BSCI392
10-22-07
The Burgess Shale and other exceptional Cambrian faunas

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The Cambrian Explosion's fuse: Prior to the discovery of the Ediacaran fauna, it seemed that animal diversity burst upon the scene during the Early and Middle Cambrian. The subset of animal diversity that was visible through the fossil record was that with hard parts, many lineages of which showed up in a short time span, but not too many to allow us to believe that the major phyla accumulated across the Cambrian and Ordovician. We now know:

Thus, in the Early Cambrian, a wide range of animals learned to secrete hard skeletons. This is the Cambrian Explosion. It has speculatively been attributed to:

But occasionally, we are reminded of how little we know. Three fossil faunas from the Early and Middle Cambrian have been extensively studied since the mid 20th century. They preserve the remains of both hard and soft bodied creatures, and contain many surprises. The best known of these is the Middle Cambrian Burgess Shale, but this lecture derives from all three:

Fossil preservation: All are places of exceptionally fine preservation of fossil material, including (at Chenjiang and Burgess Shale) carbonized soft tissue. How this occurred is best understood for the Burgess Shale which has, in addition, been studied the longest.

Located near Mt. Burgess in Yoho National Park, BC, the burgess shale is a finely laminated dark claystone. At the time of its deposition, the site was near the equator off the northwestern shore of the ancient precursor to North America.

The shales accumulated downslope from a sunlit shallow marine environment that was prone to occasional slumping. These slumps conveyed sediments and members of the fauna into deep anoxic water where the animals quickly asphyxiated (no trace - so to speak - of escape traces here). Moreover, decomposers could not reduce their flesh, which ended up being preserved as carbonized films on the surface of the layers in which they were buried. Thus, in addition to the hard-part fossils we'd normally expect we get:

Ecologically, the fauna equally preserves:

A Burgess Shale Bestiary

Things one might expect from a typical fossil site:

  • Trilobites. Note the contrast between typical calcified cuticle and the non-clacified limbs.

  • http://www.trilobites.info/

  • Archeocyathids and sponges

  • http://paleobiology.si.edu/burgess/burgessSpecimens.html

  • Brachiopods

  • http://members.cox.net/jdmount/nisusia1.html

    Soft bodied things one wouldn't typically expect but that don't really surprise us: :

  • Arthropods belonging to familiar groups but without calcified cuticle. E.G.: Canadapsis perfecta, a crustacean.

  • http://paleobiology.si.edu/burgess/burgessSpecimens.html

  • Priapulid worms. E.G.: Ottoia prolifica.

  • http://paleobiology.si.edu/burgess/burgessSpecimens.html

  • Annelid worms. E.G.: Canadia spinosa.

  • http://paleobiology.si.edu/burgess/burgessSpecimens.html

    Soft bodied things that do surprise us: :

  • Arthropods belonging to no obvious modern group E.G.: Marrella splendens. See also:

  • http://paleobiology.si.edu/burgess/burgessSpecimens.html

  • Onychophorans . E.G.: Aysheaia pedunculata. Not really expected, given that modern onychophorans are terrestrial.

  • http://paleobiology.si.edu/burgess/burgessSpecimens.html

  • Chordates . E.G.: Pikaia gracilens. Apparently a cephalochordate closely related to Branchiostoma (AKA Amphioxus). Reconstruction.

  • http://www.cartage.org.lb/en/themes/sciences/Paleontology/Paleozoology/EarlyPaleozoic/EarlyPaleozoic.htm

    Major enigmas: :

  • Lobopodans: E.G.: Opabinia regalis. Reconstruction.

    The particular idiosyncrasies of Opabinia include the five eyes, anterior "funnel" and segmented body in which each segment bears a fleshy lobe and an small jointed limb. Turns out that there are similar creatures, the Anomalocarids, that include the top predators of the Cambrian (reaching lengths exceeding one foot.) Lobopodans are known from the Burgess Shale and Chenjiang, and now, it seems, from Sirius Passet.

    A link for all of your Anomalocarid needs.


  • http://paleobiology.si.edu/burgess/burgessSpecimens.html

  • Odontogriphus . E.G.: Odontogriphus omalus. Originally reconstructed as a flattened, segmented animal with an array of tentacles around its mouth vaguely reminiscent of a lophophore.

  • http://www.rom.on.ca/collections/research/jcburgess.php

  • Wiwaxia corrugata. A little, unsegmented box of meat covered with calcareous scales reminiscent of pieces of the Small Shelly Fauna. Reconstruction.

  • http://paleobiology.si.edu/burgess/burgessSpecimens.html

  • Hallucigenia sparsa . When first discovered, a real problem. It appeared to be a tiny worm-like creature that walked on a series of spiny stilts and had a series of tentacles protruding from its back, prompting suspicions that it was only part of a larger organism. Reconstruction.

  • http://paleobiology.si.edu/burgess/burgessSpecimens.html

    Enigmas spawn controversy:

    The Burgess Shale has been studied for over a century, in three phases:

    This history of study reveals three general philosophies: The reappraisal has been greatly facilitated by the unfolding study of Chenjiang and Sirius Passet. Remember the general cladogram of Bilateria (Note: This time, you're required to):

    There are three major monophyletic groups:

    The Early-Middle Cambrian faunas seem to track their initial diversification.

    Deuterostomia: Although the presence of proper echinoderms is controversial for any of these faunas, chordates are well represented. In addition to Pikaia, the Chenjiang offers:

    Additionally, we have Vetulicolida a strange group of strange creatures argued by some to be near the common ancestry of deuterostomes.

    Ecdysozoa: Upon reflection, Arthropoda, Lobopoda, and Onychophora appear to be united by the synapomorphy of having limbs sheathed in cuticle. Arthropods and lobopodans are even closer with the additional synapomorphy of having at least some limbs with a jointed exoskeleton:

    Lophotrochozoa: Wiwaxia was very difficult to interpret. In some ways, it was like an intermediate between a chiton, a basal molllusk, and a sea mouse, - a polycheate annelid. The discovery of Halkeriids - similar creatures from Sirius Passet, enhanced this impression (but with the added tease of a vaguely brachipod-like valve at the front and back end). Could they be close to the common ancestry of annelids and mollusks? Yes, but.....

    So, we now have an evolutionary progression from Kimberella (soft bodied with no radula) to Odontogrophus (soft bodied with a radula) to Wiwaxia (with radula and many calcareous armor plates) to derived mollusks (with radula and a few highly regular plates). Cool. We await news on how halkeriids and annelids fit into the scheme.

    Take-home lesson: Our understanding of the significance of the Burgess Shale was hampered by the typology of the Linnean System, regardless of whether you saw too few or too many phyla. Understanding came with the evolutionary perspective of phylogeny reconstruction.