Skeletons in the Closet: the Evolution of Hardparts in Metazoa

(and the Cambrian Explosion)

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)

Additional Comments on Ediacaran Fauna

Pound QuartziteEdiacara Hills, north of Sydney, Australia

Metazoan Fossils Found in 1946

Discovery of Ediacaran Fauna

Classification: lumped together as “medusoids”

Dickinsonia

Mawsonites Spriggina

Details originally overlooked:A diverse assemblages of fossils

Adolph SeilacherConcept 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)

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

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.

Why make skeletons ?

Tommotian Fauna: first skeletonized organisms

Records rapid diversification of metazoa after extinction of many Ediacaran forms

(at base of Cambrian(scale bar: 1 mm)

Oldest widespread skeletons: all phosphatic !

Why Do So Many Metazoans Have Skeletons ?:

1. 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

Charles Walcott

Discovered Middle Cambrian Burgess Shale

Burgess Shale, Yoho National Park, B.C.

Exceptionally preserved soft-bodied organismsMiddle Cambrian age (shortly after “Cambrian Explosion” of Early Cambrian)A snapshot of life assemblages (in general, fossil recordonly preserves hard parts of organisms) Surprisingly diverse assemblage with very unusual forms

Vauxia -sponge Vauxia “forest” with Leanchoilia

Wiwaxia

A worm, a mollusc, or something completely different ?

Marella

An early arthropod (presumably related to trilobites)

AnomalocarisAn unusual shrimp-like arthropod ?

PeytoiaA jellyfish ?

Anomalocaris: A Case of Mistaken Identity

Anomalocaris: A composite of components previously

Believed to be separate organisms

(Peytoia)

(Original Anomalocaris)

(Peytoia)

Anomalocaris – oblique viewA “Lobopod”

Anomalocaris - reconstruction

Opabinia(Lobopod ?)

Hallucigenia Onychophoran worm or separate phylum ?)

Pikaiaearliest known Chordate

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

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

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 numberof phyla through time

Gould’s View:Sudden appearance of phyla,removal of many by mass extinction

END OF LECTURE