The Burgess Shale and other exceptional Cambrian faunas
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:
- 600- 540 ma. The Ediacaran fauna significantly predated the explosion.
- 550- 530 ma. The Small Shelly Faunas of the earliest Cambrian show that animals were experimenting with the secretion of hard skeletons for 20-30 million years prior to the Cambrian Explosion. There were minute elements resembling either small versions of the shells of better-known later creatures or small modules of such shells.
- 530 ma. During the Early Cambrian we suddenly see the appearance of the hard-parts of familiar-looking fossil organisms, brachiopods, archaeocyathids. These are joined at about 520 ma by trilobites, arthropods that calcify their cuticle.
- Remember, the Early Cambrian is the age of the Cambrian substrate revolution, when infaunal creatures began churning up sediment and eliminating the old Ediacaran algal mat meadows.
- Changes in sea-water chemistry
- Changes in mean sea temperature, facilitating the metabolism of hard parts.
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:
- The Chenjiang fauna of Yunnan, China (Early Cambria: 520-515 ma)
- The Sirius Passet fauna of Greenland (Early Cambrian: 518-505 ma)
- The Burgess Shale fauna of British Columbia, Canada (Middle Cambrian: 507 ma)
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:
- Completely soft bodied creatures
- Arthropods without calcified cuticle
Ecologically, the fauna equally preserves:
- Benthic epifaunal animals
- Benthic infaunal animals (pioneers of a new region of ecospace!)
- Nektonic animals that dies while swimming or floating above the depositional environment.
A Burgess Shale Bestiary
Enigmas spawn controversy:
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.
. E.G.: Odontogriphus omalus. Originally reconstructed as a flattened, segmented animal with an array of tentacles around its mouth vaguely reminiscent of a lophophore.
Wiwaxia corrugata. A little, unsegmented box of meat covered with calcareous scales reminiscent of pieces of the Small Shelly Fauna. Reconstruction.
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.
The Burgess Shale has been studied for over a century, in three phases:
This history of study reveals three general philosophies:
- 1909-1927: by Charles Walcott, the site's discoverer
- 1967-present: by the Cambridge University group, primarily, Harry Whittington, Derek Briggs, and Simon Conway-Morris.
- 1989: the history of Burgess Shale studies scrutinized by Stephen J. Gould for the book Wonderful Life
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):
- "Walcott's shoehorn": Walcott, studied the fossils while Secretary of the Smithsonian Institution from 1907 to 1927. He invariably interpreted these in terms of the strict typology of the established ranks and taxa of the Linnean System. Many ambiguous taxa were relegated to the hopelessly paraphyletic phylum "Vermes" - worms. The effect was to mask the unique diversity of specimens coming from the site.
- "Initial diversity, winnowing, and luck": In his 1990 book, Gould argues that Walcott's typologic style not only predisposed him not to appreciate the diversity of the Burgess Shale fauna, but blinded him to something more fundamental and paradoxical - that there had been much greater animal diversity at the beginning of the Phanerozoic than in more recent times. After all, many of the Burgess Shale critters did not fit into any established Linnean phylum. Indeed, by the accounting of the Linnean System, there were more phyla in the Cambrian than at any other time. Earth had witnessed a truly explosive initial radiation of animal diversity followed by a slow winnowing down to the phyla that dominated later faunas. Were the "winners" (i.e. the familiar phyla) better? Not at all. Gould argues that the sweepstakes was probablistic and that were we to rerun the tape, modern oceans might be ful of five-eyed descendants of Opabinia.
- "Reappraising relationships of the Burgess Shale enigmas": Simon Conway-Morris, a member of the Cambridge group, responded in 1998 with his own book, Crucible of Creation, in which he took Gould to task for failing to ask the crucial questions about the true relationships of the Burgess shale enigmas, phyla notwithstanding. Although not entirely comfortable with Phylogenetic Systematics, Conway-Morris argues that these relationships can be identified and that they undermine Gould's basic premise. (But don't believe me. Read a diologue between Conway-Morris and Gould here.)
There are three major monophyletic groups:
- Deuterostomia: I.e. Chordates, echinoderms, etc.
- Lophotrochozoa: I.e. Annelids, mollusks, brachiopods, etc.
- Ecdysozoa: I.e. Arthropods, onychophorans, etc.
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:
- The true nature of Hallucigenia was illuminated by the discovery of onychophorans from Chenjiang In these forms, the spines are sufficiently reduced in height to see that they are armored onychophorans. We had Hallucigenia upside-down.
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.