolving and diversifying during preceding millions of years, but these soft-bodied organisms had not left fossils.
There are many soft-bodied fossils from the Cambrian and before
However, this clearly is not a satisfactory explanation, because there are many examples of fossils of exclusively or predominantly soft-bodied animals from both the Cambrian (which shows that it wasn’t only the organisms with hard parts that were fossilised then) and the preceding Ediacaran (when the major diversification is supposed to have been occurring). Whilst it is true that soft tissues are preserved well at only some sites (lagerstätten), there are now many such sites known.  Soft tissues are evident in some of the images shown on this page; of particular note are the cnidarian jellyfish and ctenophore (comb jelly) which are exclusively soft-bodied organisms.
The only book written by a geologist that most biologists have read is On The Origin of Species (1859) by Charles Darwin. This great work had one unfortunate side effect: it convinced a great number of biologists that the fossil record was so incomplete that it was essentially not worth their attention. This view is still common among biologists, who, despite being exceptionally well versed in modern biological data, are often roughly 150 years out of date when it comes to palaeontological data. The Precambrian is no longer considered a wasteland barren of fossils, as it was during the nineteenth century. 
2. Ediacaran biota were intermediates
An alternative proposition is that the Cambrian diversity of life is not so explosive because we can see evidence of the diversification in the preceding Ediacaran fossils which date from about 635 Myr (mainly from about 575 Myr) ago. They are named after the Ediacara Hills in South Australia which was one of the first fossil sites to be explored, and there are now at least 40 sites where Ediacaran fossils have been found. 
The Ediacaran fossils record a group of "large (up to 2 metres), biologically complex, mostly soft-bodied organisms".  But they are enigmatic - so different from subsequent animal fossils that some authors even doubt whether they were animals (as recently as 2013 ) - and ‘Determining the position of these organisms in the tree of life is one of the biggest unresolved challenges in palaeobiology'.  Although some have been interpreted as early sponges, cnaidarians or molluscs ...
It remains true, although controversial, to say that there are still no uncontested claims of crown-group cnidarians, bilaterians or even sponges from the Ediacaran, which is fairly remarkable if animals were really diversifying just after the end of the Marinoan glaciation (c. 635 Ma).  
So the Ediacaran fossils certainly do not explain the appearance of the diverse phyla of the Cambrian explosion.
3. Environmental spurs to evolution
Another approach taken by biologists to try to explain the Cambrian explosion is to propose that there was some sort of environmental stimulus to the evolution of multicellular animal life.
The most common suggestion is that the evolution of animals was boosted by increased availability of oxygen. The rationale being that more readily available oxygen could boost metabolic rate e.g. to sustain larger body sizes, locomotion and predation. (Some also propose that it could facilitate increased body size because the ratio of volume to surface area would be less limiting, but this would only be applicable to small organisms that rely on diffusion of oxygen.) There is evidence that oxygen levels increased at about the time of the early Cambrian; although others have pointed out that there was a substantial increase in oxygen much earlier than that, so why didn’t the rapid evolution of animals occur then?
Another possibility is that the end of the preceding Marinoan glaciation period brought not only increased temperatures but also increased availability of nutrients, which together accelerated evolution.
There may also have been a diversification of ecological niches which facilitated a diversification of life; and the development of mineralized skeletons at the start of the Cambrian Explosion might have been a response to increased pressure from predators.
However, it seems to me that all theories such as these are missing the point. New body plans require new genes. Unless it can be shown (or at least come up with a sensible theory for) how increased oxygen (or whatever) can promote the origination of the genes required for assembling new body plans (in effect, a means for directing the formation of the necessary genes rather than having to rely on random mutations - see obstacles to new genes), then all such suggestions are merely futile red herrings.
Notes display in the main text when the cursor is on the Note number.
1. Legg, David A. 2015. Fossil Focus: Cambrian Arthropods, Palaeontology Online, Volume 5, Article 4, 1-11, p10.
2. Diego Garcia-Bellido and Desmond Collins. Moulting arthropod caught in the act, Nature Vol 429, 6 May 2004, p40.
3. See Wikipedia article 'Trilobite'; accessed 11 November 2017.
4. Thomas Harvey, Maria Velez and Nicholas Butterfield; 'Exceptionally preserved crustaceans from western Canada reveal a cryptic Cambrian radiation'; PNAS Vol 109(5), 1589-1594; www.pnas.org/cgi/doi/10.1073/pnas.1115244109.
