Significance of the Cambrian Explosion

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Significance of the Cambrian Explosion
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Base of Cambrian/Paleozoic/Phanerozoic
The base of the Phanerozoic Eon is defined by the
first appearance of burrows (presumably made by
worm-like organisms). This indicates the
evolution of a coelom that could be manipulated
for locomotion and feeding. Burrowing also added
an important tier to community structure (no
organisms burrowed prior to this time).
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Oldest Skeletons
Tommotian Fauna (small shelly fossils) first
skeletonized organisms Early Cambrian (scale
bar 1 mm) Typically phosphatic (but calcified
forms now known) !
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Why Do So Many Metazoans Have Skeletons ?

• Receptacles for excess mineral matter
• - note Kidney Stones and Gallstones in humans
• - warm seawater commonly saturated with calcium
  carbonate
• 2. Storehouses for scarce minerals (or means of
  slow release)
• -bones and teeth, some shells composed of
  calcium phosphate
• - phosphate rather scarce in nature, but
  essential for
• metabolism (e.g. Adenosine triphosphate-
  ATP)
• and is also key component of genetic material
• -calcium essential for heart, nerve, muscle
  functions,
• enzyme activation
• 3. Support and muscle attachment areas for
  locomotory organs
• 4. Serves as protective cage for soft internal
  organs
• 5. In shelled organisms, serves as a box to
  ensure controlled
• environment for metabolic functions

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The Pitfalls of Preservation
It is obvious that a major change occurred in the
Earths metazoan biota by Cambrian time
(particularly in the development of skeletal
tissue). But while skeletal remains give us some
indication of the magnitude of change that
occurred in the earliest Phanerozoic, the perils
of fossil preservation prevent us from seeing the
entire biota (both skeletonized and
soft-bodied). But we do have a window shortly
after this (Middle Cambrian). This window is the
Burgess Shale.
Trilobites from Middle Cambrian Burgess Shale
showing soft part preservation
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Charles Walcott
Discovered Middle Cambrian Burgess Shale
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Burgess Shale, Yoho National Park, B.C.
Exceptionally preserved soft-bodied
organisms Middle Cambrian age (shortly after
Cambrian Explosion of Early Cambrian) About 505
ma A snapshot of life assemblages (in general,
fossil record only preserves hard parts of
organisms) Surprisingly diverse assemblage with
very unusual forms
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Vauxia -sponge
Vauxia forest with Leanchoilia
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Wiwaxia
A worm, a mollusc, or something completely
different ?
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Marella
An early arthropod (presumably related to
trilobites)
Marella caught in the act of molting
Marella with squished-out guts
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Anomalocaris A Case of Mistaken Identity
Anomalocaris An unusual shrimp-like arthropod ?
Peytoia A jellyfish ?
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Anomalocaris A composite of components
previously Believed to be separate organisms
(Peytoia)
(Peytoia)
(Original Anomalocaris)
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Anomalocaris oblique view A Lobopod
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Anomalocaris - reconstruction
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Opabinia (Lobopod ?)
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The great Hallucigenia flip-out
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Hallucigenia Presently classified as onychophoran
Hallucigenia
Modern velvet worms (onychophorans) in
tropical rainforests
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An aside are arthropods just onychophorans with
exoskeletons ?
millipede
onychophoran
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Pikaia earliest known Chordate
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Sanctacaris (Santa Claws)
An undoubted arthropod (A Chelicerate without
chelicerae ?) Subphylum Chelicerata includes
spiders, mites, ticks scorpions,horseshoe crabs
all having feeding appendages called
chelicerae Makes one wonder what really defines
this group
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Chengjiang Fauna
A similar Burgess Shale- type biota has since
been found in the Chengjiang County, Yunnan
Province, China. Yu'anshan Member of the
Heilinpu Formation. These fossils are about 525
ma (Early Cambrian), and therefore slightly older
than the Burgess Shale fossils.
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Haikouella lanceolata
Of major importance in the Chengjiang biota is
the occurrence of a chordate named Haikouella
(this is older than Pikaia, which was previously
declared to be the oldest chordate). It is
decidedly lamprey-like, indicating the appearance
of jawless, fish-like chordates by the early
Cambrian (even though the oldest definite remains
of jawless fish date to the Early Ordovician)
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Note differences in community structure
Cambrian marine community
Ediacaran marine community
Carnivores Suspension feeders Deposit
feeders The first arms race !
All suspension feeders (or at least passive food
gatherers)
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Catastrophic Burial
Burgess Shale organisms living on foot of
escarpment (and possibly on edge and top of
escarpment as well) smothered by due to
slumping, Killed and buried instantly Rapid
burial low oxygen ( possible mineralization in
vicinity of cold seeps ?) exceptional
preservation
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Stephen Jay Gould Wonderful Life
Could some Burgess Shale organisms belong to
extinct phyla ? Is it possible that a phylum
could be represented by few or single species
? If so, suggests that the Cambrian Explosion
produced more phyla than are present today.
This view has been softened a bit since Goulds
publication of Wonderful Life (perhaps more
Classes than today)
extinction
Conventional view Gradual increase in number of
phyla through time
Goulds View Sudden appearance of phyla, removal
of many by mass extinction
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END OF LECTURE