| Summary: | The Middle Cambrian Burgess Shale Formation 1S a unique deposit hosting a
remarkable diversity of soft-bodied and skeletal organisms. This unit onlaps the
shallow-water dolomites of the Cathedral Formation which have been cut by a near vertical
truncation surface known as the Cathedral Escarpment, a feature continuous for
nearly 100 km.
An abundant and diverse fauna is known from the classic quarries on Fossil Ridge
between Mount Field and Mount Wapta, but other fossiliferous localities are present
along the strike of the Cathedral Escarpment. In order to understand the ecological and
paleontological basis for lateral variations in faunal composition within the Burgess
Shale, exposures 60 km to the southeast of the type area were examined at The
Monarch, a mountain straddling the British Columbia/Alberta border. In four closely
spaced sections, faunal elements are present only in the basal 30 m of the silty shales
immediately adjacent to the Cathedral Escarpment; fossil occurrence decreases
dramatically barely tens of meters away. The fauna is dominated by priapulid worms,
hyolithids, and phosphatic and calcareous brachiopods, which together constitute more
than 75% of specimens counted. Trilobites (mainly Olenoides serratus), sponges
(mainly Vauxia spp.) and phosphatic tubes (Byronia annulata) are also significant
components of the fauna.
The fauna at The Monarch is much less diverse than that of the classic Burgess Shale
on Fossil Ridge. A prominent difference is the absence of non-trilobite arthropods (e.g.
Marrella splendens and Anomalocaris canadensis) and most soft-bodied elements.
While all four sections at The Monarch are equally fossiliferous, differences between localities are evident in the distribution of the rarer biotic elements. Specifically, the
arthropod Sidneyia inexpectans, the sponge Takakkawia sp. nov., and the probable
lepadomorph barnacle Priscansermarinus barnetti occur, albeit rarely, only on the
southern exposures.
Fault-related fluid seeps along the base of the Cathedral Escarpment are a possible
explanation for the localized biota. Sporadic and variably active seeps could have
generated localized and intermittent increases in biologic productivity, especially
bacteria, which in tum would have resulted in patchy concentrations of fauna close to
the escarpment, a pattern consistent with the contrast in assemblages between The
Monarch and the type area.
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