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. Concept of “Vendozoa”:
Adolph Seilacher
Concept of “Vendozoa”:
(extinct phylum ?)
soft bodied
“quilted” structure (fluid-filled bags ?”
Dependent on microbial mats
“mat stickers”: fixed to seafloor, photosynthesizers
“mat 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. first skeletonized organisms
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)

9. 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
6. Protection from predators

10. Charles Walcott
Discovered Middle Cambrian Burgess Shale

11. 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
An unusual shrimp-like arthropod ?
Peytoia
A jellyfish ?

16. A composite of components previously Believed to be separate organisms
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. Onychophoran worm or separate phylum ?)
Hallucigenia
Onychophoran worm or separate phylum ?)

21. earliest known Chordate
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
Gould’s View:
Sudden appearance of phyla,
removal of many by mass extinction

25. END OF LECTURE