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ancestors were present during the
Cambrian Explosion. A narrow anterior
end has a pair of tentacles, while the
posterior is expanded into a fin-like tail.
The cone-in-cone arrangement of the
muscles is often clearly preserved.
mainly of sponges, such as the branching
Vauxia , and others, such as Choia and
Pirania adorned with sharp, glassy
spicules which both supported the
skeleton and protected against predators.
The sea pen, Thaumaptilon , was a rare
member of the sessile epifauna, as was the
enigmatic animal Dinomischus , projec-
ting just 1 cm (0.4 in) above the mud on a
thin stalk and looking like a small flower.
The vagrant epifauna was more varied,
but was dominated by arthropods, only a
small proportion of which were trilobites,
such as Olenoides and the soft-bodied
Naraoia . It also included the ubiquitous
'lace crab', Marrella , and a number of
small, scavenging lobopods, most notably
Hallucigenia and Aysheaia . The most
bizarre mud crawler was Wiwaxia , which
moved across the sediment using its slug-
like foot.
Animals which lived above the mud
surface were fewer in number, comprising
only about 10% of the Burgess biota,
simply because they were more able to
escape the mud flow by swimming away.
The nektobenthic animals (near-bottom
swimmers) were, however, within the
range of turbidity flows and included the
tiny chordate Pikaia . The medusoid
Eldonia , which is more probably related to
the holothurians (sea cucumbers) than to
true jellyfish, may be regarded as
nektobenthos and/or a planktonic floater.
Nekton (forms swimming above the
substrate) include the giant predator
Anomalocaris
P ALEOECOLOGY OF THE
B URGESS S HALE
The Burgess Shale represents a marine,
benthic community living in, on, or just
above the muddy seabed at the foot of a
submarine cliff, where the mud was
banked sufficiently high to be clear of
stagnant bottom waters. The marine basin
faced the open sea and was situated within
the tropical zone at about 15°N. The
presence of photosynthesizing algae
suggests that the depth was not much
more than 100 m (330 ft).
Paleoecological analysis by Conway
Morris (1986) examined over 30,000
slabs of shale with 65,000 fossils.
Approximately 10% of the biota consisted
of benthic infauna, i.e., living in the
sediment itself, this community being
dominated by the burrowing priapulid
worms such as Ottoia , Selkirkia , and
Louisella , and polychaete worms such as
Burgessochaeta and Canadia .
The vast majority of Burgess animals
(c. 75%) consisted of benthic epifauna,
living on the sediment surface, and
divided into the fixed or sessile epifauna
(c. 30%) and the vagrant epifauna
(c. 40%), that walked or crawled across
the seabed. The sessile biota consisted
and
the
enigmatic
47
47 Reconstruction of Pikaia.
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