Skeletons in the Closet:

1
Skeletons in the Closet the Evolution of
Hardparts in Metazoa (and the Cambrian Explosion)
2
Additional Comments on Ediacaran Fauna
Remember earliest complex metazoa (Ediacaran
fauna) were all soft bodied (no skeletonized
tissue). All preserved as impressions. Note
that in order for differentiated tissues to have
evolved (to allow development of organs for
different purposes), Hox genes must have
developed Hox genes dictate the duties of
particular cells (for particular purposes)
3
Discovery of Ediacaran Fauna
Pound Quartzite Ediacara Hills, north of Sydney,
Australia Metazoan Fossils Found in 1946
Classification lumped together as medusoids
4
Details originally overlooked A diverse
assemblages of fossils
Mawsonites
Spriggina
Dickinsonia
5
Adolph Seilacher Concept of Vendozoa (extinct
phylum ?)
soft bodied quilted structure (fluid-filled
bags ? Dependent on microbial mats mat
stickers fixed to seafloor, photosynthesizers m
at scratchers grazed on microbial
mats Seilacher has suggested that this was a
failed experiment in evolution and that metazoans
developed once again from scratch in the very
latest Proterozoic No carnivores ! (and no
evidence of predation until appearance of
skeletons)
6
Latest Proterozoic (Ediacaran Period) Also, note
that atmosphere and seas were well-oxygenated by
this time, so metazoa had developed the means to
cope with the high reactivity of oxygen
7
Why make skeletons ?
One might think that the development of hardparts
(by at least the earliest Cambrian) came about
due to the need for structural support or
protection from predation. But some of the
Ediacaran animals were already fairly large
(remember the frond from Newfoundland up to 2
metres long). Plus there is no evidence of
predation in the Ediacaran. So why would
skeletons have developed ? Origin of skeletons
may have to do with the chemical significance of
minerals in organisms.
8
Oldest widespread skeletons all phosphatic !
Tommotian Fauna first skeletonized organisms
Records rapid diversification of metazoa after
extinction of many Ediacaran forms (at base of
Cambrian (scale bar 1 mm)
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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

10
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) A
snapshot of life assemblages (in general, fossil
record only preserves hard parts of organisms)
Surprisingly diverse assemblage with very
unusual forms
12
Vauxia -sponge
Vauxia forest with Leanchoilia
13
Wiwaxia
A worm, a mollusc, or something completely
different ?
14
Marella
An early arthropod (presumably related to
trilobites)
15
Anomalocaris A Case of Mistaken Identity
Anomalocaris An unusual shrimp-like arthropod ?
Peytoia A jellyfish ?
16
Anomalocaris A composite of components
previously Believed to be separate organisms
(Peytoia)
(Peytoia)
(Original Anomalocaris)
17
Anomalocaris oblique view A Lobopod
18
Anomalocaris - reconstruction
19
Opabinia (Lobopod ?)
20
Hallucigenia Onychophoran worm or separate
phylum ?)
21
Pikaia earliest known Chordate
22
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
23
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
24
Stephen Jay Gould Radical Thinker
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
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