Burgues Shale

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Burgess Shale:
Cambrian Explosion in Full Bloom
James W. Hagadorn

he middle cambrian burgess shale is one of the

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world’s best-known and best-studied fossil deposits. The story of
the discovery of its fauna is a famous part of paleontological lore.
While searching in 1909 for trilobites in the Burgess Shale Formation of
the Canadian Rockies,Charles Walcott discovered a remarkable “phyllopod crustacean” on a shale slab (Yochelson 1967). Further searching
revealed a diverse suite of soft-bodied fossils that would later be described
as algae, sponges, cnidarians, ctenophores, brachiopods, hyoliths, priapulids, annelids, onychophorans, arthropods, echinoderms, hemichordates, chordates, cirripeds, and a variety of problematica. Many ofthese
fossils came from a single horizon, in a lens of shale 2 to 3 m thick, that
Walcott called the Phyllopod (leaf-foot) Bed. Subsequent collecting at
and near this site by research teams led by Walcott, P. E. Raymond, H. B.
Whittington, and D. Collins has yielded over 75,000 soft-bodied fossils,
most of which are housed at the Smithsonian Institution in Washington,
D.C., and the RoyalOntario Museum (ROM) in Toronto.
Although interest in the Burgess Shale fauna has waxed and waned
since its discovery, its importance has inspired work on other Lagerstätten and helped galvanize the paleontological community’s attention on
soft-bodied deposits in general. For example, work on the Burgess Shale
has stimulated work on the older Chengjiang fauna (Chapter 3), as well
as a number ofother Burgess Shale–type localities from around the
world (Chapter 5).
In the first descriptions of the Burgess fauna, Walcott placed most of
the new taxa (over 110 species) within existing taxonomic groups.

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Burgess Shale

Among other reasons, this was largely because his specimen analysis was
based on examination of unprepared singleshale slabs. Because many
specimens are oriented obliquely to bedding planes, folded over on
themselves, and/or contain shale intercalated between carcass segments,
subsequent dissection and analysis of part–counterpart slabs allowed
more detailed morphological reconstructions and hence more accurate
taxonomic assignments (Whittington 1971a). Together with reexcavation
of original collectingsites and documentation of new collection sites, reexamination of Burgess Shale specimens using these techniques has
yielded a variety of new interpretations about these organisms and provided more accurate phylogenetic, paleoecological, and environmental
information about Middle Cambrian marine life.
The fauna of the deposit includes both relatively common skeletonized forms and an abundance ofsoft-bodied forms seldom preserved
in typical Cambrian paleoenvironments. This conservation Lagerstätten
is thus unique because it provides the most comprehensive view of a typical benthic paleocommunity during the Cambrian explosion. Early in its
history, this deposit was recognized as exceptional (Walcott 1911b), not
only because it was the first documented example of a nearly completeCambrian paleocommunity, but because at that time the Cambrian represented the oldest accepted record of animal life. Faunas from these
strata are preserved in fine-grained obrution deposits that were transported via fluidized flows into a poorly oxygenated basin or trough adjacent to a major carbonate escarpment. Rapid burial, low oxygenation,
and early diagenetic clay replacement of carcasses allowedthe preservation of a variety of soft- and hard-bodied organisms representing most
major marine phyla, as well as several morphologically distinctive organisms of uncertain taxonomic affinity.

Geological Context
The Burgess Shale is an informal name for two fossiliferous shale members of the Burgess Shale Formation, which are well exposed near the
town of Field, British Columbia (Figure...