5. Legg, David A. 2015. Fossil Focus: Cambrian Arthropods, Palaeontology Online, Volume 5, Article 4, 1-11, p10.
6. Wikipedia artcle 'Lagerstätte' lists 4 Precambrian and 10 Cambrian major Konservat-Lagerstätten.
7. Jonathan Antcliffe. 2012. Patterns in Palaeontology: The Cambrian Explosion - Paradoxes and possible worlds, Palaeontology Online, Volume 2, Article 8, 1-12, p8. http://www.palaeontologyonline.com/articles/2012/the-cambrian-explosion-paradoxes-and-possible-worlds/
8. Mary Droser and James Gehling. The advent of animals: The view from the Ediacaran. PNAS vol 112(16), 4865-4870, p4865.
9. Dunn, F. S. and Liu, A. G. 2017. Fossil Focus: The Ediacaran Biota. Palaeontology Online, Volume 7, Article 1, 1-15, p2; http://www.palaeontologyonline.com/articles/2017/fossil-focus-ediacaran-biota
10. Gregory Retallack. Ediacaran life on land. Nature Vol 493, p89 (2013).
11. Dunn, F. S. and Liu, A. G. 2017. Fossil Focus: The Ediacaran Biota. Palaeontology Online, Volume 7, Article 1, 1-15, p1.
12. Graham Budd. At the Origin of Animals: The Revolutionary Cambrian Fossil Record, Current Genomics, 2013, Vol 14, 344-354, p352.
13. Sponges are thought to be the most primitive of animals because they have almost no cell differentiation, cnidarians the next step because of their cell differentiation; and bilaterians have bilateral symmetry (head and tail, front and back) which is true for all ‘higher’ animals.
Some of the images on this page are from the Royal Ontario Museum website burgess-shale.rom.on.ca . The source pages include a zoom facility which enables the photographs of the fossils to be magnified and viewed in fine detail.
a. Annelid Burgessochaeta setigera (ROM 61042). Photo by Jean-Bernard Caron; copyright © Royal Ontario Museum; source http://www.burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=29&ref=i& ; copyright information at http://www.burgess-shale.rom.on.ca/en/copyright.php
b. Trilobite Ogygopsis klotzi from Mount Stephen, Field, BC, Canada. Photo by Kevin Walsh; source https://www.flickr.com/photos/86624586@N00/10197098 , available via Creative Commons Attribution 2.0 Generic (CC BY 2.0).
c. Brachiopod Nisusia naturalis. Source https://commons.wikimedia.org/wiki/File:Nisusia_sp_Naturalis.JPG By Ghedoghedo (Own work) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons.
d. Chordate Metaspriggina walcotti. Photograph copyright © Royal Ontario Museum; source https://www.rom.on.ca/en/exhibitions-galleries/exhibitions/dawn-of-life-preview; copyright information at https://www.rom.on.ca/en/copyright-policy.
e. Cnidaria jellyfish, scale bar = 5 mm. Image from https://doi.org/10.1371/journal.pone.0001121.g003 , Open Access.
f. Ctenophor Ctenorhabdotus capulus (ROM 50822) Photo by Jean-Bernard Caron; copyright © Royal Ontario Museum; source http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=41&m=2&&ref=i ; copyright information at http://www.burgess-shale.rom.on.ca/en/copyright.php
g. Echinoderm Lyracystis radiata (ROM 57228) Photo by Jean-Bernard Caron; copyright © Royal Ontario Museum; source http://www.burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=76&m=4&&ref=i ; copyright information at http://www.burgess-shale.rom.on.ca/en/copyright.php
h. Mollusc Nectocaris pteryx (ROM 59660) Photo by Jean-Bernard Caron; copyright © Royal Ontario Museum; source http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=87&m=6&&ref=i ; copyright information at http://www.burgess-shale.rom.on.ca/en/copyright.php
i. Nematode Maotianshania cyliindrica, from the Cheng Jiang Mao Tian Shan Shales, Yunman Province, China. By Dwergenpaartje (Own work) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia CommonsSource https://commons.wikimedia.org/wiki/File:Maotianshania_detail_CRF_02.jpg
j. Porifera Protoprisma annulata (ROM 53565) Photo by Jean-Bernard Caron; copyright © Royal Ontario Museum; source http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=107&ref=i ; copyright information at http://www.burgess-shale.rom.on.ca/en/copyright.php
k. Arthropod Marrella splendens (ROM 56781) Photo by Jean-Bernard Caron; copyright © Royal Ontario Museum; source http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=80&m=6&&ref=i ; copyright information at http://www.burgess-shale.rom.on.ca/en/copyright.php